Publications Database
2024
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Publication: Neural Networks 170, 94 (2024)Doi: 10.1016/j.neunet.2023.10.054
Recent work has shown that machine learning (ML) models can skillfully forecast the dynamics of unknown chaotic systems. Short-term predictions of the state evolution and long-term predictions of the statistical patterns of the dynamics (“climate”) can be produced by employing a feedback loop, whereby the model is trained to predict forward only one time step, then the model output is used as input for multiple time steps. In the absence of mitigating techniques, however, this feedback can result in artificially rapid error growth (“instability”). One established mitigating technique is to add noise to the ML model training input. Based on this technique, we formulate a new penalty term in the loss function for ML models with memory of past inputs that deterministically approximates the effect of many small, independent noise realizations added to the model input during training. We refer to this penalty and the resulting regularization as Linearized Multi-Noise Training (LMNT). We systematically examine the effect of LMNT, input noise, and other established regularization techniques in a case study using reservoir computing, a machine learning method using recurrent neural networks, to predict the spatiotemporal chaotic Kuramoto–Sivashinsky equation. We find that reservoir computers trained with noise or with LMNT produce climate predictions that appear to be indefinitely stable and have a climate very similar to the true system, while the short-term forecasts are substantially more accurate than those trained with other regularization techniques. Finally, we show the deterministic aspect of our LMNT regularization facilitates fast reservoir computer regularization hyperparameter tuning.
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Publication: J. Infrared Millim. Terahz. Waves (February, 2024)Doi: 10.1007/s10762-024-00972-3
In this paper we present a comprehensive overview of the theoretical and experimental studies on gyrotrons operating at harmonics of the electron cyclotron frequency. Besides the conventional (small-orbit) gyrotrons, three other types of such devices are considered, namely large-orbit gyrotrons (LOG), double-beam gyrotrons, and gyro-devices with a frequency multiplication. Based on a comparison between them and the devices that work on the fundamental resonances, both the advantages and disadvantages of the harmonic gyrotrons are critically examined. Such an analysis is helpful for choosing between different alternative concepts in the design process of appropriate radiation sources for various applications.
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Publication: Astrophys. J. 964, 97 (2024)Doi: 10.3847/1538-4357/ad217f
The waves generated by high-energy proton and alpha particles streaming from solar flares into regions of colder plasma are explored using particle-in-cell simulations. Initial distribution functions for the protons and alphas consist of two populations: an energetic, streaming population represented by an anisotropic (T∥ > T⊥), one-sided kappa function and a cold, Maxwellian background population. The anisotropies and nonzero heat fluxes of these distributions destabilize oblique waves with a range of frequencies below the proton cyclotron frequency. These waves scatter particles out of the tails of the initial distributions along constant-energy surfaces in the wave frame. Overlap of the nonlinear resonance widths allows particles to scatter into near-isotropic distributions by the end of the simulations. The dynamics of ³He are explored using test particles. Their temperatures can increase by a factor of nearly 20. Propagation of such waves into regions above and below the flare site can lead to heating and transport of ³He into the flare acceleration region. The amount of heated ³He that will be driven into the flare site is proportional to the wave energy. Using values from our simulations, we show that the abundance of ³He driven into the acceleration region should approach that of ⁴He in the corona. Therefore, waves driven by energetic ions produced in flares are a strong candidate to drive the enhancements of ³He observed in impulsive flares.
2023
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Publication: J. Plasma Phys. 89, 945890401 (2023)Doi: 10.1017/S0022377823000326
The published article incorrectly contains a placeholder reference to an equation (“(2.2)”) on page 2, paragraph 2, line 4:
“The high-beta, β ∼ 1, regime has been explored previously in the context of asymptotic magnetohydrodynamic (MHD) equilibria by solving the Grad–Shafranov equation in the limit ε/(βq2) << 1 (Hsu, Artun & Cowley 1996) where ε is the aspect ratio of the tokamak and q is the safety factor defined in (2.2):.”
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Publication: Astrophys. J. 948, 20 (2023)Doi: 10.3847/1538-4357/acb6f1
We investigate the detailed properties of electron inflow in an electron-only reconnection event observed by the four Magnetospheric Multiscale (MMS) spacecraft in the Earth's turbulent magnetosheath downstream of the quasi-parallel bow shock. The lack of ion coupling was attributed to the small-scale sizes of the current sheets, and the observed bidirectional super-Alfvénic electron jets indicate that the MMS spacecraft crossed the reconnecting current sheet on both sides of an active X-line. Remarkably, the MMS spacecraft observed the presence of large asymmetries in the two electron inflows, with the inflows (normal to the current sheet) on the two sides of the reconnecting current layer differing by as much as a factor of four. Furthermore, even though the four MMS spacecraft were separated by less than seven electron skin depths, the degree of inflow asymmetry was significantly different at the different spacecraft. The asymmetry in the inflow speeds was larger with increasing distances downstream from the reconnection site, and the asymmetry was opposite on the two sides of the X-line. We compare the MMS observations with a 2D kinetic particle-in-cell (PIC) simulation and find that the asymmetry in the inflow speeds stems from in-plane currents generated via the combination of reconnection-mediated inflows and parallel flows along the magnetic separatrices in the presence of a large guide field.
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Publication: Phys. Rev. Lett. 131, 100801 (2023)Doi: 10.1103/PhysRevLett.131.100801
We demonstrate quantum logic enhanced sensitivity for a macroscopic ensemble of solid-state, hybrid two-qubit sensors. We achieve over a factor of 30 improvement in the single-shot signal-to-noise ratio, translating to an ac magnetic field sensitivity enhancement exceeding an order of magnitude for timeaveraged measurements. Using the electronic spins of nitrogen vacancy (NV) centers in diamond as sensors, we leverage the on-site nitrogen nuclear spins of the NV centers as memory qubits, in combination with homogeneous and stable bias and control fields, ensuring that all of the ∼109 two-qubit sensors are sufficiently identical to permit global control of the NV ensemble spin states. We find quantum logic sensitivity enhancement for multiple measurement protocols with varying optimal sensing intervals, including XY8 and DROID-60 dynamical decoupling, as well as correlation spectroscopy, using an applied ac magnetic field signal. The results are independent of the nature of the target signal and broadly applicable to measurements using NV centers and other solid-state spin ensembles. This work provides a benchmark for macroscopic ensembles of quantum
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Publication: Rev. Sci. Instrum. 94, 125109 (2023)Doi: 10.1063/5.0168764
Dielectric breakdown is an example of a natural phenomenon that occurs on very short time scales, making it incredibly difficult to capture optical images of the process. Event initiation jitter is one of the primary challenges, as even a microsecond of jitter time can cause the imaging attempt to fail. Initial attempts to capture images of dielectric breakdown using a gigahertz frame rate camera and an exploding bridge wire initiation were stymied by high initiation jitter. Subsequently, a novel optical delay line apparatus was developed in order to effectively circumvent the jitter and reliably image dielectric breakdown. The design and performance of the optical delay line apparatus are presented. The optical delay line increased the image capture success rate from 25% to 94% while also permitting enhanced temporal resolution and has application in imaging other high-jitter, extremely fast phenomena.
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Publication: Space Sci. Rev. 219, 75 (2023)Doi: 10.1007/s11214-023-01011-8
Particles are accelerated to very high, non-thermal energies during explosive energy-release phenomena in space, solar, and astrophysical plasma environments. While it has been established that magnetic reconnection plays an important role in the dynamics of Earth’s magnetosphere, it remains unclear how magnetic reconnection can further explain particle acceleration to non-thermal energies. Here we review recent progress in our understanding of particle acceleration by magnetic reconnection in Earth’s magnetosphere. With improved resolutions, recent spacecraft missions have enabled detailed studies of particle acceleration at various structures such as the diffusion region, separatrix, jets, magnetic islands (flux ropes), and dipolarization front. With the guiding-center approximation of particle motion, many studies have discussed the relative importance of the parallel electric field as well as the Fermi and betatron effects. However, in order to fully understand the particle acceleration mechanism and further compare with particle acceleration in solar and astrophysical plasma environments, there is a need for further investigation of, for example, energy partition and the precise role of turbulence.
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Publication: Astrophys. J. 957, 103 (2023)Doi: 10.3847/1538-4357/acfa77
Low-collisionality plasma in a magnetic field generically develops anisotropy in its distribution function with respect to the magnetic field direction. Motivated by the application to radiation from accretion flows and jets, we explore the effect of temperature anisotropy on synchrotron emission. We derive analytically and provide numerical fits for the polarized synchrotron emission and absorption coefficients for a relativistic bi-Maxwellian plasma (we do not consider Faraday conversion/rotation). Temperature anisotropy can significantly change how the synchrotron emission and absorption coefficients depend on observing angle with respect to the magnetic field. The emitted linear polarization fraction does not depend strongly on anisotropy, while the emitted circular polarization does. We apply our results to black hole imaging of Sgr A* and M87* by ray tracing a GRMHD simulation and assuming that the plasma temperature anisotropy is set by the thresholds of kinetic-scale anisotropydriven instabilities. We find that the azimuthal asymmetry of the 230 GHz images can change by up to a factor of 3, accentuating (T⊥ > T∥) or counteracting (T⊥ < T∥) the image asymmetry produced by Doppler beaming. This can change the physical inferences from observations relative to models with an isotropic distribution function, e.g., by allowing for larger inclination between the line of sight and spin direction in Sgr A* . The observed image diameter and the size of the black hole shadow can also vary significantly due to plasma temperature anisotropy. We describe how the anisotropy of the plasma can affect future multifrequency and photon ring observations. We also calculate kinetic anisotropy-driven instabilities (mirror, whistler, and firehose) for relativistically hot plasmas.
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Publication: Appl. Phys. Lett. 123, 240502 (2023)Doi: 10.1063/5.0181707
Microresonator Kerr optical frequency combs currently constitute a well-established research area in integrated, nonlinear, and quantum photonics. These systems have found a plethora of technological applications, while serving as an excellent platform to investigate fundamental scientific topics such as light–matter interactions, pattern formation in driven-dissipative systems, or entangled twin-photon generation. We here provide a brief overview of the topic, highlight some of the most recent advances, and discuss a few of the main challenges ahead in this field.
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Publication: Astrophys. J. Lett. 956, L33 (2023)Doi: 10.3847/2041-8213/acfdae
The presence of magnetic fields in the late inspiral of black hole–neutron star binaries could lead to potentially detectable electromagnetic precursor transients. Using general-relativistic force-free electrodynamics simulations, we investigate premerger interactions of the common magnetosphere of black hole–neutron star systems. We demonstrate that these systems can feature copious electromagnetic flaring activity, which we find depends on the magnetic field orientation but not on black hole spin. Due to interactions with the surrounding magnetosphere, these flares could lead to fast-radio-burst-like transients and X-ray emission, with LEM ≲ 1041(B∗/1012G)2 erg s−1 as an upper bound on the luminosity, where B∗ is the magnetic field strength on the surface of the neutron star.
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Publication: Astrophys. J. Lett. 959 (2023)Doi: 10.3847/2041-8213/ad12a5
Switchbacks (SBs) are localized structures in the solar wind containing deflections of the magnetic field direction relative to the background solar wind magnetic field. The amplitudes of the magnetic field deflection angles for different SBs vary from ~40 to ~160-170 degrees. Alignment of the perturbations of the magnetic field and the bulk solar wind velocity is observed inside SBs and causes spiky enhancements of the radial bulk velocity inside SBs. We have investigated the deviations of SB perturbations from Alfvénicity by evaluating the distribution of the parameter defined as the ratio of the parallel to ΔB component of ΔV to ΔVA = ΔB/4πnimi inside SBs, i.e., α = V||/|ΔVA| ( α = |ΔV| /|ΔVA| when ΔV ∼ ΔB), which quantifies the deviation of the perturbation from an Alfvénic one. Based on Parker Solar Probe (PSP) observations, we show that α inside SBs has systematically lower values than it has in the pristine solar wind: α inside SBs observed during PSP Encounter 1 were distributed in a range of 0.2-0.9. The upper limit on α is constrained by the requirement that the jump in velocity across the switchback boundary be less than the local Alfvén speed. This prevents the onset of shear flow instabilities. The consequence is that the perturbation of the proton bulk velocity in SBs with deflection greater than 60 degrees cannot reach α = 1 (the Alfvénicity condition) and the highest possible α for a SB with the full reversal of B is 0.5. These results have consequences for the interpretation of switchbacks as large amplitude Alfvén waves.
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Publication: Monthly Notices Royal Astronom. Soc. 525, 4985 (2023)Doi: 10.1093/mnras/stad2609
The nature of cosmic ray (CR) transport in the Milky Way remains elusive. The predictions of current microphysical CR transport models in magnetohydrodynamic (MHD) turbulence are drastically different from what is observed. These models usually focus on MHD turbulence with a strong guide field and ignore the impact of turbulent intermittency on particle propagation. This motivates our studying the alternative regime of large-amplitude turbulence with δB/B0 ≫ 1, in which intermittent small-scale magnetic field reversals are ubiquitous. We study particle transport in such turbulence by integrating trajectories in stationary snapshots. To quantify spatial diffusion, we use a set-up with continuous particle injection and escape, which we term the turbulent leaky box. We find that particle transport is very different from the strong guide-field case. Low-energy particles are better confined than high-energy particles, despite less efficient pitch-angle isotropization at small energies. In the limit of weak guide field, energy-dependent confinement is driven by the energy-dependent (in)ability to follow reversing magnetic field lines exactly and by the scattering in regions of ‘resonant curvature’, where the field line bends on a scale that is of the order of the local particle gyro-radius. We derive a heuristic model of particle transport in magnetic folds that approximately reproduces the energy dependence of transport found numerically. We speculate that CR propagation in the Galaxy is regulated by the intermittent field reversals highlighted here and discuss the implications of our findings for CR transport in the Milky Way.
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Publication: J. Vac. Sci. Technol. B 41, 062201 (2023)Doi: 10.1116/6.0002868
The scalloping of oxide-nitride-oxide (ONO) stacked layers on vertical sidewalls during high-aspect-ratio contact etch is commonly seen and characterized by the horizontal etching of oxide and nitride layers at different etch rates. To understand the mechanisms of ONO scalloping in complex plasma chemistry, it is crucial to examine the surface chemistry of silicon dioxide and silicon nitride processed with single fluorocarbon (FC) or hydrofluorocarbon (HFC) gases. To simulate the isotropic etching of SiO2 and Si3N4 sidewalls, we use a horizontal trench structure to study the effect of neutral radicals produced by FC (Ar/C4F8), HFC (Ar/CH3F, CH2F2, or CH3F), FC/HFC (Ar/C4F8/CH2F2), or FC/H2 (Ar/C4F8/H2), plasma for aspect-ratio (AR) up to 25. To eliminate the effect of ions, oxide and nitride trench structures were treated by inductively coupled plasma. The changes in the film thickness as a function of AR were probed by ellipsometry. Additionally, x-ray photoelectron spectroscopy (XPS) measurements on oxide and nitride substrates processed by Ar/C4F8 and Ar/CH2F2 plasma were performed at various locations: outside of the trench structure, near the trench entrance (AR = 4.3), and deeper in the trench (AR = 12.9). We find a variety of responses of the trench sidewalls including both FC deposition and spontaneous etching which reflect (1) the nature of the FC and HFC gases, (2) the nature of the surfaces being exposed, and (3) the position relative to the trench entrance. Overall, both the etching and deposition patterns varied systematically depending on the precursor gas. We found that the ONO scalloping at different ARs is plasma chemistry dependent. Oxide showed a binary sidewall profile, with either all deposition inside of the trench (with FC and FC/H2 processing) or etching (HFC and FC/HFC). Both profiles showed a steady attenuation of either the deposition or etching at higher AR. On the nitride substrate, etching was observed near the entrance for HFC precursors, and maximum net etching occurred at higher AR for high F:C ratio HFC precursors like CHF3. XPS measurements performed with Ar/C4F8 and Ar/CH2F2 treated surfaces showed that Ar/C4F8 overall deposited a fluorine-rich film outside and inside of the trench, while Ar/CH2F2 mostly deposited a cross-linked film (except near the trench entrance) with an especially thin graphitic-like film deep inside the trench.
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Publication: Astrophys. J. 959, 4 (2023)Doi: 10.3847/1538-4357/ad03e2
We compare hybrid (kinetic proton, fluid electron) and particle-in-cell (kinetic proton, kinetic electron) simulations of the solar wind termination shock with parameters similar to those observed by Voyager 2 during its crossing. The steady-state results show excellent agreement between the downstream variations in the density, plasma velocity, and magnetic field. The quasi-perpendicular shock accelerates interstellar pickup ions to a maximum energy limited by the size of the computational domain, with somewhat higher fluxes and maximal energies observed in the particle-in-cell simulation, likely due to differences in the cross-shock electric field arising from electron kinetic-scale effects. The higher fluxes may help address recent discrepancies noted between observations and large-scale hybrid simulations.
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Publication: Sensors Actuators A - Physical 363, 114729 (2023)Doi: 10.1016/j.sna.2023.114729
Certain configurations of anisotropic single crystal materials can generate a thermoelectric voltage orthogonal to an induced temperature gradient. This phenomenon is known as the Transverse Seebeck Effect (TSE) and can be leveraged to fabricate simple and robust heat flux sensors. Only a small number of materials have been considered as TSE-based transducers and, among these, few have been developed into sensors with ruggedization against chemical and mechanical degradation. Here, we report on the fabrication and characterization of a rugged TSE-based heat flux sensor using prismatic antimony single crystals. The heat flux sensor was tested under static and dynamic heating scenarios. The sensor has a linear responsivity of 16.8 µV/(W/cm2) to heat fluxes spanning more than two orders of magnitude and a time constant of 4.4 s. The sensor’s response to localized heating, probed with a laser scanning technique, validated that the transduction mechanism is primarily the TSE by ruling out a sizable contribution from the conventional Seebeck effect. Finite element analysis corroborated that components used in the sensor package are the primary determinants of the time constant and the decrement of the responsivity from its theoretical maximum. Design principles that may be applied to elicit a faster transient response or higher responsivity are proposed. The results establish single crystal antimony as a promising transducer material for heat flux measurement systems and demonstrate potential effects of ruggedization on sensor performance.
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Publication: Plasma Sources Sci. Technol. 32, 125001 (2023)Doi: 10.1088/1361-6595/ad0f47
Atmospheric pressure non-equilibrium plasmas can form nitrogen oxide (NOx) compounds directly from nitrogen and oxygen without a catalyst, and at lower catalyst temperatures than would be possible without plasma. In this work, the oxidation of plasma-produced NO from an Ar/N2/O2 non-equilibrium atmospheric-pressure plasma-jet (APPJ) over a platinum-on-alumina powder catalyst was investigated with in-situ infrared spectroscopy. Products downstream of the catalyst bed were analyzed along with catalyst surface species. The catalyst was exposed to plasma at both constant temperature and a cyclic temperature ramp in order to study long-lasting and transient surface changes. Primary incident reactive species to the catalyst were assessed to be NO and O3. Pt-Al2O3 at 350°C increased oxidation of NO relative to Al2O3 or an empty chamber. The surface state of Pt-Al2O3 evolves during plasma-effluent exposure and requires upwards of 20 min exposure for stabilization compared to Al2O3. Once stable surface conditions are achieved, thermal cycling reveals a repeatable hysteresis pattern in downstream products. At low temperature, oxygen and NOx accumulate on the catalyst surface and react at elevated temperatures to form NO2. Increasing plasma power and O2:N2 ratio increases the hysteresis of the heating relative to the cooling curves in the pattern of NO2 formation. The limitation on NO oxidation at high temperatures was assessed to be Pt-O which is depleted as the catalyst is heated. Once stored species have been depleted, NO oxidation rates are determined by incoming reactants. Two overlapping NO oxidation patterns are identified, one determined by surface reactants formed at low temperature, and the other by reactants arriving at the surface at high temperature. The plasma is responsible for providing the reactants to the catalyst surface, while the catalyst enables reaction at high temperature or storage at low temperature for subsequent reaction.
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Publication: Astrophys. J. 954, 118 (2023)Doi: 10.3847/1538-4357/acdf44
We have surveyed 21 reconnection exhaust events observed by Magnetospheric MultiScale in the low-plasma-β and high-Alfvén-speed regime of the Earth's magnetotail to investigate the scaling of electron bulk heating produced by reconnection. The ranges of inflow Alfvén speed and inflow electron βe covered by this study are 800–4000 km s−1 and 0.001–0.1, respectively, and the observed heating ranges from a few hundred electronvolts to several kiloelectronvolts. We find that the temperature change in the reconnection exhaust relative to the inflow, ΔTe, is correlated with the inflow Alfvén speed, VAx,in, based on the reconnecting magnetic field and the inflow plasma density. Furthermore, ΔTe is linearly proportional to the inflowing magnetic energy per particle, miVAx,in2, and the best fit to the data produces the empirical relation ΔTe = 0.020 miVAx,in2, i.e., the electron temperature increase is on average ∼2% of the inflowing magnetic energy per particle. This magnetotail study extends a previous magnetopause reconnection study by two orders of magnitude in both magnetic energy and electron β, to a regime that is comparable to the solar corona. The validity of the empirical relation over such a large combined magnetopause–magnetotail plasma parameter range of VA ∼ 10–4000 km s−1 and βe ∼ 0.001–10 suggests that one can predict the magnitude of the bulk electron heating by reconnection in a variety of contexts from the simple knowledge of a single parameter: the Alfvén speed of the ambient plasma.
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Publication: ACS Appl. Mater. Interfaces 15, 40409 (2023)Doi: 10.1021/acsami.3c06287
Si is known for cracking and delamination during electrochemical cycling of a battery due to the large volume change associated with Li insertion and extraction. However, it has been found experimentally that patterned Si island electrodes that are 200 nm thick and less than 7 μm wide can deform in a purely elastic manner. Inspired by this, we performed in situ Raman stress characterization of model poly-crystalline Si island electrodes using an electrochemical cell coupled with an immersion objective lens and designed for a short working distance. A 5 μm wide Si island electrode showed a parabolic stress profile during lithiation, while for a 15 μm Si island electrode, a stress plateau in the center of the electrode was observed. A continuum model with coupled electro-chemo-mechanical (ECM) physics was established to understand the stress measurement. A qualitative agreement was reached between modeling and experimental data, and the critical size effect could be explained by the Li diffusive flux as governed by competition between the Li concentration and hydrostatic stress gradients. Below the critical size, the stress gradient drives Li toward the edges, where the electrode volume is free to expand, while above the critical size, the stress plateau inhibits Li diffusion to the edge and forces destructive stress relief by cracking. This work represents a promising methodology for in situ characterization of ECM coupling in battery electrodes, with suggestions provided for further improvement.
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Publication: Astrophys. J. 959, 122 (2023)Doi: 10.3847/1538-4357/acffc6
We present the results of 3D particle-in-cell simulations that explore relativistic magnetic reconnection in pair plasma with strong synchrotron cooling and a small mass fraction of nonradiating ions. Our results demonstrate that the structure of the current sheet is highly sensitive to the dynamic efficiency of radiative cooling. Specifically, stronger cooling leads to more significant compression of the plasma and magnetic field within the plasmoids. We demonstrate that ions can be efficiently accelerated to energies exceeding the plasma magnetization parameter, ≫ σ, and form a hard power-law energy distribution, fi ∝ γ−1. This conclusion implies a highly efficient proton acceleration in the magnetospheres of young pulsars. Conversely, the energies of pairs are limited to either σ in the strong cooling regime or the radiation burnoff limit, γsyn, when cooling is weak. We find that the high-energy radiation from pairs above the synchrotron burnoff limit, εc ≈ 16 MeV, is only efficiently produced in the strong cooling regime, γsyn < σ. In this regime, we find that the spectral cutoff scales as εcut ≈ εc(σ/γsyn) and the highest energy photons are beamed along the direction of the upstream magnetic field, consistent with the phenomenological models of gamma-ray emission from young pulsars. Furthermore, our results place constraints on the reconnection-driven models of gamma-ray flares in the Crab Nebula.
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Publication: Astrophys. J. 955, 34 (2023)Doi: 10.3847/1538-4357/acebeb
Using observations from the Solar Dynamics Observatory's Atmosphere Imaging Assembly and the Ramaty High Energy Solar Spectroscopic Imager, we present novel measurements of the shear of post-reconnection flare loops (PRFLs) in SOL20141218T21:40 and study its evolution with respect to magnetic reconnection and flare emission. Two quasi-parallel ribbons form adjacent to the magnetic polarity inversion line (PIL), spreading in time first parallel to the PIL and then mostly in a perpendicular direction. We measure the magnetic reconnection rate from the ribbon evolution, and also the shear angle of a large number of PRFLs observed in extreme ultraviolet passbands (≲1 MK). For the first time, the shear angle measurements are conducted using several complementary techniques allowing for cross validation of the results. In this flare, the total reconnection rate is much enhanced before a sharp increase in the hard X-ray emission, and the median shear decreases from 60°–70° to 20°, on a timescale of 10 minutes. We find a correlation between the shear-modulated total reconnection rate and the nonthermal electron flux. These results confirm the strong-to-weak shear evolution suggested in previous observational studies and reproduced in numerical models, and also confirm that, in this flare, reconnection is not an efficient producer of energetic nonthermal electrons during the first 10 minutes when the strongly sheared PRFLs are formed. We conclude that an intermediate shear angle, ≤40°, is needed for efficient particle acceleration via reconnection, and we propose a theoretical interpretation.
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Publication: Nano Lett. 24, 319 (2023)Doi: 10.1021/acs.nanolett.3c04056
Silicon T centers present the promising possibility of generating optically active spin qubits in an all-silicon device. However, these color centers exhibit long excited state lifetimes and a low Debye–Waller factor, making them dim emitters with low efficiency into the zero-phonon line. Nanophotonic cavities can solve this problem by enhancing radiative emission into the zero-phonon line through the Purcell effect. In this work, we demonstrate cavity-enhanced emission from a single T center in a nanophotonic cavity. We achieve a 2 order of magnitude increase in the brightness of the zero-phonon line relative to waveguide-coupled emitters, a 23% collection efficiency from emitter to fiber, and an overall emission efficiency into the zero-phonon line of 63.4%. We also observe a lifetime enhancement of 5, corresponding to a Purcell factor exceeding 18 when correcting for the emission to the phonon sideband. These results pave the way toward efficient spin–photon interfaces in silicon photonics.
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Publication: Astrophys. J. 952, 26 (2023)Doi: 10.3847/1538-4357/acd769
A variety of energy sources, ranging from dynamic processes, such as magnetic reconnection and waves, to quasisteady terms, such as plasma pressure, may contribute to the acceleration of the solar wind. We utilize a combination of charged particle and magnetic field observations from the Parker Solar Probe (PSP) to attempt to quantify the steady-state contribution of the proton pressure, the electric potential, and the wave energy to the solar wind proton acceleration observed by PSP between 13.3 and ∼100 solar radii (R☉). The proton pressure provides a natural kinematic driver of the outflow. The ambipolar electric potential acts to couple the electron pressure to the protons, providing another definite proton acceleration term. Fluctuations and waves, while inherently dynamic, can act as an additional effective steady-state pressure term. To analyze the contributions of these terms, we utilize radial binning of single-point PSP measurements, as well as repeated crossings of the same stream at different distances on individual PSP orbits (i.e., fast radial scans). In agreement with previous work, we find that the electric potential contains sufficient energy to fully explain the acceleration of the slower wind streams. On the other hand, we find that the wave pressure plays an increasingly important role in the faster wind streams. The combination of these terms can explain the continuing acceleration of both slow and fast wind streams beyond 13.3 R(☉).
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Publication: Nature Nanotechnol. 18, 1389 (2023)Doi: 10.1038/s41565-023-01528-7
A future quantum internet is expected to generate, distribute, store and process quantum bits (qubits) over the world by linking different quantum nodes via quantum states of light. To facilitate long-haul operations, quantum repeaters must operate at telecom wavelengths to take advantage of both the low-loss optical fibre network and the established technologies of modern optical communications. Semiconductor quantum dots have thus far shown exceptional performance as key elements for quantum repeaters, such as quantum light sources and spin–photon interfaces, but only in the near-infrared regime. Therefore, the development of high-performance telecom-band quantum dot devices is highly desirable for a future solid-state quantum internet based on fibre networks. In this Review, we present the physics and technological developments towards epitaxial quantum dot devices emitting in the telecom O- and C-bands for quantum networks, considering both advanced epitaxial growth for direct telecom emission and quantum frequency conversion for telecom-band down-conversion of near-infrared quantum dot devices. We also discuss the challenges and opportunities for future realization of telecom quantum dot devices with improved performance and expanded functionality through hybrid integration.
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Publication: Nature Commun. 14, 7493 (2023)Doi: 10.1038/s41467-023-43412-x
Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm−2.
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Publication: Phys. Rev. Appl. 20, 034026 (2023)Doi: 10.1103/PhysRevAplied.20.034026
The effect of laser polarization on the laser wakefield acceleration (LWFA) of electrons has been investigated in the bubble regime, in particular when assisted by ionization injection. By utilizing linear and circular laser polarizations, we discover that circular polarization leads to a dramatic increase in the electron reproducibility rate and also increases the electron charge and beam divergence, while linear polarization yields higher electron peak energy and more stable beam pointing. Our experimental findings are also supported by three-dimensional particle-in-cell simulations. Our study highlights the potential of laser polarization as a simple and effective tool in controlling LWFA and electron-beam properties depending on applications.
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Publication: Optics Exp. 31, 37574 (2023)Doi: 10.1364/OE.496081
The heterogeneous integration of silicon with III-V materials provides a way to overcome silicon’s limited optical properties toward a broad range of photonic applications. Hybrid modes are a promising way to integrate such heterogeneous Si/III-V devices, but it remains unclear how to utilize these modes to achieve photonic crystal cavities. Herein, using 3D finite-difference time-domain simulations, we propose a hybrid Si-GaAs photonic crystal cavity design that operates at telecom wavelengths and can be fabricated without requiring careful alignment. The hybrid cavity consists of a patterned silicon waveguide that is coupled to a wider GaAs slab featuring InAs quantum dots. We show that by changing the width of the silicon cavity waveguide, we can engineer the hybrid modes and control the degree of coupling to the active material in the GaAs slab. This provides the ability to tune the cavity quality factor while balancing the device’s optical gain and nonlinearity. With this design, we demonstrate cavity mode confinement in the GaAs slab without directly patterning it, enabling strong interaction with the embedded quantum dots for applications such as low-power-threshold lasing and optical bistability (156 nW and 18.1 µW, respectively). This heterogeneous integration of an active III-V material with silicon via a hybrid cavity design suggests a pro
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Publication: Astrophys. J. 945, 28 (2023)Doi: 10.3847/1538-4357/acaf6c
We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite-polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere.
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Publication: Computer Methods Appl. Mech. Eng. 418, 116504 (2023)Doi: 10.1016/j.cma.2023.116504
Topology optimization, a technique to determine where material should be placed within a predefined volume in order to minimize a physical objective, is used across a wide range of scientific fields and applications. A general application for topology optimization is inverse magnetostatics; a desired magnetic field is prescribed, and a distribution of steady currents is computed to produce that target field. In the present work, electromagnetic coils are designed by magnetostatic topology optimization, using volume elements (voxels) of electric current, constrained so the current is divergence-free. Compared to standard electromagnet shape optimization, our method has the advantage that the nonlinearity in the Biot-Savart law with respect to position is avoided, enabling convex cost functions and a useful reformulation of topology optimization as sparse regression. To demonstrate, we consider the application of designing electromagnetic coils for a class of plasma experiments known as stellarators. We produce topologically-exotic coils for several new stellarator designs and show that these solutions can be interpolated into a filamentary representation and then further optimized.
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Publication: ACS Photon. 10, 3844 (2023)Doi: 10.1021/acsphotonics.3c01142
Color centers in Si could serve as both efficient quantum emitters and quantum memories with long coherence times in an all-silicon platform. Of the various known color centers, the T center holds particular promise because it possesses a spin ground state that has long coherence times. But this color center exhibits a long excited state lifetime which results in a low photon emission rate, requiring methods to extract photon emission with high efficiency. We demonstrate high-efficiency single photon emission from a single T center using a nanobeam. The nanobeam efficiently radiates light in a mode that is well-matched to a lensed fiber, enabling us to collect over 70% of the T center emission directly into a single mode fiber. This efficiency enables us to directly demonstrate single photon emission from the zero phonon line, which represents the coherent emission from the T center. Our results represent an important step towards silicon-integrated spin-photon interfaces for quantum computing and quantum networks.
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Publication: J. Plasma Phys. 89, 905890518 (2023)Doi: 10.1017/S0022377823000995
We demonstrate a fast adjoint-based method to optimise tokamak and stellarator equilibria against a pressure-driven instability known as the infinite-n ideal ballooning mode. We present three finite-β (the ratio of thermal to magnetic pressure) equilibria: one tokamak equilibrium and two stellarator equilibria that are unstable against the ballooning mode. Using the self-adjoint property of ideal magnetohydrodynamics, we construct a technique to rapidly calculate the change in the eigenvalue, a measure of ideal ballooning instability. Using the SIMSOPT optimisation framework, we then implement our fast adjoint gradient-based optimiser to minimise the eigenvalue and find stable equilibria for each of the three originally unstable equilibria.
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Publication: J. Plasma Phys. 89, 90580503 (2023)Doi: 10.1017/S0022377823000788
In designing stellarators, any design decision ultimately comes with a trade-off. Improvements in particle confinement, for instance, may increase the burden on engineers to build more complex coils, and the tightening of financial constraints may simplify the design and worsen some aspects of transport. Understanding trade-offs in stellarator designs is critical in designing high performance devices that satisfy the multitude of physical, engineering and financial criteria. In this study, we show how multi-objective optimization (MOO) can be used to investigate trade-offs and develop insight into the role of design parameters. We discuss the basics of MOO, as well as practical solution methods for solving MOO problems. We apply these methods to bring insight into the selection of two common design parameters: the aspect ratio of an ideal magnetohydrodynamic equilibrium and the total length of the electromagnetic coils.
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Publication: AVS Quantum Sci. 5, 044403 (2023)Doi: 10.1116/5.0176317
Wide-field imaging of magnetic signals using ensembles of nitrogen-vacancy (NV) centers in diamond has garnered increasing interest due to its combination of micron-scale resolution, millimeter-scale field of view, and compatibility with diverse samples from across the physical and life sciences. Recently, wide-field NV magnetic imaging based on the Ramsey protocol has achieved uniform and enhanced sensitivity compared to conventional measurements. Here, we integrate the Ramsey-based protocol with spin-bath driving to extend the NV spin dephasing time and improve magnetic sensitivity. We also employ a high-speed camera to enable dynamic wide-field magnetic imaging. We benchmark the utility of this quantum diamond microscope (QDM) by imaging magnetic fields produced from a fabricated wire phantom. Over a 270 × 270 μm2 field of view, a median per-pixel magnetic sensitivity of 4.1(1) nT /Hz is realized with a spatial resolution ≲ 10 μm and sub-millisecond temporal resolution. Importantly, the spatial magnetic noise floor can be reduced to the picotesla scale by time-averaging and signal modulation, which enables imaging of a magnetic-field pattern with a peak-to-peak amplitude difference of about 300 pT. Finally, we discuss potential new applications of this dynamic QDM in studying biomineralization and electrically active cells.
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Publication: Nature Photon. 7, 561 (2023)Doi: 10.1038/s41566-023-01217-w
Memristors in electronics have shown the potential for a range of applications, ranging from circuit elements to neuromorphic computing. In recent years, the ability to vary the conductance of a channel in electronics has enabled in-memory computing, thus leading to substantial interest in memristors. Optical analogues will require modulation of the transmission of light in a semicontinuous and nonvolatile manner. With the proliferation of photonic computing, such an optical analogue, which involves modulating the optical response in integrated circuits while maintaining the modulated state afterwards, is being pursued using a range of functional materials. Here we review recent progress in this important and emerging aspect of photonic integrated circuits and provide an overview of the current state of the art. Optical memristors are of particular interest for applications in high-bandwidth neuromorphic computing, machine learning hardware and artificial intelligence, as these optical analogues of memristors allow for technology that combines the ultrafast, high-bandwidth communication of optics with local information processing.
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Publication: Geophys. Res. Lett. 50, e2022GL102649 (2023)Doi: 10.1029/2022GL102649
It is shown that a recently developed hybrid modeling approach that combines machine learning (ML) with an atmospheric global circulation model (AGCM) can serve as a basis for capturing atmospheric processes not captured by the AGCM. This power of the approach is illustrated by three examples from a decades-long climate simulation experiment. The first example demonstrates that the hybrid model can produce sudden stratospheric warming, a dynamical process of nature not resolved by the low resolution AGCM component of the hybrid model. The second and third example show that introducing 6-hr cumulative precipitation and sea surface temperature (SST) as ML-based prognostic variables improves the precipitation climatology and leads to a realistic ENSO signal in the SST and atmospheric surface pressure.
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Publication: Nucl. Fusion 63, 014002 (2023)Doi: 10.1088/1741-4326/aca10d
We present a technique that can be used to design stellarators with a high degree of experimental flexibility. For our purposes, flexibility is defined by the range of values the rotational transform can take on the magnetic axis of the vacuum field while maintaining satisfactory quasisymmetry. We show that accounting for configuration flexibility during the modular coil design improves flexibility beyond that attained by previous methods. Careful placement of planar control coils and the incorporation of an integrability objective enhance the quasisymmetry and nested flux surface volume of each configuration. We show that it is possible to achieve flexibility, quasisymmetry, and nested flux surface volume to reasonable degrees with a relatively simple coil set through an NCSX-like example. This example coil design is optimized to achieve three rotational transform targets and nested flux surface volumes in each magnetic configuration larger than the NCSX design plasma volume. Our work suggests that there is a tradeoff between flexibility, quasisymmetry, and volume of nested flux surfaces.
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Mode Power Spectrum for Laguerre-Gauss Beams in Kolmogorov Turbulence: Erratum (Vol. 47, 3447, 2022)
Publication: Optics Lett. 48, 606 (2023)Doi: 10.1364/OL.481303We present an erratum to our Letter [Opt. Lett. 47, 3447 (2022) [CrossRef] ]. In the Letter we provided an example calculation for how to use our results to predict the signal-to-noise ratio for an OAM-multiplexed communication system. This erratum corrects the parameter name for which numerical values are provided. The calculations in the original Letter were performed using the correct values for all parameters; therefore, this correction does not affect the results and conclusions of the original Letter.
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Publication: Commun. Phys. 6, 117 (2023)Doi: 10.1038/s42005-023-01225-w
Kerr optical frequency combs are expected to play a major role in photonic technology, with applications related to spectroscopy, sensing, aerospace, and communication engineering. Most of these applications are related to the metrological performance of Kerr combs, which is ultimately limited by their noise-driven fluctuations. For this reason, it is of high importance to understand the influence of random noise on the comb dynamics. In this communication, we theoretically investigate a model where Gaussian white noise is added to the coupled-mode equations governing the comb dynamics. This stochastic model allows us to characterize the noise-induced broadening of the spectral lines. Moreover, this study permits to determine the phase noise spectra of the microwaves generated via comb photodetection. In this latter case, our analysis indicates that the low-frequency part of the spectra is dominated by pattern drift while the high-frequency part is dominated by pattern deformation. The theoretical results are found to be in excellent agreement with numerical simulations.
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Publication: IEEE Trans. Appl. Superconductivity 33, 1100605 (2023)Doi: 10.1109/TASC.2023.3243492
This article presents a high-rejection, thin-film high-temperature superconductor, microstrip bandstop filter to prevent a local, and high-power radio-frequency interference (RFI) source from interfering with NASA Goddard Geophysical Astronomical Observatory (GGAO)’s very long baseline interferometry (VLBI) global observing system (VGOS) cryogenic receiver. This filter has an excellent 2.7%, 50-dB-fractional-bandwidth, and a center stopband frequency of 9.41 GHz. It does not contain any narrow or interdigital features as found in some designs, which reduces the fringing electric fields and improves its power handling capability. The YBCO films were grown on 435-μm-thick R-plane sapphire substrate and the anisotropic behavior was modeled and simulated with a high degree of accuracy. The device was tested while cooled to 77 K and the measurements agree well with simulation.
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Publication: J. Optics 25, 103001 (2023)Doi: 10.1088/2040-8986/acea92
Structured waves are ubiquitous for all areas of wave physics, both classical and quantum, where the wavefields are inhomogeneous and cannot be approximated by a single plane wave. Even the interference of two plane waves, or of a single inhomogeneous (evanescent) wave, provides a number of nontrivial phenomena and additional functionalities as compared to a single plane wave. Complex wavefields with inhomogeneities in the amplitude, phase, and polarization, including topological structures and singularities, underpin modern nanooptics and photonics, yet they are equally important, e.g. for quantum matter waves, acoustics, water waves, etc. Structured waves are crucial in optical and electron microscopy, wave propagation and scattering, imaging, communications, quantum optics, topological and non-Hermitian wave systems, quantum condensed-matter systems, optomechanics, plasmonics and metamaterials, optical and acoustic manipulation, and so forth. This Roadmap is written collectively by prominent researchers and aims to survey the role of structured waves in various areas of wave physics. Providing background, current research, and anticipating future developments, it will be of interest to a wide cross-disciplinary audience.
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Publication: Phys. Rev. E 108, 015203 (2023)Doi: 10.1103/PhysRevE.108.015203
In this paper, we analyze and numerically simulate mechanisms for generating directed rf radiation by a low-intensity laser pulse train (LPT) propagating in air. The LPT ionizes the air, forming a plasma filament. The ionization process relies on the background level of radioactivity which plays an important role in initiating a collisional ionization process. In our model a low-intensity LPT photoionizes background negative ions (produced by ambient ionizing radiation) and provides the seed electrons necessary to initiate collisional ionization. The intensity of the LPT is far below tunneling ionization levels. The ponderomotive forces associated with the LPT and self-fields drive plasma oscillations predominately in the radial direction. The driven radial electron currents in turn generate directed rf radiation. As the plasma density builds up on axis, the later portion of the LPT can defocus and limit the interaction length. The spectrum of the rf radiation consists of the fundamental frequency associated with the pulse separation time as well as harmonics. The rf generation mechanism is analyzed using fluid equations which incorporate, among other things, the effects of background radioactivity, photoionization, collisional ionization, ponderomotive and space-charge effects, and electron attachment–recombination processes. As an example, for a specific set of parameters, the rf spectrum and intensity are compared to experimental data.
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Publication: IEEE Trans. Plasma Sci. 51, 888 (2023)Doi: 10.1109/TPS.2023.3241542
Interest is increasing in high-power terahertz (THz) sources of radiation. The terminology is fluid, but researchers in the field typically refer to frequencies ranging from about 300 GHz to 10 THz as THz radiation. In this article, we present a description of design considerations for a compact, high-average power free-electron laser. At present, THz radiation is generated by a variety of mechanisms, including laser-based sources and electron-beam-based sources. We provide a short description of current THz source technology to give background against which to compare the present concept; however, this should not be considered a comprehensive discussion of such technologies.
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Publication: Astrophys. J. 954, 21 (2023)Doi: 10.3847/1538-4357/ace59e
We conduct two-dimensional particle-in-cell simulations to investigate the scattering of electron heat flux by self-generated oblique electromagnetic waves. The heat flux is modeled as a bi-kappa distribution with a T∥ > T⊥ temperature anisotropy maintained by continuous injection at the boundaries. The anisotropic distribution excites oblique whistler waves and filamentary-like Weibel instabilities. Electron velocity distributions taken after the system has reached a steady state show that these instabilities inhibit the heat flux and drive the total distributions toward isotropy. Electron trajectories in velocity space show a circular-like diffusion along constant energy surfaces in the wave frame. The key parameter controlling the scattering rate is the average speed, or drift speed vd, of the heat flux compared with the electron Alfvén speed vAe, with higher drift speeds producing stronger fluctuations and a more significant reduction of the heat flux. Reducing the density of the electrons carrying the heat flux by 50% does not significantly affect the scattering rate. A scaling law for the electron scattering rate versus vd/vAe is deduced from the simulations. The implications of these results for understanding energetic electron transport during energy release in solar flares are discussed.
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Publication: ACS Appl. Mater. Interfaces 15, 10752 (2023)Doi: 10.1021/acsami.2c22530
This work presents a stress characterization of crystalline Si electrodes using micro-Raman spectroscopy. First, the phase heterogeneity in the c-Si electrodes after initial lithiation was investigated by scanning electron microscopy (SEM) and other complementary techniques. A surprising three-phase layer structure, with a-LixSi (x = 2.5), c-LixSi (x = 0.3–2.5), and c-Si layers, was observed, and its origin was attributed to the electro–chemo–mechanical (ECM) coupling effect in the c-Si electrodes. Then, a Raman scan was performed to characterize stress distribution in lithiated c-Si electrodes. The results showed that the maximum tensile stress occurred at the interface between c-LixSi and c-Si layers, indicating a plastic flow behavior. The yield stress increased with total lithium charge, and the relationship showed consistency with a prior multibeam optical sensor (MOS) study. Lastly, stress distribution and structural integrity of the c-Si electrodes after initial delithiation and further cycling were studied, and a comprehensive picture of the failure mechanism of the c-Si electrode was obtained.
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Publication: Phys. Rev. Res. 5, 033195 (2023)Doi: 10.1103/PhysRevResearch.5.033195
We present the theoretical framework required to describe the statistics of microwave networks that serve to model quantum graphs. The networks are described by impedance and admittance matrices relating the voltages and currents at the network's ports. As we show, these matrices can be calculated in a number of ways. Normal modes of the network are characterized by a discrete set of wave numbers corresponding to the propagation constants on the network's bonds for which the determinant of the admittance matrix vanishes. The distribution of the spacings between adjacent eigenmode wave numbers is found to depend on the nature of the way bonds are connected at nodes. The critical quantity is the reflection coefficient presented at a node to a wave on a bond. As the reflection coefficient increases, the spacing distribution changes from one characteristic of the spacing of eigenvalues of a Gaussian orthogonal ensemble matrix to a Poisson distribution. The effect of loss is studied, and the scaling of the variance of the impedance values on network size, degree distribution, and other parameters is characterized. We attempted to find universal scaling relations for the distribution of impedance values for networks of different sizes. Finally, we compare the distribution of impedance values predicted by the model with those measured in a network of cables.
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Publication: J. Vac. Sci. Technol. A 41, 013004 (2023)Doi: 10.1116/6.0002234
Electron-stimulated etching of surfaces functionalized by remote plasma is a flexible and novel approach for material removal. In comparison with plasma dry etching, which uses the ion-neutral synergistic effect to control material etching, electron beam-induced etching (EBIE) uses an electron-neutral synergistic effect. This approach appears promising for the reduction of plasma-induced damage (PID), including atomic displacement and lateral straggling, along with the potential for greater control and lateral resolution. One challenge for EBIE is the limited selection of chemical precursor molecules that can be used to produce functionalized materials suitable for etching under electron beam irradiation. In this work, we studied a new experimental approach that utilizes a remote plasma source to functionalize substrate surfaces in conjunction with electron beam irradiation by an electron flood gun. Etching rates (ERs) of SiO2, Si3N4, and poly-Si are reported in a broad survey of processing conditions. The parametric dependence of the ER of these Si-based materials on the operating parameters of the flood gun and the remote plasma source is evaluated. We also identified the processing parameters that enable the realization of material selective removal, i.e., the etching selectivity of Si3N4 over SiO2 and poly-Si over SiO2. Additionally, surface characterization of etched materials is used to clarify the effects of the co-introduction of particle fluxes from the remote plasma and flood gun sources on surface chemistry.
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Publication: Nano Lett. 23, 1781 (2023)Doi: 10.1021/acs.nanolett.2c04552
Noise spectroscopy elucidates the fundamental noise sources in spin systems, thereby serving as an essential tool toward developing spin qubits with long coherence times for quantum information processing, communication, and sensing. But existing techniques for noise spectroscopy that rely on microwave fields become infeasible when the microwave power is too weak to generate Rabi rotations of the spin. Here, we demonstrate an alternative all-optical approach to performing noise spectroscopy. Our approach utilizes coherent Raman rotations of the spin state with controlled timing and phase to implement Carr–Purcell–Meiboom–Gill pulse sequences. Analyzing the spin dynamics under these sequences enables us to extract the noise spectrum of a dense ensemble of nuclear spins interacting with a single spin in a quantum dot, which has thus far been modeled only theoretically. By providing spectral bandwidths of over 100 MHz, our approach enables studies of spin dynamics and decoherence for a broad range of solid-state spin qubits.
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Publication: J. Plasma Phys. 89, 905890112 (2023)Doi: 10.1017/S0022377823000107
High-power-density tokamaks offer a potential solution to design cost-effective fusion devices. One way to achieve high power density is to operate at a high β value (the ratio of thermal to magnetic pressure), i.e., β∼1. However, a β∼1 state may be unstable to various pressure- and current-driven instabilities or have unfavourable microstability properties. To explore these possibilities, we generate β∼1 equilibria and investigate their stability. First, we demonstrate the generation of high-β equilibria with the computer code VMEC. We then analyse these equilibria to determine their stability against the infinite-n ideal-ballooning mode. We follow that by engaging in a detailed microstability study using the GS2 code, beginning with assessments of electrostatic ion-temperature-gradient and trapped election mode instabilities. We observe interesting behaviour for the high-β equilibria – stabilization of these modes through two distinct mechanisms – large negative local shear and reversal of electron precession drift. Finally, we perform electromagnetic gyrokinetic simulations and observe enhanced stability in the outer core of high-β equilibria and absence of kinetic ballooning modes in the negative-triangularity, high-β equilibria. The enhanced outer-core stability of high-β equilibria is different from their lower-β counterparts and offers an alternative, potentially favourable regime of tokamak operation.
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Publication: Phys. Today 76, 54 (2023)Doi: 10.1063/PT.3.5297
The electric field of a tightly focused milliwatt laser pointer can reach an impressive 50 000 V/m, a gradient that could, in principle, accelerate the electrons in a dental x-ray tube. With lots of cheap laser pointers and some assembly time at your kitchen table, can you build a 10 million V/m compact electron accelerator? Sadly, it wouldn’t work, and you’d needlessly annoy the neighbors.
Three main problems undermine the laser-accelerator project. First, although the light wave in a laser beam is a coherent electromagnetic field oscillation, with peaks and valleys aligned in lockstep, the train of peaks and valleys from the second laser is randomly phase shifted in time with respect to the first. So the fields from multiple lasers would interfere, as peaks from one laser cancel valleys from another.
If one has N such randomly phased laser beams, each with intensity I, the peak intensity from combining the beams would be NI. If all of them were in phase, with their wave trains aligned, however, the peak intensity would be N2I. But, alas, that dividend of coherent superposition is unavailable to the well-meaning hobbyist who just purchased a wheelbarrow full of $5 laser pointers.
Suppose you bypass the problem by simply buying a laser that, when tightly focused, gives an electric field of 107 V/m. Then the second problem can be expressed this way: “Hey genius, where’s my meter?” The heckler is pointing out that the focused laser beam is, at most, tens of microns wide—far less than 1 m. So the field is better expressed as the ratio 200 V/(20 µm), and your laser purchase would provide (at most) only 200 V of accelerating potential across the focal spot. Actually, it wouldn’t even remotely do that, courtesy of the third problem.
That problem is that laser light in free space is a high-frequency transverse electromagnetic wave, with fields orthogonal to the beam direction: An electron would do a high-frequency sideways shimmy, with negligible net acceleration and energy gain.
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Publication: Eur. Phys. J. A 59, 247 (2023)Doi: 10.1140/epja/s10050-023-01159-5
Petawatt-class lasers can produce multi-GeV electron beams through laser wakefield electron acceleration. As a by-product, the accelerated electron beams can generate synchrotron-like radiation known as betatron radiation. In the present work, we measure the properties of the radiation produced from 2 GeV, 215 pC electron beams, which shows a broad radiation spectrum with a critical energy of 515 keV, extending up to MeV photon energies and 10 mrad divergence. Due to its high energy and flux, such radiation is an ideal candidate for x-ray radiography of dense objects. We employed a compact betatron radiation setup operated at relatively high-repetition rates (0.1 Hz) and used it to scan cm-sized objects: a DRAM circuit, BNC and SMA connectors, a padlock and a gas jet nozzle. GEANT4 simulations were carried out to reproduce the radiograph of the gas jet. The setup and the radiation source can reveal the interior structure of the objects at the sub-mm level, proving that it can further be applied to diagnose cracks or holes in various components. The radiation source presented here is a valuable tool for non-destructive inspection and for applications in high-energy-density physics such as nuclear fusion.
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Publication: IEEE Trans. Electromagnetic Compatibility 65, 454 (2023)Doi: 10.1109/TEMC.2023.3235824
Wave energy distribution within enclosures with irregular boundaries is a common phenomenon in many branches of electromagnetics. If the wavelength of the injected wave is small compared with the structure size, the scattering properties of the enclosure will be extremely sensitive to small changes in geometry or wave frequency. In this case, statistical models are sought. The random coupling model (RCM) is one such model that has been explored through experiments and theory. Previous studies were conducted by injecting waves into high Q cavities in a nearly omnidirectional manner. In this article, a directed beam approach is taken, and relatively low Q cavities are considered. The goal is to determine when the so-called “random plane wave hypothesis,” a fundamental basis of the RCM formulation, breaks down. Results show that injecting such directed beams leads to large deviations in the wave statistics for single realizations of the enclosure geometry. The expected statistics are restored to some degree when multiple realizations are considered.
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Publication: Astrophys. J. Lett. 959, L5 (2023)Doi: 10.3847/2041-8213/accf1f
The processes controlling the complex clump structure, phase distribution, and magnetic field geometry that develop across a broad range of scales in the turbulent interstellar medium (ISM) remain unclear. Using unprecedentedly high-resolution 3D magnetohydrodynamic simulations of thermally unstable turbulent systems, we show that large current sheets unstable to plasmoid-mediated reconnection form regularly throughout the volume. The plasmoids form in three distinct environments: (i) within cold clumps, (ii) at the asymmetric interface of the cold and warm phases, and (iii) within the warm, volume-filling phase. We then show that the complex magnetothermal phase structure is characterized by a predominantly highly magnetized cold phase, but that regions of high magnetic curvature, which are the sites of reconnection, span a broad range in temperature. Furthermore, we show that thermal instabilities change the scale-dependent anisotropy of the turbulent magnetic field, reducing the increase in eddy elongation at smaller scales. Finally, we show that most of the mass is contained in one contiguous cold structure surrounded by smaller clumps that follow a scale-free mass distribution. These clumps tend to be highly elongated and exhibit a size versus internal velocity relation consistent with supersonic turbulence and a relative clump distance–velocity scaling consistent with subsonic motion. We discuss the striking similarity of cold plasmoids to observed tiny-scale atomic and ionized structures and H i fibers and consider how the presence of plasmoids will modify the motion of charged particles, thereby impacting cosmic-ray transport and thermal conduction in the ISM and other similar systems.
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Publication: Phys. Plasmas 30, 042511 (2023)Doi: 10.1063/5.0129716
We present a robust optimization algorithm for the design of electromagnetic coils that generate vacuum magnetic fields with nested flux surfaces and precise quasi-symmetry. The method is based on a bilevel optimization problem, where the outer coil optimization is constrained by a set of inner least squares optimization problems whose solutions describe magnetic surfaces. The outer optimization objective targets coils that generate a field with nested magnetic surfaces and good quasi-symmetry. The inner optimization problems identify magnetic surfaces when they exist, and approximate surfaces in the presence of magnetic islands or chaos. We show that this formulation can be used to heal islands and chaos, thus producing coils that result in magnetic fields with precise quasi-symmetry. We show that the method can be initialized with coils from the traditional two-stage coil design process, as well as coils from a near-axis expansion optimization. We present a numerical example where island chains are healed and quasi-symmetry is optimized up to surfaces with aspect ratio 6. Another numerical example illustrates that the aspect ratio of nested flux surfaces with optimized quasi-symmetry can be decreased from 6 to approximately 4. The last example shows that our approach is robust and a cold-start using coils from a near-axis expansion optimization.
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Publication: Phys. Rev. Lett. 130, 245201 (2023)Doi: 10.1103/PhysRevLett.130.245201
It is an open question whether and how gravitational wave events involving neutron stars can be preceded by electromagnetic counterparts. This Letter shows that the collision of two neutron stars with magnetic fields well below magnetar-level strengths can produce millisecond fast-radio-burst-like transients. Using global force-free electrodynamics simulations, we identify the coherent emission mechanism that might operate in the common magnetosphere of a binary neutron star system prior to merger. We predict that the emission show have frequencies in the range of 10-20 GHz for magnetic fields of B∗=1011 G at the surfaces of the stars.
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Publication: Nucl. Fusion 63, 036016 (2023)Doi: 10.1088/1741-4326/acb4a9
A number of scientific fields rely on placing permanent magnets in order to produce a desired magnetic field. We have shown in recent work that the placement process can be formulated as sparse regression. However, binary, grid-aligned solutions are desired for realistic engineering designs. We now show that the binary permanent magnet problem can be formulated as a quadratic program with quadratic equality constraints, the binary, grid-aligned problem is equivalent to the quadratic knapsack problem with multiple knapsack constraints, and the single-orientation-only problem is equivalent to the unconstrained quadratic binary problem. We then provide a set of simple greedy algorithms for solving variants of permanent magnet optimization, and demonstrate their capabilities by designing magnets for stellarator plasmas. The algorithms can a-priori produce sparse, grid-aligned, binary solutions. Despite its simple design and greedy nature, we provide an algorithm that compares with or even outperforms the state-of-the-art algorithms while being substantially faster, more flexible, and easier to use.
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Publication: J. Electrochem. Soc. 170, 063507 (2023)Doi: 10.1149/1945-7111/acd87b
Anodized aluminum oxide (AAO) has been used as nanotemplates for nanomaterials and nanodevice fabrications. Microfabrication techniques are attracting attention for nanodevice synthesis. However, AAO requires a microfabrication-compatible substrate due to its brittleness. While there are studies that already show AAO on compatible substrates, the pore sizes may not be applicable for multicomponent nanodevices. In this study, wide pore AAOs with ohmic bottom contacts are fabricated on 76 mm Si wafers. Sputtering was used to deposit Al along with supporting layers to achieve this goal. A quiescent electropolishing technique was used to smooth the surface of Al. Standard photolithography was used to define the active area on the Al for anodization. Then 195 V two-step anodization was performed to fabricate wide pore AAOs with pore diameters ranging from 130 ± 32 nm to 400 ± 31 nm with interpore distance of 480 ± 47 nm. It also showed that the ordering of the pores depended on the current density over the more conventional anodization time.
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Publication: Nature Rev. Phys. 5, 157 (2023)Doi: 10.1038/s42254-023-00558-3
Quantum sensors are finding their way from laboratories to the real world, as witnessed by the increasing number of start-ups in this field. The atomic length scale of quantum sensors and their coherence properties enable unprecedented spatial resolution and sensitivity. Biomedical applications could benefit from these quantum technologies, but it is often difficult to evaluate the potential impact of the techniques. This Review sheds light on these questions, presenting the status of quantum sensing applications and discussing their path towards commercialization. The focus is on two promising quantum sensing platforms: optically pumped atomic magnetometers, and nitrogen–vacancy centres in diamond. The broad spectrum of biomedical applications is highlighted by four case studies ranging from brain imaging to single-cell spectroscopy.
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Publication: Phys. Rev. Lett. 130, 115201 (2023)Doi: 10.1103/PhysRevLett.130.115201
We study the accretion of collisionless plasma onto a rotating black hole from first principles using axisymmetric general-relativistic particle-in-cell simulations. We carry out a side-by-side comparison of these results to analogous general-relativistic magnetohydrodynamic simulations. Although there are many similarities in the overall flow dynamics, three key differences between the kinetic and fluid simulations are identified. Magnetic reconnection is more efficient, and rapidly accelerates a nonthermal particle population, in our kinetic approach. In addition, the plasma in the kinetic simulations develops significant departures from thermal equilibrium, including pressure anisotropy that excites kinetic-scale instabilities, and a large field-aligned heat flux near the horizon that approaches the free-streaming value. We discuss the implications of our results for modeling event-horizon scale observations of Sgr A* and M87 by GRAVITY and the Event Horizon Telescope.
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Publication: Phys. Plasmas 30, 033906 (2023)Doi: 10.1063/5.0139039
Many scientific problems can be formulated as sparse regression, i.e., regression onto a set of parameters when there is a desire or expectation that some of the parameters are exactly zero or do not substantially contribute. This includes many problems in signal and image processing, system identification, optimization, and parameter estimation methods such as Gaussian process regression. Sparsity facilitates exploring high-dimensional spaces while finding parsimonious and interpretable solutions. In the present work, we illustrate some of the important ways in which sparse regression appears in plasma physics and point out recent contributions and remaining challenges to solving these problems in this field. A brief review is provided for the optimization problem and the state-of-the-art solvers, especially for constrained and high-dimensional sparse regression.
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Publication: ACS Appl. Mater. Interfaces 15, 24271 (2023)Doi: 10.1021/acsami.2c23256
Nanostructured solid-state batteries (SSBs) are poised to meet the demands of next-generation energy storage technologies by realizing performance competitive to their liquid-based counterparts while simultaneously offering improved safety and expanded form factors. Atomic layer deposition (ALD) is among the tools essential to fabricate nanostructured devices with challenging aspect ratios. Here, we report the fabrication and electrochemical testing of the first nanoscale sodium all-solid-state battery (SSB) using ALD to deposit both the V2O5 cathode and NaPON solid electrolyte followed by evaporation of a thin-film Na metal anode. NaPON exhibits remarkable stability against evaporated Na metal, showing no electrolyte breakdown or significant interphase formation in the voltage range of 0.05–6.0 V vs Na/Na+. Electrochemical analysis of the SSB suggests intermixing of the NaPON/V2O5 layers during fabrication, which we investigate in three ways: in situ spectroscopic ellipsometry, time-resolved X-ray photoelectron spectroscopy (XPS) depth profiling, and cross-sectional cryo-scanning transmission electron microscopy (cryo-STEM) coupled with electron energy loss spectroscopy (EELS). We characterize the interfacial reaction during the ALD NaPON energy loss spectroscopy (EELS). We characterize the interfacial reaction during the ALD NaPON deposition on V2O5 to be twofold: (1) reduction of V2O5 to VO2 and (2) Na+ insertion into VO2 to form NaxVO2. Despite the intermixing of NaPON–V2O5, we demonstrate that NaPON-coated V2O5 electrodes display enhanced electrochemical cycling stability in liquid-electrolyte coin cells through the formation of a stable electrolyte interphase. In all-SSBs, the Na metal evaporation process is found to intensify the intermixing reaction, resulting in the irreversible formation of mixed interphases between discrete battery layers. Despite this graded composition, the SSB can operate for over 100 charge–discharge cycles at room temperature and represents the first demonstration of a functional thin-film solid-state sodium-ion battery.
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Publication: Physica D: Nonlinear Phenomena 445, 133616 (2023)Doi: 10.1016/j.physd.2022.133616
The dissipation of kinetic energy to heat in viscous flows has significant implications in nature and technology. Here we experimentally examine the scaling of dissipation in rotating turbulent shear flows as measured in laboratory experiments via torque measurements. The motivation is to better understand natural rotating turbulence in atmospheres, oceans and liquid planetary cores, as well as to also understand the approach to the asymptotic Kolmogorov–Constantin–Doering limit where the small, but non-zero, viscosity becomes irrelevant. In both cylindrical and spherical Couette flows, differential rotation can either enhance or reduce the observed dissipation. As well, we document new results in the increase in scaling exponents expected, and here observed, for rough spherical Couette flows.
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Publication: Plasma Phys. Control. Fusion 65, 045011 (2023)Doi: 10.1088/1361-6587/acb9ba
In magnetic confinement fusion devices, the ratio of the plasma pressure to the magnetic field energy, β, can become sufficiently large that electromagnetic microinstabilities become unstable, driving turbulence that distorts or reconnects the equilibrium magnetic field. In this paper, a theory is proposed for electromagnetic, electron-driven linear instabilities that have current layers localised to mode-rational surfaces and binormal wavelengths comparable to the ion gyroradius. The model retains axisymmetric toroidal geometry with arbitrary shaping, and consists of orbit-averaged equations for the mode-rational surface layer, with a ballooning space kinetic matching condition for passing electrons. The matching condition connects the current layer to the large scale electromagnetic fluctuations, and is derived in the limit that β is comparable to the square root of the electron-to-ion-mass ratio. Electromagnetic fluctuations only enter through the matching condition, allowing for the identification of an effective β that includes the effects of equilibrium flux surface shaping. The scaling predictions made by the asymptotic theory are tested with comparisons to results from linear simulations of micro-tearing and electrostatic microinstabilities in MAST discharge #6252, showing excellent agreement. In particular, it is demonstrated that the effective β can explain the dependence of the local micro-tearing mode (MTM) growth rate on the ballooning parameter θ0–possibly providing a route to optimise local flux surfaces for reduced MTM-driven transport.
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Publication: IEEE Trans. Electromagnetic Compatibility 65, 436 (2023)Doi: 10.1109/TEMC.2023.3234912
This article presents a physics-oriented, mathematically tractable, statistical wave model for analyzing the wave physics of high-frequency reverberation in complex cavity environments. The key ingredient is a vector dyadic stochastic Green's function (SGF) method that is derived from the Wigner's random matrix theory and Berry's random wave hypothesis. The SGF statistically replicates multipath, ray-chaotic communication between vector sources and vectorial electromagnetic fields at displaced observation points using generic, macroscopic parameters of the cavity environment. The work establishes a physics-based modeling and simulation capability that predicts the probabilistic behavior of backdoor coupling to complex electronic enclosures. Experimental results are supplied to validate the proposed work.
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Publication: Nature 618, 252 (2023)Doi: 10.1038/s41586-023-05955-3
Quantum sensors are finding their way from laboratories to the real world, as witnessed by the increasing number of start-ups in this field. The atomic length scale of quantum sensors and their coherence properties enable unprecedented spatial resolution and sensitivity. Biomedical applications could benefit from these quantum technologies, but it is often difficult to evaluate the potential impact of the techniques. This Review sheds light on these questions, presenting the status of quantum sensing applications and discussing their path towards commercialization. The focus is on two promising quantum sensing platforms: optically pumped atomic magnetometers, and nitrogen–vacancy centres in diamond. The broad spectrum of biomedical applications is highlighted by four case studies ranging from brain imaging to single-cell spectroscopy.
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Publication: Astrophys. J. 943, 105 (2023)Doi: 10.3847/1538-4357/acab05
Some of the most energetic pulsars exhibit rotation-modulated γ-ray emission in the 0.1–100 GeV band. The luminosity of this emission is typically 0.1%–10% of the pulsar spin-down power (γ-ray efficiency), implying that a significant fraction of the available electromagnetic energy is dissipated in the magnetosphere and reradiated as high-energy photons. To investigate this phenomenon we model a pulsar magnetosphere using 3D particle-in-cell simulations with strong synchrotron cooling. We particularly focus on the dynamics of the equatorial current sheet where magnetic reconnection and energy dissipation take place. Our simulations demonstrate that a fraction of the spin-down power dissipated in the magnetospheric current sheet is controlled by the rate of magnetic reconnection at microphysical plasma scales and only depends on the pulsar inclination angle. We demonstrate that the maximum energy and the distribution function of accelerated pairs is controlled by the available magnetic energy per particle near the current sheet, the magnetization parameter. The shape and the extent of the plasma distribution is imprinted in the observed synchrotron emission, in particular, in the peak and the cutoff of the observed spectrum. We study how the strength of synchrotron cooling affects the observed variety of spectral shapes. Our conclusions naturally explain why pulsars with higher spin-down power have wider spectral shapes and, as a result, lower γ-ray efficiency.
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Publication: IEEE J. ST Quantum Electron. 29, 6100812 (2023)Doi: 10.1109/JSTQE.2023.3239918
Key to recent successes in the field of artificial intelligence (AI) has been the ability to train a growing number of parameters which form fixed connectivity matrices between layers of nonlinear nodes. This “deep learning” approach to AI has historically required an exponential growth in processing power which far exceeds the growth in computational throughput of digital hardware as well as trends in processing efficiency. New computing paradigms are therefore required to enable efficient processing of information while drastically improving computational throughput. Emerging strategies for analog computing in the photonic domain have the potential to drastically reduce latency but require the ability to modify optical processing elements according to the learned parameters of the neural network. In this point-of-view article, we provide a forward-looking perspective on both optical and electrical memories coupled to integrated photonic hardware in the context of AI. We also show that for programmed memories, the READ energy-latency-product of photonic random-access memory (PRAM) can be orders of magnitude lower compared to electronic SRAMs. Our intent is to outline path for PRAMs to become an integral part of future foundry processes and give these promising devices relevance for emerging AI hardware.
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Publication: Front. Astron. Space Sci. 10, (2023)Doi: 10.3389/fspas.2023.1143909
Most stars generate winds and move through the interstellar medium that surrounds them. This movement creates a cocoon formed by the deflection of these winds that envelops and protects the stars. We call these “cocoons” astrospheres. The Sun has its own cocoon, the heliosphere. The heliosphere is an immense shield that protects the Solar System from harsh, galactic radiation. The radiation that enters the heliosphere affects life on Earth as well as human space exploration. Galactic cosmic rays are the dominant source of radiation and principal hazard affecting space missions within our Solar System. Current global heliosphere models do not successfully predict the radiation environment at all locations or under different solar conditions. To understand the heliosphere’s shielding properties, we need to understand its structure and large-scale dynamics. A fortunate confluence of missions has provided the scientific community with a treasury of heliospheric data. However, fundamental features remain unknown. The vision of the Solar wind with Hydrogen Ion charge Exchange and Large-Scale Dynamics (SHIELD) DRIVE Science Center is to understand the nature and structure of the heliosphere. Through four integrated research thrusts leading to the global model, SHIELD will: 1) determine the global nature of the heliosphere; 2) determine how pickup ions evolve from “cradle to grave” and affect heliospheric processes; 3) establish how the heliosphere interacts with and influences the Local Interstellar Medium (LISM); and 4) establish how cosmic rays are filtered by and transported through the heliosphere. The key deliverable is a comprehensive, self-consistent, global model of the heliosphere that explains data from all relevant in situ and remote observations and predicts the radiation environment. SHIELD will develop a “digital twin” of the heliosphere capable of: (a) predicting how changing solar and LISM conditions affect life on Earth, (b) understanding the radiation environment to support long-duration space travel, and (c) contributing toward finding life elsewhere in the Galaxy. SHIELD also will train the next-generation of heliophysicists, a diverse community fluent in team science and skilled working in highly transdisciplinary collaborative environments.
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Publication: Phys. Rev. Appl. 19, 064052 (2023)Doi: 10.1103/PhysRevApplied.19.064052
Electromagnetic wave scattering in electrically large, irregularly shaped, environments is a common phenomenon. The deterministic, or first-principles, study of this process is usually computationally expensive and the results exhibit extreme sensitivity to scattering details. For this reason, the deterministic approach is often dropped in favor of a statistical one. The random coupling model (RCM) Hemmady et al. [Phys. Rev. Lett. 94, 014102 (2005).] is one such approach that has found great success in providing a statistical characterization for wave chaotic systems in the frequency domain. Here we aim to transform the RCM into the time domain and generalize it to alternative situations. The proposed time-domain RCM (TDRCM) method can treat a wave chaotic system with multiple ports and modes. Two features are now possible with the time-domain approach for chaotic resonators: the incorporation of early time short-orbit transmission path effects between the ports, and the inclusion of arbitrary nonlinear or time-varying port load impedances. We conduct short-pulse time-domain experiments in wave chaotic enclosures, and use the TDRCM to simulate the corresponding experimental setup. We also examine a diode-loaded port and compare experimental results with a numerical TDRCM treatment and find agreement.
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Publication: Plasma Phys. Control. Fusion 65, 065012 (2023)Doi: 10.1088/1361-6587/acd141
Confining energetic ions such as alpha particles is a prime concern in the design of stellarators. However, directly measuring alpha confinement through numerical simulation of guiding-center trajectories has been considered to be too computationally expensive and noisy to include in the design loop, and instead has been most often used only as a tool to assess stellarator designs post hoc. In its place, proxy metrics, simplified measures of confinement, have often been used to design configurations because they are computationally more tractable and have been shown to be effective. Despite the success of proxies, their correlation with direct trajectory calculations is known to be imperfect. In this study, we optimize stellarator designs for improved alpha particle confinement without the use of proxy metrics. In particular, we numerically optimize an objective function that measures alpha particle losses by simulating alpha particle trajectories. While this method is computationally expensive, we find that it can be used successfully to generate configurations with low losses.
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Publication: Front. Astron. Space Sci. 10, 2060618 (2023)Doi: 10.3389/fspas.2023.1060618
We highlight the importance of magnetic reconnection at the heliopause, both as one of the key processes driving the interaction between solar and interstellar media, but also as an element of the definition of the heliopause itself. We highlight the main observations that have fed the current debates on the definition, location and shape of the heliopause. We explain that discriminating between the current interpretations of plasma and magnetic field structures near the heliopause necessitates appropriate measurements which are lacking on Voyager 1 and 2, and describe some of the ensuing requirements for thermal plasma measurements on a future Interstellar Probe. The content of this article was submitted as a white paper contribution to the Decadal Survey for Solar and Space Physics 2024–2033 of the National Academy of Sciences.
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Publication: ACS Appl. Energy Mater. 6, 4538 (2023)Doi: 10.1021/acsaem.2c03683
A combination of depth-resolved cathodoluminescence spectroscopy (DRCLS) and X-ray photoemission depth profiling (XPS) measured the pronounced changes in both the electronic density of states and lithium composition near the nanoscale LixV2O5/LiPON interface. DRCLS studies of electrochemically lithiated bare V2O5 and the sputter-deposited V2O5 plus LiPON overlayer electrochemically lithiated in stages both showed that in the bulk the luminescence intensity of the “split-off” hybridized bonding density of states was anticorrelated with XPS-measured Li content, decreasing as the Li content increased. However, the LiPON overlayer was found to modify the band structure of the underlying LixV2O5 (LVO) to a depth of at least 30 nm beneath the V2O5 interface. DRCLS spectra near the electrochemically lithiated LiPON/LVO interface showed a significant intensity of the split-off band, implying a low Li content. However, XPS depth profiling revealed a pronounced negative gradient of Li extending from a maximum Li content at the intimate LiPON boundary to its lowest content of ∼30 nm into the V2O5 in the same region, indicating a strong interaction between band structure and Li electrochemical potential near this heterojunction. These results provide evidence for substantial effects on the local band structure near an electrolyte/cathode interface and insights into the electrochemical interface behavior of solid-state batteries in general.
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Publication: Phys. Rev. Appl. 19, 034001 (2023)Doi: 10.1103/PhysRevApplied.19.034001
Trapped ions are promising candidates for nodes of a scalable quantum network due to their long-lived qubit coherence times and high-fidelity single- and two-qubit gates. Future quantum networks based on trapped ions will require a scalable way to route photons between different nodes. Photonic integrated circuits from fabrication foundries provide a compact solution to this problem. However, these circuits typically operate at telecommunication wavelengths that are incompatible with the strong dipole emissions of trapped ions. In this work, we demonstrate the routing of single photons from a trapped ion using a photonic integrated circuit. We employ quantum frequency conversion to match the emission of the ion to the operating wavelength of a foundry-fabricated silicon nitride photonic integrated circuit, achieving a total transmission of 31.0% ± 0.9% through the device. Using programmable phase shifters, we switch the single photons between the output channels of the circuit and demonstrate a 50:50 beam splitting condition. These results constitute an important step towards programmable routing and entanglement distribution in large-scale quantum networks and distributed quantum computers.
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Publication: Plasma Phys. Control. Fusion 65, 075003 (2023)Doi: 10.1088/1361-6587/acd957
We introduce a novel approach for the simultaneous optimization of plasma physics and coil engineering objectives using fixed-boundary equilibria that is computationally efficient and applicable to a broad range of vacuum and finite plasma pressure scenarios. Our approach treats the plasma boundary and coil shapes as independently optimized variables, penalizing the mismatch between the two using a quadratic flux term in the objective function. Four use cases are presented to demonstrate the effectiveness of the approach, including simple and complex stellarator geometries. As shown here, this method outperforms previous two-stage approaches, achieving smaller plasma objective function values when coils are taken into account.
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Publication: Phys. Rev. A 107, 022225 (2023)Doi: 10.1103/PhysRevA.107.022225
A general technique for experimentally characterizing the effect of qubit-preserving impairments on the Clauser-Horne-Shimony-Holt parameter is introduced. This technique is independent of the underlying qubit encoding and is theoretically demonstrated for specific example impairments in polarization-encoded quantum-optical qubits. Included in this analysis is how spectrotemporal impairments can be incorporated into this technique.
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Publication: NPJ Quantum Info. 9, 77 (2023)Doi: 10.1038/s41534-023-00743-3
Strong interactions between defect spins in many-body solid-state quantum systems are a crucial resource for exploring non-classical states. However, they face the key challenge of controlling interactions between the defect spins, since they are spatially fixed inside the host lattice. In this work, we present a dressed state approach to control the effective dipolar coupling between solid-state spins and demonstrate this scheme experimentally using two strongly-coupled nitrogen vacancy (NV) centers in diamond. Through Ramsey spectroscopy on the sensor spin, we detect the change of the effective dipolar field generated by the control spin prepared in different dressed states. To observe the change of interaction dynamics, we deploy spin-lock-based polarization transfer measurements between the two NV spins in different dressed states. This scheme allows us to control the distribution of interaction strengths in strongly interacting spin systems, which can be a valuable tool for generating multi-spin correlated states for quantum-enhanced sensing.
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Publication: Opt. Lett. 48, 578 (2023)Doi: 10.1364/OL.476647
We report experimental observation of subharmonic mode excitation in primary Kerr optical frequency combs generated using crystalline whispering-gallery mode resonators. We show that the subcombs can be controlled and span a single or multiple free spectral ranges around the primary comb modes. In the spatial domain, the resulting multiscale combs correspond to an amplitude modulation of intracavity roll patterns. We perform a theoretical analysis based on eigenvalue decomposition that evidences the mechanism leading to the excitation of these combs.
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Publication: ACS Photon. 10, 2861 (2023)Doi: 10.1021/acsphotonics.3c00581
Trapped ions are one of the leading candidates for scalable and long-distance quantum networks because of their long qubit coherence time, high-fidelity single- and two-qubit gates, and their ability to generate photons entangled with the qubit state of the ion. One method for creating ion-photon entanglement is to exploit optical transitions from the 6P1/2 to 6S1/2 levels, which naturally emit spin-photon entangled states. But these optical transitions typically lie in the ultraviolet and visible wavelength regions of the spectrum. These wavelengths exhibit significant fiber-optic propagation loss, thereby limiting the transfer of quantum information to tens of meters. Quantum frequency conversion is essential to convert these photons to telecom wavelengths so that they can propagate over long distances in fiber-based networks, as well as for compatibility with the vast number of telecom-based optoelectronic components. Here, we generate O-band telecom photons via a low-noise quantum frequency conversion scheme from photons emitted from the 6P1/2 to 6S1/2 dipole transition of a trapped barium ion. We use a two-stage quantum frequency conversion scheme to achieve a frequency shift of 375.4 THz between the input visible photon and the output telecom photon, achieving a conversion efficiency of 11%. We attain a signal-to-background ratio of over 100 for the converted O-band telecom photon with background noise of less than 15 counts/s. These results are an important step toward achieving trapped ion quantum networks over long distances for distributed quantum computing and quantum communication.
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Publication: Light: Sci. Appl. 12, 37 (2023)Doi: 10.1038/s41377-022-01068-0
High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in the laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 mJ of energy was produced at <10 THz with a laser-to-terahertz conversion efficiency of ~0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and the plasma wakefields on the time scale of the laser pulse duration and plasma period. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.
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Publication: IEEE Access 11, 26729 (2023)Doi: 10.1109/ACCESS.2023.3255514
A technique for the simulation of multimode quantum optical interferometry and protocols in quantum communications is introduced. This technique is very efficient at simulating in the single-photon-counting regime. This works by treating the photons in the system as members of a multiphoton pulse and reducing the computation of measurable quantities to overlap integrals that may be precomputed and combined in a recursive algorithm. The simulation of a Mach-Zehnder interferometer and the Hong-Ou-Mandel effect are demonstrated using this technique. The results of these simulations perfectly agree with the theoretical results. Additionally, since the effects of the components in the system can be integrated into the quantum operators involved, the technique is agnostic to the components introduced into the system.
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Publication: NPJ Quantum Info. 9, 66 (2023)Doi: 10.1038/s41534-023-00732-6
Chern numbers characterize topological phases in a wide array of physical systems. However, the resilience of system topology to external perturbations makes it challenging experimentally to investigate transitions between different phases. In this study, we demonstrate the transitions of a Chern number from 0 to 3, synthesized in an electronic-nuclear spin system associated with the nitrogen-vacancy (NV) centre in diamond. The Chern number is characterized by the number of degeneracies enclosed in a control Hamiltonian parameter sphere. Topological transitions between different phases are realized by varying the radius and offset of the sphere such that the Chern number changes. We show that the measured topological phase diagram is consistent with numerical calculations and can also be mapped onto an interacting three-qubit system. The NV system may also allow access to even higher Chern numbers, which could be applied to exploring exotic topology or topological quantum information.
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Publication: Joule 7, 652 (2023)Doi: 10.1016/j.joule.2023.03.001
Lithium metal solid-state batteries (LiSSBs) present new challenges in the measurement of material, component, and cell mechanical behaviors and in the measurement and theory of fundamental mechanical-electrochemical (thermodynamics, transport, and kinetics) couplings. Here, we classify the major mechanical and electrochemical-mechanical (ECM) studies underway and provide an overview of major mechanical testing platforms. We emphasize key distinctions among testing platforms, including tip- vs. platen-based sample compression, surface- vs. volume-based analysis, ease of integration with a vacuum or inert atmosphere environment, the ability to control and measure force/displacement over long periods of time, ranges of force and contact area, and others. Among the techniques we review, nanoindentation platforms offer some unique benefits associated with being able to use both tip-based nanoindentation techniques as well as platen-based compression over areas approaching 1 mm2. Sample design is also important: while most efforts are particle-based (i.e., using particles of solid electrolyte and cathode-active materials and densifying them using sintering or pressure), the resulting electrochemical response is from the overall collection of particles present. In contrast, thin-film (<1 μm) solid-state battery materials (e.g., Li, LiPON, LCO) provide well defined and uniform structures well suited for fundamental electrochemical-mechanical studies and offer an important opportunity to drive underlying scientific advances in LiSSB and other areas. We believe there are exciting opportunities to advance the measurement of both mechanical properties and electrochemical-mechanical couplings through the careful and novel co-design of test structures and experimental approaches for LiSSB materials, components, and cells.
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Publication: J. Kor. Phys. Soc. 82, 455 (2023)Doi: 10.1007/s40042-023-00730-z
We measure X-rays emitted perpendicular to the laser propagation direction in petawatt-laser-driven wakefield acceleration of electrons in a gas cell. Multi-mega-electronvolt electrons are ejected in the transverse direction by laser-driven plasma wakefields, generating bremsstrahlung X-rays when they encounter a dense medium such as a gas-cell window. The X-rays, detected and characterized by two separate filter-stack spectrometers containing a series of imaging plates, exhibit peak energy fluences at ~ 150–200 keV. The mechanism of electron acceleration in the transverse direction and subsequent bremsstrahlung X-ray generation is also examined and confirmed using particle-in-cell and Monte Carlo FLUKA simulations.
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Publication: Opt. Express 31, 2316 (2023)Doi: 10.1364/OE.475465
Quantum measurements that use the entangled photons' polarization to encode quantum information require calibration and alignment of the measurement bases between spatially separate observers. Because of the changing birefringence in optical fibers arising from temperature fluctuations or external mechanical vibrations, the polarization state at the end of a fiber channel is unpredictable and time-varying. Polarization tracking and stabilization methods originally developed for classical optical communications cannot be applied to polarization-entangled photons, where the separately detected photons are statistically unpolarized, yet quantum mechanically correlated. We report here a fast method for automatic alignment and dynamic tracking of the polarization measurement bases between spatially separated detectors. The system uses the Nelder-Mead simplex method to minimize the observed coincidence rate between non-locally measured entangled photon pairs, without relying on classical wavelength-multiplexed pilot tones or temporally interleaved polarized photons. Alignment and control is demonstrated in a 7.1 km deployed fiber loop as well as in a controlled drifting scenario.
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Publication: J. Phys. D - Appl. Phys. 56 [22], (2023)Doi: 10.1088/1361-6463/acbe4c
Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a 'snapshot' introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.
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Publication: Chaos 33, 023143 (2023)Doi: 10.1063/5.0131787
The ability of machine learning (ML) models to “extrapolate” to situations outside of the range spanned by their training data is crucial for predicting the long-term behavior of non-stationary dynamical systems (e.g., prediction of terrestrial climate change), since the future trajectories of such systems may (perhaps after crossing a tipping point) explore regions of state space which were not explored in past time-series measurements used as training data. We investigate the extent to which ML methods can yield useful results by extrapolation of such training data in the task of forecasting non-stationary dynamics, as well as conditions under which such methods fail. In general, we find that ML can be surprisingly effective even in situations that might appear to be extremely challenging, but do (as one would expect) fail when “too much” extrapolation is required. For the latter case, we show that good results can potentially be obtained by combining the ML approach with an available inaccurate conventional model based on scientific knowledge.
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Publication: ACS Appl. Energy Mater. 6, 8266 (2023)Doi: 10.1021/acsaem.3c01397
The family of thio-LISICON solid-state electrolytes (SSEs) is one of the most promising material systems for the realization of fully solid state batteries due to comparable performance with liquid electrolyte-based counterparts. Among this SSE family, Li10GeP2S12 (LGPS) is one of the most promising candidates due to its high theoretical ionic conductivity (1 × 10–2 S cm–1). However, the narrow electrochemical and chemical stability windows of LGPS make it unstable in direct contact with both Li metal and conventional transition metal oxide cathode materials, leading to dramatic degradation during battery cycling and even during battery storage prior to battery operation. In this study, we employ an elastomeric artificial solid electrolyte interphase (ASEI) as a protective layer grown directly on Li metal by electrochemically polymerizing 1,3-dioxolane prior to assembling Li/LGPS/Li test cells. This ASEI serves as a Li+-conducting interlayer capable of halving the chemical degradation rate as compared to untreated pristine Li at the Li/LGPS interface, while also significantly lowering the absolute impedance and overpotential of Li/LGPS/Li symmetric cells during galvanostatic cycling at 0.1 mA h cm–2. The elemental composition and spatial structure of this ASEI layer were investigated using X-ray photoelectron spectroscopy and scanning electron microscopy characterization techniques. Density functional theory calculations were performed to understand the impact of the elastomeric ASEI layer on chemical aging at the Li/LGPS interface.
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Publication: J. Power Sources 575, 233056 (2023)Doi: 10.1016/j.jpowsour.2023.233056
Multi-layer ceramic capacitors have been used for high frequency decoupling application due to a lower overall impedance leading to fast current response. However, high parasitic inductance limits the application of these capacitors in ultra-high frequency domain. Thus, Multlayer Ceramic Capacitors (MLCCs) are placed close to the IC to improve circuit efficiency and reduce inductance. With next generation applications, the demand for frequency range has further increased which not only requires enhanced capacitor material but improved manufacturing techniques to limit the inductive path. Here, we demonstrate ALD of two different polymorphs of ultra-thin film lithium phosphorus oxynitride (LiPON) as an inorganic solid state electrolyte (SSE) for on chip capacitors for decoupling application. Both the LiPON capacitors shows an electric double layer behavior with a capacitance of 15 μF/cm2 and a low leakage current (< 20 nA/cm2) at 2V. The LiPON shows EDLC behavior up to 10 kHz and beyond, both the polymorphs show an electrostatic behavior with a high dielectric constant (14). This dual frequency behavior along with low parasitic inductance and on chip integration allows for operation in extended frequency ranges.
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Publication: IEEE Photonics Technol. Lett. 35, 701 (2023)Doi: 10.1109/LPT.2023.3273412
Microwave photonic circuits are capable of processing large bandwidths of radiometric data ( > 100 GHz) in an unprecedented number of analog spectrometer channels ( > 100) at narrow spectral resolutions (< 100 MHz) and a small device footprint. However, simultaneously processing this entire bandwidth requires a large dynamic range. In this work, a heterodyne photonic radiometer is assembled and tested with a microwave thermal noise source. We model and demonstrate the 1 dB output power compression point occurs at approximately half of the average input power for a thermal noise signal compared to a continuous wave signal. These results have a significant impact on future photonic radiometer design considerations.
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Publication: ACS Photon. 10, 1104 (2023)Doi: 10.1021/acsphotonics.2c01835
Quantum memories are a key building block for optical quantum computers and quantum networks. Rare-earth ion-doped crystals are a promising material to achieve quantum memory using an atomic frequency comb protocol. However, current atomic frequency comb memories typically use bulk materials or waveguides with large cross sections or rely on fabrication techniques not easily adaptable to wafer scale processing. Here, we demonstrate a compact chip-integrated atomic frequency comb in rare-earth-doped thin-film lithium niobate. Our optical memory exhibits a broad storage bandwidth exceeding 100 MHz and optical storage time as long as 250 ns. The enhanced optical confinement in this device leads to three orders of magnitude reduction in optical power required for a coherent control as compared to ion-diffused waveguides. These compact atomic frequency comb memories pave the way toward scalable, highly efficient, electro-optically tunable quantum photonic systems that can store and manipulate light on a compact chip.
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Publication: Photon. Res. 11, 917 (2023)Doi: 10.1364/PRJ.484727
Backward stimulated Brillouin scattering (SBS) is widely exploited for various applications in optics and optoelectronics. It typically features a narrow gain bandwidth of a few tens of megahertz in fluoride crystals. Here we report a hundredfold increase of SBS bandwidth in whispering-gallery mode resonators. The crystalline orientation results in a large variation of the acoustic phase velocity upon propagation along the periphery, from which a broad Brillouin gain is formed. Over 2.5 GHz wide Brillouin gain profile is theoretically found and experimentally validated. SBS phenomena with Brillouin shift frequencies ranging from 11.73 to 14.47 GHz in ultrahigh Q Z-cut magnesium fluoride cavities pumped at the telecommunication wavelength are demonstrated. Furthermore, the Brillouin–Kerr comb in this device is demonstrated. Over 400 comb lines spanning across a spectral window of 120 nm are observed. Our finding paves a new way for tailoring and harnessing the Brillouin gain in crystals.
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Publication: J. Plasma Phys. 89, 905890304 (2023)Doi: 10.1017/S0022377823000430
We present a new neoclassical transport model for large aspect ratio tokamaks where the gradient scale lengths are of the size of the ion poloidal gyroradius. Previous work on neoclassical transport across transport barriers assumed large density and potential gradients but a small temperature gradient, or neglected the gradient of the mean parallel flow. Using large aspect ratio and low collisionality expansions, we relax these restrictive assumptions. We define a new set of variables based on conserved quantities, which simplifies the drift kinetic equation whilst keeping strong gradients, and derive equations describing the transport of particles, parallel momentum and energy by ions in the banana regime. The poloidally varying parts of density and electric potential are included. Studying contributions from both passing and trapped particles, we show that the resulting transport is dominated by trapped particles. We find that a non-zero neoclassical particle flux requires parallel momentum input which could be provided through interaction with turbulence or impurities. We derive upper and lower bounds for the energy flux across a transport barrier in both temperature and density and present example profiles and fluxes.
2017
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Publication: Phys. Plasmas 24, 103116 (2017)Doi: 10.1063/1.4986006
For mobile ionospheric heaters, it is necessary to develop highly efficient RF sources capable of delivering radiation in the frequency range from 3 to 10 MHz with an average power at a megawatt level. A promising source, which is capable of offering these parameters, is a grid-less version of the inductive output tube (IOT), also known as a klystrode. In this paper, studies analyzing the efficiency of grid-less IOTs are described. The basic trade-offs needed to reach high efficiency are investigated. In particular, the trade-off between the peak current and the duration of the current micro-pulse is analyzed. A particle in the cell code is used to self-consistently calculate the distribution in axial and transverse momentum and in total electron energy from the cathode to the collector. The efficiency of IOTs with collectors of various configurations is examined. It is shown that the efficiency of IOTs can be in the 90% range even without using depressed collectors.
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Publication: Phys. Plasmas 24, 043109 (2017)Doi: 10.1063/1.4981218
The excitation of Terahertz (THz) radiation by the interaction of an ultrashort laser pulse with the modes of a miniature corrugated plasma waveguide is considered. The axially corrugated waveguide supports the electromagnetic modes with appropriate polarization and subluminal phase velocities that can be phase matched to the ponderomotive potential associated with the laser pulse, making significant THz generation possible. This process is studied via full format Particle-in-Cell simulations that, for the first time, model the nonlinear dynamics of the plasma and the self-consistent evolution of the laser pulse in the case where the laser pulse energy is entirely depleted. It is found that the generated THz is characterized by lateral emission from the channel, with a spectrum that may be narrow or broad depending on the laser intensity. A range of realistic laser pulse and plasma parameters is considered with the goal of maximizing the conversion efficiency of optical energy to THz radiation. As an example, a fixed drive pulse (0.55 J) with a spot size of 15 μm and a duration of 15 fs produces a THz radiation of 37.8 mJ of in a 1.5 cm corrugated plasma waveguide with an on axis average density of 1.4 × 1018 cm−3.
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Publication: Phys. Rev. E 95, 050201(R) (2017)Doi: 10.1103/PhysRevE.95.050201
Through experiments and numerical simulations we explore the behavior of rf SQUID (radio frequency superconducting quantum interference device) metamaterials, which show extreme tunability and nonlinearity. The emergent electromagnetic properties of this metamaterial are sensitive to the degree of coherent response of the driven interacting SQUIDs. Coherence suffers in the presence of disorder, which is experimentally found to be mainly due to a dc flux gradient. We demonstrate methods to recover the coherence, specifically by varying the coupling between the SQUID meta-atoms and increasing the temperature or the amplitude of the applied rf flux.
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Publication: Chaos 27, 033102 (2017)Doi: 10.1063/1.4977514
We derive a mean-field approximation for the macroscopic dynamics of large networks of pulse-coupled theta neurons in order to study the effects of different network degree distributions and degree correlations (assortativity). Using the ansatz of Ott and Antonsen [Chaos 18, 037113 (2008)], we obtain a reduced system of ordinary differential equations describing the mean-field dynamics, with significantly lower dimensionality compared with the complete set of dynamical equations for the system. We find that, for sufficiently large networks and degrees, the dynamical behavior of the reduced system agrees well with that of the full network. This dimensional reduction allows for an efficient characterization of system phase transitions and attractors. For networks with tightly peaked degree distributions, the macroscopic behavior closely resembles that of fully connected networks previously studied by others. In contrast, networks with highly skewed degree distributions exhibit different macroscopic dynamics due to the emergence of degree dependent behavior of different oscillators. For nonassortative networks (i.e., networks without degree correlations), we observe the presence of a synchronously firing phase that can be suppressed by the presence of either assortativity or disassortativity in the network. We show that the results derived here can be used to analyze the effects of network topology on macroscopic behavior in neuronal networks in a computationally efficient fashion.
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Publication: Chem. Mater. 29, 6298 (2017)Doi: 10.1021/acs.chemmater.7b01496
Li metal is among the most attractive anode materials for secondary batteries, with a theoretical specific capacity > 3800 mAh g–1. However, its extremely low electrochemical potential is associated with high chemical reactivity that results in undesirable reduction of electrolyte species on the lithium surface, leading to spontaneous formation of a solid electrolyte interphase (SEI) with uncontrolled composition, morphology, and physicochemical properties. Here, we demonstrate a new approach to stabilize Li metal anodes using a hybrid organic/inorganic artificial solid electrolyte interphase (ASEI) deposited directly on the Li metal surface by self-healing electrochemical polymerization (EP) and atomic layer deposition (ALD). This hybrid protection layer is thin, flexible, ionically conductive, and electrically insulating. We show that Li metal protected by the hybrid protection layer gives rise to very stable cycling performance for over 300 cycles at current density 1 mA/cm2 and over 110 cycles at current density 2 mA/cm2, well above the threshold for dendrite growth at unprotected Li. Our strategy for protecting Li metal anodes by hybrid organic/inorganic ASEI represents a new approach to mitigating or eliminating dendrite formation at reactive metal anodes—illustrated here for Li—and may expedite the realization of a “beyond-Li-ion” battery technology employing Li metal anodes (e.g., Li–S).
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Publication: Nano Lett. 17, 5811 (2017)Doi: 10.1021/acs.nanolett.7b02931
Photoconductive antennas are widely used for time-resolved detection of terahertz (THz) pulses. In contrast to photothermoelectric or bolometric THz detection, the coherent detection allows direct measurement of the electric field transient of a THz pulse, which contains both spectral and phase information. In this Letter, we demonstrate for the first time photoconductive detection of free-space propagating THz radiation with thin flakes of a van der Waals material. Mechanically exfoliated flakes of black phosphorus are combined with an antenna that concentrates the THz fields to the small flake (∼10 μm). Similar performance is reached at gating wavelengths of 800 and 1550 nm, which suggests that the narrow bandgap of black phosphorus could allow operation at wavelengths as long as 4 μm. The detected spectrum peaks at 60 GHz, where the signal-to-noise ratio is of the order of 40 dB, and the detectable signal extends to 0.2 THz. The measured signal strongly depends on the polarization of the THz field and the gating pulse, which is explained by the role of the antenna and the anisotropy of the black phosphorus flake, respectively. We analyze the limitations of the device and show potential improvements that could significantly increase the efficiency and bandwidth.
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Publication: Science 357, 1354 (2017)Doi: 10.1126/science.aao2437
Information networks must transmit data over long distances and store it for later retrieval. A quantum network operates in an analogous way but uses signals contained in a quantum system, such as single photons or atoms (1). Although rapid progress in the generation of quantum light provides hope for future realizations of quantum networks, the ability to store photons remains a critical limitation. A number of approaches offer the possibility to store photons (2), but they typically suffer from short storage times, low efficiencies, or large footprints that are incompatible with compact integrated devices. On page 1392 of this issue, Zhong et al. (3) report a major step toward the goal of storing light in a chip-scale atomic memory. They exploit the enhancement of optical cavities to transfer light efficiently to excitations in rare-earth ions. The device fits on a micrometer-sized footprint, allowing storage of light in a very tiny box.
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Publication: IEEE Trans. Plasma Sci. 45, 2945 (2017)Doi: 10.1109/TPS.2017.2759269
A cyclotron resonance maser source using low-effective-mass conduction electrons in graphene, if successful, would allow for generation of far infrared (FIR) and terahertz (THz) radiation without requiring magnetic fields running into the tens of tesla. In order to investigate this possibility, we consider a situation in which electrons are effectively injected via pumping from the valence band to the conduction band using an IR laser source, subsequently gyrate in a magnetic field applied perpendicular to the plane of the graphene, and give rise to gain for an FIR/THz wave crossing the plane of the graphene. The treatment is classical, and includes on equal footing the electron interaction with the radiation field and the decay in electron energy due to collisional processes. Gain is found even though there is no inversion of the energy distribution function. Gain can occur for electron damping times as short as hundreds of femtoseconds.
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Publication: Nucl. Fusion 57, 046003 (2017)Doi: 10.1088/1741-4326/aa57d4
Several fast methods for computing stellarator coil shapes are compared, including the classical NESCOIL procedure (Merkel 1987 Nucl. Fusion 27 867), its generalization using truncated singular value decomposition, and a Tikhonov regularization approach we call REGCOIL in which the squared current density is included in the objective function. Considering W7-X and NCSX geometries, and for any desired level of regularization, we find the REGCOIL approach simultaneously achieves lower surface-averaged and maximum values of both current density (on the coil winding surface) and normal magnetic field (on the desired plasma surface). This approach therefore can simultaneously improve the free-boundary reconstruction of the target plasma shape while substantially increasing the minimum distances between coils, preventing collisions between coils while improving access for ports and maintenance. The REGCOIL method also allows finer control over the level of regularization, it preserves convexity to ensure the local optimum found is the global optimum, and it eliminates two pathologies of NESCOIL: the resulting coil shapes become independent of the arbitrary choice of angles used to parameterize the coil surface, and the resulting coil shapes converge rather than diverge as Fourier resolution is increased. We therefore contend that REGCOIL should be used instead of NESCOIL for applications in which a fast and robust method for coil calculation is needed, such as when targeting coil complexity in fixed-boundary plasma optimization, or for scoping new stellarator geometries.
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Publication: J. Phys. D - Appl. Phys. 50, 490201 (2017)Doi: 10.1088/1361-6463/aa95c8
State-of-the-art technologies are increasingly demanding materials and thin film processing technologies that offer control at atomistic length scales. These requirements are pushing plasma-based processing techniques towards fundamental limits with regard to modification, deposition, and etching of materials. The goal of achieving atomistic control in plasma–material interactions may be viewed as a grand challenge of low-temperature plasma science and technology. It involves the need to control the interaction of multiple particle fluxes that are characteristic of the plasma state, including electrons, ions, radicals, excited neutrals and photons, with surfaces and to regulate the consequences of these interactions towards desired atomistic outcomes.
The challenge of atomistic control in plasma–material interactions provides the unifying perspective of this Special Issue. The collection of invited reviews and current research articles of the issue illustrates various aspects of the overall challenge. The presentation of a related number of coordinated topics was intended to (1) illustrate the achievements and state of the art of fundamental research and technical capabilities in different areas of application, (2) identify areas where we either lack sufficient understanding to achieve this goal or where our current plasma-surface interaction approaches provide insufficient control, and (3) clarify the scientific knowledge and potential advances required for low temperature plasma-based methods to be successful against the goal of achieving atomistic control in plasma–surface interactions.
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Publication: Appl. Phys. Lett. 111, 221104 (2017)Doi: 10.1063/1.5000248
We demonstrate coupling of lead halide perovskite nanocrystals to a nanophotonic cavity. From photoluminescence measurements, we observe a factor of 10 enhancement in brightness from the cavity mode emission. We perform room temperature time-resolved lifetime measurements that demonstrate an average spontaneous emission rate enhancement of 2.9 for perovskite nanocrystals within the cavity as compared to those located on the unpatterned surfaces. Our method provides a way towards realizing efficient light emitters and low-threshold lasers, as well as fast nonlinear optical devices, using solution processable materials.
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Publication: Phys. Plasmas 24, 092110 (2017)Doi: 10.1063/1.4986211
Magnetic reconnection is an important driver of energetic particles in many astrophysical phenomena. Using kinetic particle-in-cell simulations, we explore the impact of three-dimensional reconnection dynamics on the efficiency of particle acceleration. In two-dimensional systems, Alfvénic outflows expel energetic electrons into flux ropes where they become trapped and disconnected from acceleration regions. However, in three-dimensional systems these flux ropes develop an axial structure that enables particles to leak out and return to acceleration regions. This requires a finite guide field so that particles may move quickly along the flux rope axis. We show that greatest energetic electron production occurs when the guide field is of the same order as the reconnecting component: large enough to facilitate strong transport, but not so large as to throttle the dominant Fermi mechanism responsible for efficient electron acceleration. This suggests a natural explanation for the envelope of electron acceleration during the impulsive phase of eruptive flares.
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Publication: Opt. Exp. 25, 12666 (2017)Doi: 10.1364/OE.25.012666
Two-dimensional black phosphorus is a new material that has gained widespread interest as an active material for optoelectronic applications. It features high carrier mobility that allows for efficient free-carrier absorption of terahertz radiation, even though the photon energy is far below the bandgap energy. Here we present an efficient and ultrafast terahertz detector, based on exfoliated multilayer flakes of black phosphorus. The device responsivity is about 1 mV/W for a 2.5 THz beam with a diameter of 200 μm, and is primarily limited by the small active area of the device in comparison to the incident beam area. The intrinsic responsivity is determined by Joule heating experiments to be about 44 V/W, which is in agreement with predictions from the Drude conductivity model. Time resolved measurements at a frequency of 0.5 THz reveal an ultrafast response time of 20 ps, making black phosphorus a candidate for high performance THz detection at room temperature.
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Publication: Astrophys. J. Lett. 839, L12 (2017)Doi: 10.3847/2041-8213/aa692f
Based on the difference between the orientation of the interstellar BISM and the solar magnetic fields, there was an expectation that the magnetic field direction would rotate dramatically across the heliopause (HP). However, the Voyager 1 spacecraft measured very little rotation across the HP. Previously, we showed that the BISM twists as it approaches the HP and acquires a strong T component (east–west). Here, we establish that reconnection in the eastern flank of the heliosphere is responsible for the twist. On the eastern flank the solar magnetic field has twisted into the positive N direction and reconnects with the southward pointing component of the BISM. Reconnection drives a rotational discontinuity (RD) that twists the BISM into the −T direction and propagates upstream in the interstellar medium toward the nose. The consequence is that the N component of BISM is reduced in a finite width band upstream of the HP. Voyager 1 currently measures angles (delta = sin-1(BN/B)) close to solar values. We present MHD simulations to support this scenario, suppressing reconnection in the nose region while allowing it in the flanks, consistent with recent ideas about reconnection suppression from diamagnetic drifts. The jump in plasma β (the plasma to magnetic pressure) across the nose of HP is much greater than in the flanks because the heliosheath β is greater there than in the flanks. Large-scale reconnection is therefore suppressed in the nose but not at the flanks. Simulation data suggest that BISM will return to its pristine value 10–15 au past the HP.
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Publication: Nature Commun. 8, 143 (2017)Doi: 10.1038/s41467-017-00198-z
Colloidal semiconductor nanocrystals have emerged as promising active materials for solution-processable optoelectronic and light-emitting devices. In particular, the development of nanocrystal lasers is currently experiencing rapid progress. However, these lasers require large pump powers, and realizing an efficient low-power nanocrystal laser has remained a difficult challenge. Here, we demonstrate a nanolaser using colloidal nanocrystals that exhibits a threshold input power of less than 1 μW, a very low threshold for any laser using colloidal emitters. We use CdSe/CdS core-shell nanoplatelets, which are efficient nanocrystal emitters with the electronic structure of quantum wells, coupled to a photonic-crystal nanobeam cavity that attains high coupling efficiencies. The device achieves stable continuous-wave lasing at room temperature, which is essential for many photonic and optoelectronic applications. Our results show that colloidal nanocrystals are suitable for compact and efficient optoelectronic devices based on versatile and inexpensive solution-processable materials.
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Publication: 2D Mater. 4, 2 (2017)Doi: 10.1088/2053-1583/aa5c75
Exploiting the underdeveloped terahertz range (~1012–1013 Hz) of the electromagnetic spectrum could advance many scientific fields (e.g. medical imaging for the identification of tumors and other biological tissues, non-destructive evaluation of hidden objects or ultra-broadband communication). Despite the benefits of operating in this regime, generation, detection and manipulation have proven difficult, as few materials have functional interactions with THz radiation. In contrast, graphene supports resonances in the THz regime through structural confinement of surface plasmons, which can lead to enhanced absorption. In prior work, the achievable plasmon resonances in such structures have been limited by multiple electron scattering mechanisms (i.e., large carrier scattering rates) which greatly broaden the resonance (>100 cm−1; 3 THz). We report the narrowest room temperature Drude response to-date, 30 cm−1 (0.87 THz), obtained using quasi-free standing bilayer epitaxial graphene (QFS BLG) synthesized on (0 0 0 1)6H–SiC. This narrow response is due to a 4-fold increase in carrier mobility and improved thickness and electronic uniformity of QFS BLG. Moreover, QFS BLG samples patterned into microribbons targeting 1.8–5.7 THz plasmon resonances also exhibit low scattering rates (37–53 cm−1). Due to the improved THz properties of QFS BLG, the effects of e-beam processing on carrier scattering rates was determined and we found that fabrication conditions can be tuned to minimize the impact on optoelectronic properties. In addition, electrostatic gating of patterned QFS BLG shows narrow band THz amplitude modulation. Taken together, these properties of QFS BLG should facilitate future development of THz optoelectronic devices for monochromatic applications.
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Publication: Chem. Mater. 29, 8780 (2017)Doi: 10.1021/acs.chemmater.7b03058
High-energy conversion electrodes undergo successive Li insertion and conversion during lithiation. A primary scientific obstacle to harnessing the potentially high lithium storage capabilities of conversion electrode materials has been the formation of insulating new phases throughout the conversion reactions. These new phases are chemically stable, and electrochemically irreversible if formed in large amounts with coarsening. Herein, we synthesized FeOF conversion material as a model system and mechanistically demonstrate that a thin solid electrolyte [lithium phosphorus oxynitride (LiPON)] atomic layer deposition-deposited on the composite electrode extends the Li insertion process to higher concentrations, delaying the onset of a parasitic chemical conversion reaction and rendering the redox reaction of the protected conversion electrode electrochemically reversible. Reversibility is demonstrated to at least 100 cycles, with the LiPON protective coating increasing capacity retention from 29 to 89% at 100 cycles. Pursuing the chemical mechanism behind the boosted electrochemical reversibility, we conducted electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and electrochemical measurements that unrevealed the suppression of undesired phase formation and extended lithium insertion of the coated electrode. Support for the delayed consequences of the conversion reaction is also obtained by high-resolution transmission electron microscopy. Our findings strongly suggest that undesired new phase formation upon lithiation of electrode materials can be suppressed in the presence of a thin protection layer not only on the surface of the coated electrode but also in the bulk of the material through mechanical confinement that modulates the electrochemical reaction.
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Publication: Chaos 27, 051101 (2017)Doi: 10.1063/1.4983470
A common observation is that large groups of oscillatory biological units often have the ability to synchronize. A paradigmatic model of such behavior is provided by the Kuramoto model, which achieves synchronization through coupling of the phase dynamics of individual oscillators, while each oscillator maintains a different constant inherent natural frequency. Here we consider the biologically likely possibility that the oscillatory units may be capable of enhancing their synchronization ability by adaptive frequency dynamics. We propose a simple augmentation of the Kuramoto model which does this. We also show that, by the use of a previously developed technique [Ott and Antonsen, Chaos 18, 037113 (2008)], it is possible to reduce the resulting dynamics to a lower dimensional system for the macroscopic evolution of the oscillator ensemble. By employing this reduction, we investigate the dynamics of our system, finding a characteristic hysteretic behavior and enhancement of the quality of the achieved synchronization.
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Publication: Chaos 27, 103114 (2017)Doi: 10.1063/1.4986499
Concepts from the field of wave chaos have been shown to successfully predict the statistical properties of linear electromagnetic fields in electrically large enclosures. The Random Coupling Model (RCM) describes these properties by incorporating both universal features described by Random Matrix Theory and the system-specific features of particular system realizations. In an effort to extend this approach to the nonlinear domain, we add an active nonlinear frequency-doubling circuit to an otherwise linear wave chaotic system, and we measure the statistical properties of the resulting second harmonic fields. We develop an RCM-based model of this system as two linear chaotic cavities coupled by means of a nonlinear transfer function. The harmonic field strengths are predicted to be the product of two statistical quantities and the nonlinearity characteristics. Statistical results from measurement-based calculation, RCM-based simulation, and direct experimental measurements are compared and show good agreement over many decades of power.
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Publication: Phys. Rev. A 96, 043867 (2017)Doi: 10.1103/PhysRevA.96.043867
The nonlinear polarization response and plasma generation produced by intense optical pulses, modeled by the metastable-electronic-state approach, are verified against space-and-time resolved measurements with single-shot supercontinuum spectral interferometry. This first of a kind theory-experiment comparison is done in the intensity regime typical for optical filamentation, where self-focusing and plasma generation play competing roles. Excellent agreement between the theory and experiment shows that the self-focusing nonlinearity can be approximated by a single resonant state. Moreover, we demonstrate that inclusion of the post-adiabatic corrections, previously tested only in theoretic models, provides a viable description of the ionization rate in real gases.
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Publication: Astrophys. J. 837, 159 (2017)Doi: 10.3847/1538-4357/aa6304
The misalignment of the solar rotation axis and the magnetic axis of the Sun produces a periodic reversal of the Parker spiral magnetic field and the sectored solar wind. The compression of the sectors is expected to lead to reconnection in the heliosheath (HS). We present particle-in-cell simulations of the sectored HS that reflect the plasma environment along the Voyager 1 and 2 trajectories, specifically including unequal positive and negative azimuthal magnetic flux as seen in the Voyager data. Reconnection proceeds on individual current sheets until islands on adjacent current layers merge. At late time, bands of the dominant flux survive, separated by bands of deep magnetic field depletion. The ambient plasma pressure supports the strong magnetic pressure variation so that pressure is anticorrelated with magnetic field strength. There is little variation in the magnetic field direction across the boundaries of the magnetic depressions. At irregular intervals within the magnetic depressions are long-lived pairs of magnetic islands where the magnetic field direction reverses so that spacecraft data would reveal sharp magnetic field depressions with only occasional crossings with jumps in magnetic field direction. This is typical of the magnetic field data from the Voyager spacecraft. Voyager 2 data reveal that fluctuations in the density and magnetic field strength are anticorrelated in the sector zone, as expected from reconnection, but not in unipolar regions. The consequence of the annihilation of subdominant flux is a sharp reduction in the number of sectors and a loss in magnetic flux, as documented from the Voyager 1 magnetic field and flow data.
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Publication: Nanoscale 9, 11566 (2017)Doi: 10.1039/c7nr02151h
Here we report the high performance and cyclability of an asymmetric full cell nanopore battery, comprised of V2O5 as the cathode and prelithiated SnO2 as the anode, with integrated nanotubular Pt current collectors underneath each nanotubular storage electrode, confined within an anodized aluminium oxide (AAO) nanopore. Enabled by atomic layer deposition (ALD), this coaxial nanotube full cell is fully confined within a high aspect ratio nanopore (150 nm in diameter, 50 μm in length), with an ultra-small volume of about 1 fL. By controlling the amount of lithium ion prelithiated into the SnO2 anode, we can tune the full cell output voltage in the range of 0.3 V to 3 V. When tested as a massively parallel device (∼2 billion cm−2), this asymmetric nanopore battery array displays exceptional rate performance and cyclability: when cycled between 1 V and 3 V, capacity retention at the 200C rate is ∼73% of that at 1C, and at 25C rate only 2% capacity loss occurs after more than 500 charge/discharge cycles. With the increased full cell output potential, the asymmetric V2O5–SnO2 nanopore battery shows significantly improved energy and power density over the previously reported symmetric cell, 4.6 times higher volumetric energy and 5.2 times higher power density – an even more promising indication that controlled nanostructure designs employing nanoconfined environments with large electrode surface areas present promising directions for future battery technology.
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Publication: Chaos 27, 121102 (2017)Doi: 10.1063/1.5010300
We use recent advances in the machine learning area known as “reservoir computing” to formulate a method for model-free estimation from data of the Lyapunov exponents of a chaotic process. The technique uses a limited time series of measurements as input to a high-dimensional dynamical system called a “reservoir.” After the reservoir's response to the data is recorded, linear regression is used to learn a large set of parameters, called the “output weights.” The learned output weights are then used to form a modified autonomous reservoir designed to be capable of producing an arbitrarily long time series whose ergodic properties approximate those of the input signal. When successful, we say that the autonomous reservoir reproduces the attractor's “climate.” Since the reservoir equations and output weights are known, we can compute the derivatives needed to determine the Lyapunov exponents of the autonomous reservoir, which we then use as estimates of the Lyapunov exponents for the original input generating system. We illustrate the effectiveness of our technique with two examples, the Lorenz system and the Kuramoto-Sivashinsky (KS) equation. In the case of the KS equation, we note that the high dimensional nature of the system and the large number of Lyapunov exponents yield a challenging test of our method, which we find the method successfully passes.
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Publication: Geophys. Res. Lett. 44, 2978 (2017)Doi: 10.1002/2016GL072493
Observations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large-amplitude parallel electric fields (E||). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large-amplitude (~100 mV/m) E|| in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process.
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Publication: Chaos 27, 041102 (2017)Doi: 10.1063/1.4979665
Deducing the state of a dynamical system as a function of time from a limited number of concurrent system state measurements is an important problem of great practical utility. A scheme that accomplishes this is called an “observer.” We consider the case in which a model of the system is unavailable or insufficiently accurate, but “training” time series data of the desired state variables are available for a short period of time, and a limited number of other system variables are continually measured. We propose a solution to this problem using networks of neuron-like units known as “reservoir computers.” The measurements that are continually available are input to the network, which is trained with the limited-time data to output estimates of the desired state variables. We demonstrate our method, which we call a “reservoir observer,” using the Rössler system, the Lorenz system, and the spatiotemporally chaotic Kuramoto–Sivashinsky equation. Subject to the condition of observability (i.e., whether it is in principle possible, by any means, to infer the desired unmeasured variables from the measured variables), we show that the reservoir observer can be a very effective and versatile tool for robustly reconstructing unmeasured dynamical system variables.
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Publication: J. Geophys. Res. - Space Phys. 122, 11086 (2017)Doi: 10.1002/2017JA024227
We present detailed analysis of the turbulence observed in three-dimensional particle-in-cell simulations of magnetic reconnection at the magnetopause. The parameters are representative of an electron diffusion region encounter of the Magnetospheric Multiscale (MMS) mission. The turbulence is found to develop around both the magnetic X line and separatrices, is electromagnetic in nature, is characterized by a wave vector k given by kρe∼(meTe/miTi)0.25 with ρe the electron Larmor radius, and appears to have the ion pressure gradient as its source of free energy. Taken together, these results suggest the instability is a variant of the lower hybrid drift instability. The turbulence produces electric field fluctuations in the out-of-plane direction (the direction of the reconnection electric field) with an amplitude of around ±10 mV/m, which is much greater than the reconnection electric field of around 0.1 mV/m. Such large values of the out-of-plane electric field have been identified in the MMS data. The turbulence in the simulations controls the scale lengths of the density profile and current layers in asymmetric reconnection, driving them closer to sqrt(ρeρi ) than the pe or de scalings seen in 2-D reconnection simulations, and produces significant anomalous resistivity and viscosity in the electron diffusion region.
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Publication: IEEE Trans. Plasma Sci. 45, 1051 (2017)Doi: 10.1109/TPS.2017.2699925
A tunable electrically small antenna (ESA) designed to be naturally resonant at 100 MHz is evaluated for its range of tuning and feasibility for use in a mobile ionospheric heating (MIH) setup. The overarching goal is to match the ionospheric heating performance of the 180 element array at the high frequency active auroral research program (HAARP), which occupies approximately 1.2 × 10 5 m 2 of land in Gakona, Alaska. While each HAARP crossed dipole element occupies 440 m 2 of land and is tunable in the range of 2.7-10 MHz using automatic matching networks, the presented ESA approach is aimed toward enabling the fabrication of a transportable MIH array platform capable of high continuous wave (cw) power, albeit with a linear dimension five to ten times smaller than that of an equivalent dipole antenna. It is elucidated that the capacitively tuned ESA is continuously tunable to a frequency about 50% lower than that of the ESA's base frequency, albeit the resonant antenna structure carries a fractional bandwidth of merely 1%-2%.
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Publication: J. Vac. Sci. Technol. A 35, 05C315 (2017)Doi: 10.1116/1.5000691
In this paper, the authors studied atmospheric pressure plasma–surface interactions using a well-characterized radio-frequency Ar/O2 plasma jet with polystyrene (PS) polymer films in controlled gas environments as a model system. A number of plasma processing parameters, such as the treatment distance, environmental gas composition, and substrate temperature, were investigated by evaluating both the changes in the thickness and the surface chemical composition of PS after treatment. The authors found that the polymer average etch rate decayed exponentially with the nozzle–surface distance, whereas the surface oxygen composition increased to a maximum and then decreased. Both the exponential decay constant and the oxidation maximum depended on the composition of the gaseous environment which introduced changes in the density of reactive species. The authors previously reported a linear relationship between measured average etch rates and estimated atomic O flux based on measured gas phase atomic O density. In this work, the authors provided additional insights into the kinetics of surface reaction processes. The authors measured the substrate temperature dependence of the PS etch rate and found that the apparent activation energy (Ea) of the PS etching reaction was in the range of 0.10–0.13 eV. Higher values were obtained with a greater nozzle-to-surface distance. This relatively low Ea value suggests that additional energetic plasma species might be involved in the etching reactions, which is also consistent with the different behavior of etching and surface oxidation modification reactions at the polymer surface as the treatment distance is varied.
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Publication: Astron. J. 154, 82 (2017)Doi: 10.3847/1538-3881/aa7d54
To gain information on the abiotic synthesis of the building blocks of life from simple molecules, and their subsequent chemical evolution to biological systems, the starting point is the identification of target species in Titan-like planets; i.e., planets that resemble the primitive Earth, as well as in Earth-like planets in the habitable zone of their star, namely planets where life can be already originated. In this scenario, molecular spectroscopy plays a crucial role because spectroscopic signatures are at the basis of an unequivocal proof for the presence of these target molecules. Thanks to advances in many different techniques and NASA's successful Kepler exoplanet transit mission, thousands of diverse planets outside of our solar system have been discovered. The James Webb Space Telescope (JWST), scheduled to be launched in 2018, will be very helpful in the identification of biosignature gases in Earth-like planets' atmospheres and prebiotic molecule signatures in Titan-like atmospheres, by observing their absorption during transits. Although the search for key-target molecules in exoplanet atmospheres can be carried out by the JWST Transit Spectroscopy in the infrared (IR) region (0.6–29 μm wavelength range), opportunities for their detection in protostellar cores, protoplanetary disks, and on Titan are also offered by interferometric high spectral and spatial resolution observations using the Atacama Large Millimeter/submillimeter Array. In the present work, target molecules have been selected, and their spectroscopic characterization presented in view of supporting their infrared and complementary millimeter/submillimeter-wave spectral observations. In detail, the selected target molecules include: (1) the three-membered oxygen-containing heterocycles, oxirane and protonated oxirane; (2) the cyclopropenyl cation and its methyl derivative; (3) two examples of ortho- and peri-fused tri-cyclic aromatic rings, i.e., the phenalenyl cation (C13H9+) and anion (C13H9-); and (4) uracil, a specific RNA base.
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Publication: Chaos 27, 121103 (2017)Doi: 10.1063/1.5016047
We report a new experimental approach using an optoelectronic feedback loop to investigate the dynamics of oscillators coupled on large complex networks with arbitrary topology. Our implementation is based on a single optoelectronic feedback loop with time delays. We use the space-time interpretation of systems with time delay to create large networks of coupled maps. Others have performed similar experiments using high-pass filters to implement the coupling; this restricts the network topology to the coupling of only a few nearest neighbors. In our experiment, the time delays and coupling are implemented on a field-programmable gate array, allowing the creation of networks with arbitrary coupling topology. This system has many advantages: the network nodes are truly identical, the network is easily reconfigurable, and the network dynamics occur at high speeds. We use this system to study cluster synchronization and chimera states in both small and large networks of different topologies.
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Publication: Geophys. Res. Lett. 44, 3028 (2017)Doi: 10.1002/2017GL072784
Onset of reconnection in a tail-like equilibrium with a finite Bz magnetic field component is studied using 3-D explicit particle-in-cell simulations. Due to a region of a tailward Bz gradient the onset develops spontaneously as the magnetic flux release instability with dominant earthward ion flows. The instability drives the change of magnetic field topology internally, without any external forcing. The distinctive features of this regime are: previously unreported Hall magnetic field patterns; energy conversion near the dipolarization front prior to the X line formation; asymmetry of the energy conversion, plasma heating, and anisotropy relative to the X line, with regions of ion and electron heating out of phase both along and across the tail. These features distinguish the internally driven reconnection regime from similar processes in antiparallel magnetic field configurations as well as interchange and externally driven magnetotail reconnection regimes and can be used to identify the different regimes in upcoming Magnetospheric Multiscale (MMS) mission tail season observations.
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Publication: J. Electromagnetic Waves Applications 31, 1786 (2017)Doi: 10.1080/09205071.2017.1360214
A mobile heater for ionospheric modification studies requires a new megawatt (MW) class radio frequency (RF) source operating with an antenna array 1/20 the area of the High-Frequency Active Auroral Research Program (HAARP). To deliver an effective power density comparable to HAARP, the total source power must be in the range of 16 MW, thus demanding highly efficient sources. While the development of a whole multi-megawatt system for mobile ionospheric heaters is a complex engineering problem, in the present paper we describe only the work of our group on studying main features of a prototype MW-class vacuum electronics RF source for such system. The source design we are currently pursuing assumes class D operation using a modified version of the inductive output tube. The electron beam is a thin annular beam, switched on and off by a mod-anode as opposed to a grid. The beam is then passed through a decelerating gap, and its kinetic energy is extracted using a tunable resonant circuit that presents a constant impedance in the range of 3–10 MHz. With this design the beam is almost completely decelerated at all frequencies, thus achieving high efficiency.
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Publication: APL Photon. 2, 090901 (2017)Doi: 10.1063/1.5000056
The never-ending quest to improve the security of digital information combined with recent improvements in hardware technology has caused the field of random number generation to undergo a fundamental shift from relying solely on pseudo-random algorithms to employing optical entropy sources. Despite these significant advances on the hardware side, commonly used statistical measures and evaluation practices remain ill-suited to understand or quantify the optical entropy that underlies physical random number generation. We review the state of the art in the evaluation of optical random number generation and recommend a new paradigm: quantifying entropy generation and understanding the physical limits of the optical sources of randomness. In order to do this, we advocate for the separation of the physical entropy source from deterministic post-processing in the evaluation of random number generators and for the explicit consideration of the impact of the measurement and digitization process on the rate of entropy production. We present the Cohen-Procaccia estimate of the entropy rate h(ϵ,τ) as one way to do this. In order to provide an illustration of our recommendations, we apply the Cohen-Procaccia estimate as well as the entropy estimates from the new NIST draft standards for physical random number generators to evaluate and compare three common optical entropy sources: single photon time-of-arrival detection, chaotic lasers, and amplified spontaneous emission.
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Publication: J. Chem. Phys. 146, 052801 (2017)Doi: 10.1063/1.4961458
With the increasing interest in establishing directional etching methods capable of atomic scale resolution for fabricating highly scaled electronic devices, the need for development and characterization of atomic layer etching processes, or generally etch processes with atomic layer precision, is growing. In this work, a flux-controlled cyclic plasma process is used for etching of SiO2 and Si at the Angstrom-level. This is based on steady-state Ar plasma, with periodic, precise injection of a fluorocarbon (FC) precursor (C4F8 and CHF3) and synchronized, plasma-based Ar+ ion bombardment [D. Metzler et al., J. Vac. Sci. Technol., A 32, 020603 (2014) and D. Metzler et al., J. Vac. Sci. Technol., A 34, 01B101 (2016)]. For low energy Ar+ ion bombardment conditions, physical sputter rates are minimized, whereas material can be etched when FC reactants are present at the surface. This cyclic approach offers a large parameter space for process optimization. Etch depth per cycle, removal rates, and self-limitation of removal, along with material dependence of these aspects, were examined as a function of FC surface coverage, ion energy, and etch step length using in situ real time ellipsometry. The deposited FC thickness per cycle is found to have a strong impact on etch depth per cycle of SiO2 and Si but is limited with regard to control over material etching selectivity. Ion energy over the 20–30 eV range strongly impacts material selectivity. The choice of precursor can have a significant impact on the surface chemistry and chemically enhanced etching. CHF3 has a lower FC deposition yield for both SiO2 and Si and also exhibits a strong substrate dependence of FC deposition yield, in contrast to C4F8. The thickness of deposited FC layers using CHF3 is found to be greater for Si than for SiO2. X-ray photoelectron spectroscopy was used to study surface chemistry. When thicker FC films of 11 Å are employed, strong changes of FC film chemistry during a cycle are seen whereas the chemical state of the substrate varies much less. On the other hand, for FC film deposition of 5 Å for each cycle, strong substrate surface chemical changes are seen during an etching cycle. The nature of this cyclic etching with periodic deposition of thin FC films differs significantly from conventional etching with steady-state FC layers since surface conditions change strongly throughout each cycle.
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Publication: Chaos 27, 083121 (2017)Doi: 10.1063/1.4986957
In the last decade, it has been shown that a large class of phase oscillator models admit low dimensional descriptions for the macroscopic system dynamics in the limit of an infinite number N of oscillators. The question of whether the macroscopic dynamics of other similar systems also have a low dimensional description in the infinite N limit has, however, remained elusive. In this paper, we show how techniques originally designed to analyze noisy experimental chaotic time series can be used to identify effective low dimensional macroscopic descriptions from simulations with a finite number of elements. We illustrate and verify the effectiveness of our approach by applying it to the dynamics of an ensemble of globally coupled Landau-Stuart oscillators for which we demonstrate low dimensional macroscopic chaotic behavior with an effective 4-dimensional description. By using this description, we show that one can calculate dynamical invariants such as Lyapunov exponents and attractor dimensions. One could also use the reconstruction to generate short-term predictions of the macroscopic dynamics.
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Publication: Nano Lett. 17, 6564 (2017)Doi: 10.1021/acs.nanolett.7b02222
Coupling of an atom-like emitter to surface plasmons provides a path toward significant optical nonlinearity, which is essential in quantum information processing and quantum networks. A large coupling strength requires nanometer-scale positioning accuracy of the emitter near the surface of the plasmonic structure, which is challenging. We demonstrate the coupling of single localized defects in a tungsten diselenide (WSe2) monolayer self-aligned to the surface plasmon mode of a silver nanowire. The silver nanowire induces a strain gradient on the monolayer at the overlapping area, leading to the formation of localized defect emission sites that are intrinsically close to the surface plasmon. We measured an average coupling efficiency with a lower bound of 26% ± 11% from the emitter into the plasmonic mode of the silver nanowire. This technique offers a way to achieve efficient coupling between plasmonic structures and localized defects of two-dimensional semiconductors.
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Publication: Appl. Phys. Lett. 110, 024101 (2017)Doi: 10.1063/1.4973642
We propose and analyze a mechanism to accelerate protons in a low-phase-velocity wakefield. The wakefield is shock-excited by the interaction of two counter-propagating laser pulses in a plasma density gradient. The laser pulses consist of a forward-propagating short pulse (less than a plasma period) and a counter-propagating long pulse. The beating of these pulses generates a slow forward-propagating wakefield that can trap and accelerate protons. The trapping and acceleration is accomplished by appropriately tapering both the plasma density and the amplitude of the backward-propagating pulse. An example is presented in which the trapping and accelerating wakefield has a phase velocity varying from Vph ≈ 0 to ≈ 0.15 c (∼10 MeV proton energy) over a distance of ∼1 cm. The required laser intensities, pulse durations, pulse energies, and plasma densities are relatively modest. Instabilities such as the Raman instability are mitigated because of the large plasma density gradients. Numerical solutions of the wakefield equation and simulations using turboWAVE are carried out to support our model.
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Publication: J. Phys. D - Appl. Phys. 50, 254006 (2017)Doi: 10.1088/1361-6463/aa71f1
The evaluation of a plasma-based atomic layer etching (ALE) approach for native oxide surface removal from Si substrates is described. Objectives include removal of the native oxide while minimizing substrate damage, surface residues and substrate loss. Oxide thicknesses were measured using in situ ellipsometry and surface chemistry was analyzed by x-ray photoelectron spectroscopy. The cyclic ALE approach when used for removal of native oxide SiO2 from a Si substrate did not remove native oxide to the extent required. This is due to the high reactivity of the silicon substrate during the low-energy (<40 eV) ion bombardment phase of the cyclic ALE approach which leads to reoxidation of the silicon surface. A modified process, which used continuously biased Ar plasma with periodic CF4 injection, achieved significant oxygen removal from the Si surface, with some residual carbon and fluorine. A subsequent H2/Ar plasma exposure successfully removed residual carbon and fluorine while passivating the silicon surface. The combined treatment reduced oxygen and carbon levels to about half compared to as received silicon surfaces. The downside of this process sequence is a net loss of about 40 Å of Si. A generic insight of this work is the importance of the substrate and final surface chemistry in addition to precise etch control of the target film for ALE processes. By a fluorocarbon-based ALE technique, thin SiO2 layer removal at the Ångstrom level can be precisely performed from an inert substrate, e.g., a thick SiO2 layer. However, from a reactive substrate, like Si, complete removal of the thin SiO2 layer is prevented by the high reactivity of low energy Ar+ ion bombarded Si. The Si surfaces are reoxidized during the ALE ion bombardment etch step, even for very clean and ultra-low O2 process conditions.
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Publication: J. Plasma Phys. 83, 905830204 (2017)Doi: 10.1017/S0022377817000204
Relativistic electron beam transport through a high-density, magnetized plasma is studied numerically and theoretically. An electron beam injected into a cold plasma excites Weibel and two-stream instabilities that heat the beam and saturate. In the absence of an applied magnetic field, the heated beam continues to propagate. However, when a magnetic field of particular strength is applied along the direction of beam propagation, a secondary instability of off-angle whistler modes is excited. These modes then couple nonlinearly creating a large amplitude parallel-propagating whistler that stops the beam. Here, we will show these phenomena in detail and explain the mechanism of whistler mediated beam stagnation.
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Publication: J. Geophys. Res. - Space Phys. 122, 11523 (2017)Doi: 10.1002/2017JA024555
We use theory and simulations to study how the out-of-plane (guide) magnetic field strength modifies the location where the energy conversion rate between the electric field and the plasma is appreciable during asymmetric magnetic reconnection, motivated by observations (Genestreti et al., 2017). For weak guide fields, energy conversion is maximum on the magnetospheric side of the X line, midway between the X line and electron stagnation point. As the guide field increases, the electron stagnation point gets closer to the X line, and energy conversion occurs closer to the electron stagnation point. We motivate one possible nonrigorous approach to extend the theory of the stagnation point location to include a guide field. The predictions are compared to two-dimensional particle-in-cell (PIC) simulations with vastly different guide fields. The simulations have upstream parameters corresponding to three events observed with Magnetospheric Multiscale (MMS). The predictions agree reasonably well with the simulation results, capturing trends with the guide field. The theory correctly predicts that the X line and stagnation points approach each other as the guide field increases. The results are compared to MMS observations, Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations of each event, and a global resistive-magnetohydrodynamics simulation of the 16 October 2015 event. The PIC simulation results agree well with the global observations and simulation but differ in the strong electric fields and energy conversion rates found in MMS observations. The observational, theoretical, and numerical results suggest that the strong electric fields observed by MMS do not represent a steady global reconnection rate.
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Publication: Nano Lett. 17, 7394 (2017)Doi: 10.1021/acs.nanolett.7b03220
Scalable quantum photonic systems require efficient single photon sources coupled to integrated photonic devices. Solid-state quantum emitters can generate single photons with high efficiency, while silicon photonic circuits can manipulate them in an integrated device structure. Combining these two material platforms could, therefore, significantly increase the complexity of integrated quantum photonic devices. Here, we demonstrate hybrid integration of solid-state quantum emitters to a silicon photonic device. We develop a pick-and-place technique that can position epitaxially grown InAs/InP quantum dots emitting at telecom wavelengths on a silicon photonic chip deterministically with nanoscale precision. We employ an adiabatic tapering approach to transfer the emission from the quantum dots to the waveguide with high efficiency. We also incorporate an on-chip silicon-photonic beamsplitter to perform a Hanbury-Brown and Twiss measurement. Our approach could enable integration of precharacterized III–V quantum photonic devices into large-scale photonic structures to enable complex devices composed of many emitters and photons.
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Publication: ACS Photon. 4, 316 (2017)Doi: 10.1021/acsphotonics.6b00751
One of the major difficulties in the development of optoelectronic THz modulators is finding an active material that allows for large modulation depth. Graphene is a promising candidate because in the terahertz regime it behaves as a Drude metal with conductivity that can be electrostatically tuned through the application of a gate voltage. However, the maximum absorption incurred when a terahertz signal passes through a monolayer of graphene is still only on the order of 10–20%, even for the highest practically achievable carrier concentrations. We demonstrate here a THz modulator that overcomes this fundamental limitation by incorporating a graphene sheet on the surface of a passive silicon dielectric waveguide, in which the evanescent field penetrates the graphene sheet. By applying a gate voltage to the graphene sheet, a modulation depth of up to 50% was achieved. The performance of the modulator is confirmed through electromagnetic simulations, which give further insights into the spatial structure of the guided mode and polarization dependence of the modulation. We show both theoretically and experimentally that the modulation depth can be increased to over 90% by integrating the graphene sheet at the center of the waveguide.
2012
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Publication: J. Royal Soc. Interface 9, 1625 (2012)Doi: 10.1098/rsif.2011.0585
If one gene regulates another, those two genes are likely to be involved in many of the same biological functions. Conversely, shared biological function may be suggestive of the existence and nature of a regulatory interaction. With this in mind, we develop a measure of functional similarity between genes based on annotations made to the Gene Ontology in which the magnitude of their functional relationship is also indicative of a regulatory relationship. In contrast to other measures that have previously been used to quantify the functional similarity between genes, our measure scales the strength of any shared functional annotation by the frequency of that function's appearance across the entire set of annotations. We apply our method to both Escherichia coli and Saccharomyces cerevisiae gene annotations and find that the strength of our scaled similarity measure is more predictive of known regulatory interactions than previously published measures of functional similarity. In addition, we observe that the strength of the scaled similarity measure is correlated with the structural importance of links in the known regulatory network. By contrast, other measures of functional similarity are not indicative of any structural importance in the regulatory network. We therefore conclude that adequately adjusting for the frequency of shared biological functions is important in the construction of a functional similarity measure aimed at elucidating the existence and nature of regulatory interactions. We also compare the performance of the scaled similarity with a high-throughput method for determining regulatory interactions from gene expression data and observe that the ontology-based approach identifies a different subset of regulatory interactions compared with the gene expression approach. We show that combining predictions from the scaled similarity with those from the reconstruction algorithm leads to a significant improvement in the accuracy of the reconstructed network.
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Publication: Optics Lett. 37, 256 (2012)Doi: 10.1364/OL.37.000256
We report that low-loss ridge waveguides are directly written on nanoporous silicon layers by using an argon-ion laser at 514 nm up to 100 mW. Optical characterization of the waveguides indicates light propagation loss lower than 0.5 dB/cm0.5 dB/cm at 1550 nm after oxidation. A Mach–Zehnder interferometer sensor is experimentally demonstrated using the waveguide in its sensing branch, and analytical results indicate that very high sensitivity can be achieved. With large internal surface area, versatile surface chemistry, and adjustable index of refraction of porous silicon, the ridge waveguides can be used to configure Mach–Zehnder interferometers, Young’s interferometers, and other photonic devices for highly sensitive optical biosensors and chemical sensors as well as other applications.
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Publication: Astrophys. J. 757, 20 (2012)Doi: 10.1088/0004-637x/757/1/20
During flares and coronal mass ejections, energetic electrons from coronal sources typically have very long lifetimes compared to the transit times across the systems, suggesting confinement in the source region. Particle-in-cell simulations are carried out to explore the mechanisms of energetic electron transport from the corona to the chromosphere and possible confinement. We set up an initial system of pre-accelerated hot electrons in contact with ambient cold electrons along the local magnetic field and let it evolve over time. Suppression of transport by a nonlinear, highly localized electrostatic electric field (in the form of a double layer) is observed after a short phase of free-streaming by hot electrons. The double layer (DL) emerges at the contact of the two electron populations. It is driven by an ion-electron streaming instability due to the drift of the back-streaming return current electrons interacting with the ions. The DL grows over time and supports a significant drop in temperature and hence reduces heat flux between the two regions that is sustained for the duration of the simulation. This study shows that transport suppression begins when the energetic electrons start to propagate away from a coronal acceleration site. It also implies confinement of energetic electrons with kinetic energies less than the electrostatic energy of the DL for the DL lifetime, which is much longer than the electron transit time through the source region.
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Publication: Phys. Rev. E 86, 056212 (2012)Doi: 10.1103/PhysRevE.86.056212
Recent numerical experiments of Pecora et al. [Phys. Rev. E 83, 065201 (2011)] have investigated tunneling between two-dimensional symmetric double wells separated by a tunneling barrier. The wells were bounded by hard walls and by the potential barrier which was created by a step increase from the zero potential within a well to a uniform barrier potential within the barrier region, which is a situation potentially realizable in the context of quantum dots. Numerical results for the splitting of energy levels between symmetric and antisymmetric eigenstates were calculated. It was found that the splittings vary erratically from state to state, and the statistics of these variations were studied for different well shapes with the fluctuation levels being much less in chaotic wells than in comparable nonchaotic wells. Here we develop a quantitative theory for the statistics of the energy level splittings for chaotic wells. Our theory is based on the random plane wave hypothesis of Berry. While the fluctuation statistics are very different for chaotic and nonchaotic well dynamics, we show that the mean splittings of differently shaped wells, including integrable and chaotic wells, are the same if their well areas and barrier parameters are the same. We also consider the case of tunneling from a single well into a region with outgoing quantum waves.
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Publication: Phys. Rev. E 85, 066131 (2012)Doi: 10.1103/PhysRevE.85.066131
We characterize the distributions of size and duration of avalanches propagating in complex networks. By an avalanche we mean the sequence of events initiated by the externally stimulated excitation of a network node, which may, with some probability, then stimulate subsequent excitations of the nodes to which it is connected, resulting in a cascade of excitations. This type of process is relevant to a wide variety of situations, including neuroscience, cascading failures on electrical power grids, and epidemiology. We find that the statistics of avalanches can be characterized in terms of the largest eigenvalue and corresponding eigenvector of an appropriate adjacency matrix that encodes the structure of the network. By using mean-field analyses, previous studies of avalanches in networks have not considered the effect of network structure on the distribution of size and duration of avalanches. Our results apply to individual networks (rather than network ensembles) and provide expressions for the distributions of size and duration of avalanches starting at particular nodes in the network. These findings might find application in the analysis of branching processes in networks, such as cascading power grid failures and critical brain dynamics. In particular, our results show that some experimental signatures of critical brain dynamics (i.e., power-law distributions of size and duration of neuronal avalanches) are robust to complex underlying network topologies.
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Publication: J. Vac. Sci. Technol. B 30, 061601 (2012)Doi: 10.1116/1.4757287
A process is presented for patterning vias into thick amorphous fluoropolymer layers for a novel In bump fabrication process, achieved using a hot embossing technique. The technique uses a patterned Si stamp that employs a two-step etching process to obtain pillars with a controlled positive sidewall angle. After embossing with the Si stamp, vias are formed in amorphous fluoropolymer layers. A postembossing blanket reactive ion etch step is then used to remove excess fluoropolymer from the bottoms of the vias, exposing a Ni film. Successful electroplating of In bumps into vias initiated at the Ni layer is demonstrated, confirming complete removal of excess fluoropolymer.
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Publication: J. Geophys. Res.-Space Phys. 117, A09201 (2012)Doi: 10.1029/2012JA017877
[1] When analyzing data from an array of spacecraft (such as Cluster or MMS) crossing a site of magnetic reconnection, it is desirable to be able to accurately determine the orientation of the reconnection site. If the reconnection is quasi-two dimensional, there are three key directions, the direction of maximum inhomogeneity (the direction across the reconnection site), the direction of the reconnecting component of the magnetic field, and the direction of rough invariance (the “out of plane” direction). Using simulated spacecraft observations of magnetic reconnection in the geomagnetic tail, we extend our previous tests of the direction-finding method developed by Shi et al. (2005) and the method to determine the structure velocity relative to the spacecraft Vstr. These methods require data from four proximate spacecraft. We add artificial noise and calibration errors to the simulation fields, and then use the perturbed gradient of the magnetic field B and perturbed time derivative dB/dt, as described by Denton et al. (2010). Three new simulations are examined: a weakly three-dimensional, i.e., quasi-two-dimensional, MHD simulation without a guide field, a quasi-two-dimensional MHD simulation with a guide field, and a two-dimensional full dynamics kinetic simulation with inherent noise so that the apparent minimum gradient was not exactly zero, even without added artificial errors. We also examined variations of the spacecraft trajectory for the kinetic simulation. The accuracy of the directions found varied depending on the simulation and spacecraft trajectory, but all the directions could be found within about 10° for all cases. Various aspects of the method were examined, including how to choose averaging intervals and the best intervals for determining the directions and velocity. For the kinetic simulation, we also investigated in detail how the errors in the inferred gradient directions from the unmodified Shi et al. method (using the unperturbed gradient) depended on the amplitude of the calibration errors. For an accuracy of 3° for the maximum gradient direction, the calibration errors could be as large as 3% of reconnection magnetic field, while for the same accuracy for the minimum gradient direction, the calibration errors could only be as large as 0.03% of the reconnection magnetic field. These results suggest that the maximum gradient direction can normally be determined by the unmodified Shi et al. method, while the modified method or some other method must be used to accurately determine the minimum gradient direction. The structure velocity was found with magnitude accurate to 2% and direction accurate to within 5%.
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Publication: Phys. Rev. E 86, 046204 (2012)Doi: 10.1103/PhysRevE.86.046204
The flow of electromagnetic wave energy through a chain of coupled cavities is considered. The cavities are assumed to be of sufficiently irregular shape that their eigenmodes are described by random matrix theory. The cavities are coupled by electrically short single mode transmission lines. Approximate expressions for the power coupled into successive cavities are derived, and the predictions are compared with Monte Carlo simulations. The analytic formulas separate into a product of factors. Consequently, the distribution of power in the last cavity of a very long chain approaches lognormal. For lossless cavities, signatures of Anderson localization, similar to those of the conductances of quantum wires, are observed.
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Publication: EPL 98, 40007 (2012)Doi: 10.1209/0295-5075/98/40007
We study a system of M ≫ 1 nonidentical laser oscillators coupled with time delay through a central laser oscillator in a star-coupling topology. In the large-M limit we show that this system undergoes novel transitional behavior which represents a new generic type of emergent behavior in large systems of coupled laser oscillators. Specifically, we observe a sequence of two dynamical transitions ("a double transition"). A first transition occurs when the effective threshold of the central oscillators is reached leading to an increase of the order parameter to a level far above the noise level. As the control parameter is raised further a second transition occurs leading to a further increase of the order parameter to a level of order M1/2 above the level achieved past the first transition. A scaling analysis and numerical experiments reveal the underlying mechanism of this scenario. We suggest that double transitions, discovered here in the context of a specific system, are expected to occur in diverse situations involving large coupled-oscillator networks.
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Publication: IEEE Electron Device Lett. 33, 23 (2012)Doi: 10.1109/LED.2011.2171031
Nanocrystalline diamond (NCD) thin films are deposited as a heat-spreading capping layer on AlGaN/GaN HEMT devices. Compared to a control sample, the NCD-capped HEMTs exhibited approximately 20% lower device temperature from 0.5 to 9 W/mm dc power device operation. Temperature measurements were performed by Raman thermography and verified by solving the 2-D heat equation within the device structure. NCD-capped HEMTs exhibited: (1) improved carrier density NS, sheet resistance RSH; (2) stable Hall mobility μH and threshold voltage VT ; and (3) degraded on-state resistance RON , contact resistance RC, transconductance Gm, and breakdown voltage VBR .
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Publication: Phys. Rev. Lett. 108, 255005Doi: 10.1103/PhysRevLett.108.255005
Magnetic islands or flux ropes produced by magnetic reconnection have been observed on the magnetopause, in the magnetotail, and in coronal current sheets. Particle-in-cell simulations of magnetic reconnection with a guide field produce elongated electron current layers that spontaneously produce secondary islands. Here, we explore the seed mechanism that gives birth to these islands. The most commonly suggested theory for island formation is the tearing instability. We demonstrate that in our simulations these structures typically start out, not as magnetic islands, but as electron flow vortices within the electron current sheet. When some of these vortices first form, they do not coincide with closed magnetic field lines, as would be the case if they were islands. Only after they have grown larger than the electron skin depth do they couple to the magnetic field and seed the growth of finite-sized islands. The streaming of electrons along the magnetic separatrix produces the flow shear necessary to drive an electron Kelvin-Helmholtz instability and produce the initial vortices. The conditions under which this instability is the dominant mechanism for seeding magnetic islands are explored.
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Publication: IEEE Trans. Electromagnetic Compatibility 54, 758 (2012)Doi: 10.1109/TEMC.2011.2177270
We consider induced voltages on electronic components housed inside complicated enclosures and subjected to high-frequency radiation. The enclosure is assumed to be large compared to the wavelength in which case there is strong dependence of wave properties (eigenvalues, eigenfunctions, scattering, and impedance matrices, etc.) on small perturbations. The source(s) and sink(s) of radiation are treated as generalized ports and their coupling to the enclosure is quantified by an appropriate nonstatistical radiation impedance matrix. The field fluctuations within the enclosure are described in a statistical sense using random matrix theory. The random matrix theory approach implies that the wave fluctuations have “universal” properties in the sense that the statistical description of these properties depends only upon the value of a single, experimentally accessible, dimensionless loss parameter. We formulate a statistical prediction algorithm for the induced voltages at specific points within complicated enclosures when subjected to short-wavelength electromagnetic (EM) energy from either external or internal sources. The algorithm is tested and verified by measurements on a computer box. The insights gained from this model suggest design guidelines for enclosures to make them more resistant to disruptive effects produced by a short-wavelength EM radiation.
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Publication: Physica D-Nonlinear Phenomena 241, 472 (2012)Doi: 10.1016/j.physd.2011.11.002
Groups of animals often tend to arrange themselves in flocks that have characteristic spatial attributes and temporal dynamics. Using a dynamic continuum model for a flock of individuals, we find equilibria of finite spatial extent where the density goes continuously to zero at a well-defined flock edge, and we discuss conditions on the model that allow for such solutions. We also demonstrate conditions under which, as the flock size increases, the interior density in our equilibria tends to an approximately uniform value. Motivated by observations of starling flocks that are relatively thin in a direction transverse to the direction of flight, we investigate the stability of infinite, planar-sheet flock equilibria. We find that long-wavelength perturbations along the sheet are unstable for the class of models that we investigate. This has the conjectured consequence that sheet-like flocks of arbitrarily large transverse extent relative to their thickness do not occur. However, we also show that our model admits approximately sheet-like, ‘pancake-shaped’, three-dimensional ellipsoidal equilibria with definite aspect ratios (transverse length-scale to flock thickness) determined by anisotropic perceptual/response characteristics of the flocking individuals, and we argue that these pancake-like equilibria are stable to the previously mentioned sheet instability.
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Publication: IEEE J. Quantum Electron. 48, 760 (2012)Doi: 10.1109/JQE.2012.2187881
We discuss recent experiments and calculations of the high-intensity optical nonlinearity in gases. Spectral interferometry measurements of the nonlinear optical response of air constituents to laser intensities near the ionization threshold are performed. A calculation of the phase shift caused by a plasma grating created by interference between the pump and probe beams in a transient birefringence measurement suggests that experimental techniques measuring cross phase modulation of a probe pulse by a strong pump pulse are unreliable for studying the optical nonlinearity when the pump and probe pulses are of the same wavelength. An interferometric measurement of the electron density in a filament is also performed. The peak elec-tron density measured is consistent with a model that includes plasma defocusing, but not higher-order Kerr terms. These tech-niques promise to improve the quantitative understanding of nonlinear optics near the ionization threshold and filamentation.
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Publication: Astrophys. J. Lett. 750, L30 (2012)Doi: 10.1088/2041-8205/750/2/L30
Current sheets thinner than the ion inertial length are unstable to the tearing instability and will develop magnetic islands that grow due to magnetic reconnection. We investigate whether the growth of magnetic islands in a current sheet can continue indefinitely, or in the case of the heliosheath, until reaching a neighboring current sheet, and at what rate the islands grow. We investigate the development and growth of magnetic islands using a particle-in-cell code, starting from particle noise. Performing a scaling of the growth of magnetic islands versus the system size, we find that the growth rate is independent of the system size up to the largest simulation we were able to complete. The islands are able to continue growing as long as they merge with each other and maintain a high aspect ratio. Otherwise, there is not enough magnetic tension to sustain reconnection. When applied to the sectored magnetic fields in the heliosheath, we show that the islands can continue growing until they reach the sector width and do so in much less time than it takes for the islands to convect through the heliosheath.
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Publication: IEEE Trans. Plasma Sci. 40, 1575 (2012)Doi: 10.1109/TPS.2012.2188547
A new computationally efficient 2-D model of the beam-wave interaction in coupled-cavity traveling-wave tubes (CC-TWTs) has been developed. The model provides self-consistent time-dependent solutions of Maxwell's equations together with a fully relativistic solution of the electron equations of motion. The model is based on different treatments of the RF fields in the region occupied by an electron beam and in the region of the coupled-cavity structure. The RF fields inside the beam tunnel are represented as a sum of eigenmodes of the local cross section of the beam tunnel. The fields outside the beam tunnel are represented as a superposition of modes of an equivalent circuit with lumped capacitors, inductors, and resistors. The model has been implemented in the TESLA-CC code. The results of the code predictions agree well with measured data for a wideband CC-TWT operating in the Ka-band. The code also shows good agreement with predictions of the 1-D code CHRISTINE-CC in regimes in which a 1-D approximation is applicable. A numerical study of CC-TWT operation shows that, in the small-signal regime, the code is able to predict a gain enhancement due to transverse motion at focusing magnetic fields comparable with Brillouin equilibrium values, which is the major 2-D effect. In the large-signal regime, the code is also capable of treating cases in which the transverse displacement of electrons becomes large and of determining the dependence of the spent beam energy distribution on radial position.
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Publication: PLOS One 7, e31969 (2012)Doi: 10.1371/journal.pone.0031969
Various methods of reconstructing transcriptional regulatory networks infer transcriptional regulatory interactions (TRIs) between strongly coexpressed gene pairs (as determined from microarray experiments measuring mRNA levels). Alternatively, however, the coexpression of two genes might imply that they are coregulated by one or more transcription factors (TFs), and do not necessarily share a direct regulatory interaction. We explore whether and under what circumstances gene pairs with a high degree of coexpression are more likely to indicate TRIs, coregulation or both. Here we use established TRIs in combination with microarray expression data from both Escherichia coli (a prokaryote) and Saccharomyces cerevisiae (a eukaryote) to assess the accuracy of predictions of coregulated gene pairs and TRIs from coexpressed gene pairs. We find that coexpressed gene pairs are more likely to indicate coregulation than TRIs for Saccharomyces cerevisiae, but the incidence of TRIs in highly coexpressed gene pairs is higher for Escherichia coli. The data processing inequality (DPI) has previously been applied for the inference of TRIs. We consider the case where a transcription factor gene is known to regulate two genes (one of which is a transcription factor gene) that are known not to regulate one another. According to the DPI, the non-interacting gene pairs should have the smallest mutual information among all pairs in the triplets. While this is sometimes the case for Escherichia coli, we find that it is almost always not the case for Saccharomyces cerevisiae. This brings into question the usefulness of the DPI sometimes employed to infer TRIs from expression data. Finally, we observe that when a TF gene is known to regulate two other genes, it is rarely the case that one regulatory interaction is positively correlated and the other interaction is negatively correlated. Typically both are either positively or negatively correlated.
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Publication: Phys. Rev. Lett. 109, 065003 (2012)Doi: 10.1103/PhysRevLett.109.065003
We report experimental confirmation of the ionization-grating-induced transient birefringence predicted by Wahlstrand and Milchberg [Opt. Lett. 36, 3822 (2011)] and discuss its impact on the higher-order Kerr effect interpretation by Loriot et al. of pump-probe transient birefringence measurements made at 800 nm [Opt. Express 17, 13429 (2009)]. Measurement of the transient birefringence in air at 400 nm shows a negative contribution to the index of refraction at zero delay for frequencies within the pump bandwidth, the degenerate case, and no negative contribution for frequencies exceeding the pump bandwidth, the nondegenerate case. Our findings suggest that a reevaluation of the higher-order Kerr effect hypothesis of Loriot et al. is necessary.
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Publication: Photonic Network Commun. 23, 285 (2012)Doi: 10.1007/s11107-011-0359-2
This paper proposes and evaluates a four-wave mixing (FWM) aware evolutionary programming algorithm for dynamically setting up lightpaths in an optical wavelength division multiplexed network (WDM network). The proposed algorithm also considers the effect of amplified spontaneous emission noise (ASE noise) on a lightpath during propagation of the optical signal from any source to the intended destination. As crosstalk due to FWM and ASE noise are two transmission impairments that degrade the quality of optical signal even at low to medium data rates, it is mandatory for an algorithm for dynamic routing and wavelength assignment in a WDM network to consider the effect of these two impairments on the lightpath to be established. The distinguishing feature of the proposed algorithm is that it is based on an initial population of a single individual and uses a fitness function that is expressed in terms of the number of hops, path cost, variance contributions due to FWM crosstalk, amplifier noise, and different beat noises at the receiver. The performance of a newly introduced FWM aware priority-based wavelength assignment technique is compared with few of the existing wavelength assignment techniques in the present work.
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Publication: Nature Nanotechnol. 7, 419 (2012)Doi: 10.1038/nnano.2012.113
One-dimensional strings of metal nanocubes can be precisely self-assembled with the help of polymer chemistry.
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Publication: J. Vac. Sci. Technol. B 30, 041811 (2012)Doi: 10.1116/1.4736979
Metallic masking materials are promising candidates for plasma-based pattern transfer into low-k materials for fabricating integrated circuits. Improving etching selectivity (ES) between the low-k and hardmask material requires a fundamental understanding of material erosion in fluorocarbon (FC) plasmas. The authors have previously reported on the erosion mechanism and plasma parametric dependencies of Ti etch in FC discharges. The present work focuses on elucidating differences in the erosion behavior between Ti and TiN hardmasks. The authors studied erosion of Ti, TiN, and organosilicate glass (OSG), a reference low-k material, in CF4/Ar and C4F8/Ar plasmas. Changes in surface composition, FC surface reaction layer thicknesses, erosion rates, and corresponding ES were established by x-ray photoelectron spectroscopy and in situ ellipsometry. The authors found that the erosion stages and plasma parameter dependent surface compositions were similar for Ti and TiN. The previously established dependence of Ti erosion rates on FC layer thickness and energy deposition on the hardmask surface by ions generally holds for TiN as well. However, TiN etch rates (volumetric removal rates) and etch yields (atomic removal rates) were increased by a factor of 1–1.4 compared to Ti. This difference can be explained by the rapid removal of N from the TiN surface, increasing the Ti atom number density at the surface above values of the Ti hardmask. The resulting increase in surface reactivity is in good agreement with the enhanced erosion rates compared to Ti. Differences in erosion rates have a direct impact on the ES and the highest ES relative to OSG (up to 15) were achieved for Ti hardmasks in CF4/Ar plasmas with low ion energy.
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Publication: Optics Express 20, 16968 (2012)Doi: 10.1364/OE.20.016968
The angular dependence of terahertz (THz) emission from birefringent crystals can differ significantly from that of cubic crystals. Here we consider optical rectification in uniaxial birefringent materials, such as chalcopyrite crystals. The analysis is verified in (110)-cut ZnGeP2 and compared to (zincblende) GaP. Although the crystals share the same nonzero second-order tensor elements, the birefringence in chalcopyrite crystals cause the pump pulse polarization to evolve as it propagates through the crystal, resulting in a drastically different angular dependence in chalcopyrite crystals. The analysis is extended to {012}- and {114}-cut chalcopyrite crystals and predicts more efficient conversion for the {114} crystal cut over the {012}- and {110}-cuts.
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Publication: J. Nanotechnol. 2012, 870378 (2012)Doi: 10.1155/2012/870378
Small clusters of nanoparticles are ideal substrates for SERS measurements, but the SERS signal enhancement by a particular cluster is strongly dependent on its structural characteristics and the measurement conditions. Two methods for high-throughput assembly of silver nanocubes into small clusters at predetermined locations on a substrate are presented. These fabrication techniques make it possible to study both the structure and the plasmonic properties of hundreds of nanoparticle clusters. The variations in SERS enhancement factors from cluster to cluster were analyzed and correlated with cluster size and configuration, and laser frequency and polarization. Using Raman instruments with 633 nm and 785 nm lasers and linear clusters of nanocubes, an increase in the reproducibility of the enhancement and an increase in the average enhancement values were achieved by increasing the number of nanocubes in the cluster, up to 4 nanocubes per cluster. By examining the effect of cluster configuration, it is shown that linear clusters with nanocubes attached in a face-to-face configuration are not as effective SERS substrates as linear clusters in which nanocubes are attached along an edge.
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Publication: Nanotechnol. 23, 065301 (2012)Doi: 10.1088/0957-4484/23/6/065301
Processes that combine nanoparticle suspensions with micromechanical or microelectronics platforms can reveal new phenomena unique to nanoscale objects. We report that silver nanoparticles react with silicon wafers that have been patterned by reactive ion etching (RIE) in SF6/O2 plasma. This reaction results in the localized deposition of silver on the patterns. Through the modification of the reaction conditions, the reaction mechanism was explored. Redeposition of the sputtered RIE products is suggested as the key to this transformation. The new silver deposition process was utilized to localize the growth of gold nanoparticles and silicon nanowires on the vertical sidewalls of patterns in silicon, demonstrating a simple route to the fabrication of overhanging nanoscale objects.
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Publication: J. Vac. Sci. Technol. B 30, 031207 (2012)Doi: 10.1116/1.3696731
To meet future demands of free electron lasers, reliable and long lasting photocathodes are needed. With current photocathodes lacking a sufficient combination of both lifetime and high quantum efficiency, research continues to explore ways to formulate improved photocathodes. In this work, we investigate the photoemissive and thermal stability properties of CsAu on a porous tungsten substrate. Quantum efficiency (QE) and robustness characterization of evaporatively deposited metal cathodes, such as gold, opens possibilities for future long-lived high QE dispenser photocathodes on both conductive and insulating diffusion barriers.
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Publication: Phys. Rev. A 85, 043820 (2012)Doi: 10.1103/PhysRevA.85.043820
The absolute time-dependent nonlinear response of O2, N2, N2O, and Ar to intense nonionizing, ultrashort optical pump pulses is measured with single-shot spectral interferometry. The instantaneous and delayed rotational responses are distinguished as a function of pump-pulse duration and probe central wavelength. Our measurements are central to the modeling and understanding of nonlinear propagation of intense ultrashort laser pulses in gases.
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Publication: Phys. Rev. E 85, 015202 (2012)Doi: 10.1103/PhysRevE.85.015202
Fading is the time-dependent variation in transmitted signal strength through a complex medium due to interference or temporally evolving multipath scattering. In this paper we use random matrix theory (RMT) to establish a first-principles model for fading, including both universal and nonuniversal effects. This model provides a more general understanding of the most common statistical models (Rayleigh fading and Rice fading) and provides a detailed physical basis for their parameters. We also report experimental tests on two ray-chaotic microwave cavities. The results show that our RMT model agrees with the Rayleigh and Rice models in the high-loss regime, but there are strong deviations in low-loss systems where the RMT approach describes the data well.
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Publication: Appl. Phys. Lett. 100, 242106 (2012)Doi: 10.1063/1.4729381
The size-dependence of the thermoelectric power factor of thin-films and nanowires is theoretically investigated from the electric quantum limit (EQL) to the bulk-like regime. Different functional forms of the energy-dependent relaxation time τ(E) are incorporated in the model to account for carrier scattering mechanisms typical in semiconductor nanostructures. The calculations show that the steeper the increase in the relaxation time with carrier energy, the higher the power factor-to-average scattering time ratio, PF / <τ>, confirming the benefits of the preferential scattering of low-energy carriers to thermoelectric performance. However, outside the EQL, the power factor values are lower in the low-dimensional structures than in their three-dimensional counterparts. Thus, the power factor is more readily improved by modifications of the scattering rates than by quantization of the energy states.
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Publication: IEEE Trans. Plasma Sci. 40, 3420 (2012)Doi: 10.1109/TPS.2012.2223236
A circular waveguide loaded with periodic dielectrics is considered. Two theoretical methodologies have been developed to investigate the wave dispersion characteristics for such structure. The feasibility and accuracy of the theoretical approaches have been verified by high frequency structural simulator (HFSS) simulations. Using the theoretical approaches, effects of the material properties and structure dimensions on the dispersion characteristics, power distribution, coupling impedance, and wave attenuation have been analyzed, and results obtained for different structures have been compared. Examples of W-band structures with periodic dielectric loading that can be used in various microwave devices have been given. The presented methodologies and results can be used for design optimization of microwave devices with dielectric loading.
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Publication: Phys. Rev. E 86, 026304 (2012)Doi: 10.1103/PhysRevE.86.026304
Spherical Couette flow (flow between concentric rotating spheres) is one of flows under consideration for the laboratory magnetic dynamos. Recent experiments have shown that such flows may excite Coriolis restored inertial modes. The present work aims to better understand the properties of the observed modes and the nature of their excitation. Using numerical solutions describing forced inertial modes of a uniformly rotating fluid inside a spherical shell, we first identify the observed oscillations of the Couette flow with nonaxisymmetric, retrograde, equatorially antisymmetric inertial modes, confirming first attempts using a full sphere model. Although the model has no differential rotation, identification is possible because a large fraction of the fluid in a spherical Couette flow rotates rigidly. From the observed sequence of the excited modes appearing when the inner sphere is slowed down by step, we identify a critical Rossby number associated with a given mode, below which it is excited. The matching between this critical number and the one derived from the phase velocity of the numerically computed modes shows that these modes are excited by an instability likely driven by the critical layer that develops in the shear layer, staying along the tangent cylinder of the inner sphere.
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Publication: Tellus Series A-Dynamic Meteorology and Oceanography 64, 18407 (2012)Doi: 10.3402/tellusav64i0.18407
We propose a data assimilation scheme that simultaneously produces the analyses for a global model and an embedded limited-area model, considering forecast information from both models. The purpose of the proposed approach is twofold. First, we expect that the global analysis will benefit from incorporation of information from the higher resolution limited-area model. Second, our method is expected to produce a limited-area analysis that is more strongly constrained by the large-scale flow than a conventional limited-area analysis. The proposed scheme minimises a cost function in which the control variable is the joint state of the global and the limited-area models. In addition, the cost function includes a constraint term that penalises large differences between the global and the limited-area state estimates. The proposed approach is tested by idealised experiments, using ‘toy’ models introduced by Lorenz in 2005. The results of these experiments suggest that the proposed approach improves the global analysis within and near the limited-area domain and the regional analysis near the lateral boundaries. These analysis improvements lead to forecast improvements in both the global and the limited-area models.
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Publication: Optics Lett. 37, 4693 (2012)Doi: 10.1364/OL.37.004693
We describe a technique for accurately measuring the ratio between the imaginary and real parts of the third-order nonlinearity in optical waveguides. Unlike most other methods, it does not depend on precise knowledge of the coupling efficiencies, optical propagation loss, or optical pulse shape. We apply the method to characterize a silicon waveguide, a GaAs waveguide, and AlGaAs waveguides with different alloy concentrations.
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Publication: Rev. Sci. Instrum. 83, 085101 (2012)Doi: 10.1063/1.4739837
High-Reynolds-number flows are common both in nature and industrial applications, but are difficult to attain in laboratory settings using standard test fluids such as air and water. To extend the Reynolds number range, water and air have been replaced at times by low-viscosity fluids such as pressurized air, sulfur hexafluoride, and cryogenic nitrogen gas, as well as liquid and gaseous helium. With a few exceptions, liquid nitrogen has been neglected despite the fact that it has a kinematic viscosity of about a fifth of that of water at room temperature. We explore the use of liquid nitrogen here. In particular, we study the use of frozen particles for flow visualization and velocimetry in liquid nitrogen. We create particles in situ by injecting a gaseous mixture of room-temperature nitrogen and an additional seeding gas into the flow. We present a systematic study of potential seeding gases to determine which create particles with the best fidelity and optical properties. The technique has proven capable of producing sub-micrometer sized tracers that allow particle tracking and particle image velocimetry. We review possible high-Reynolds-number experiments using this technique, and discuss the merits and challenges of using liquid nitrogen as a test fluid.
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Publication: Chaos 22, 033129 (2012)Doi: 10.1063/1.4745854
We consider an alternate definition of community structure that is functionally motivated. We define network community structure based on the function the network system is intended to perform. In particular, as a specific example of this approach, we consider communities whose function is enhanced by the ability to synchronize and/or by resilience to node failures. Previous work has shown that, in many cases, the largest eigenvalue of the network’s adjacency matrix controls the onset of both synchronization and percolation processes. Thus, for networks whose functional performance is dependent on these processes, we propose a method that divides a given network into communities based on maximizing a function of the largest eigenvalues of the adjacency matrices of the resulting communities. We also explore the differences between the partitions obtained by our method and the modularity approach (which is based solely on consideration of network structure). We do this for several different classes of networks. We find that, in many cases, modularity-based partitions do almost as well as our function-based method in finding functional communities, even though modularity does not specifically incorporate consideration of function.
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Publication: Nature Phys. 8, 658 (2012)Doi: 10.1038/NPHYS2372
Networks of nonlocally coupled phase oscillators1 can support chimera states in which identical oscillators evolve into distinct groups that exhibit coexisting synchronous and incoherent behaviours despite homogeneous coupling2,3,4,5,6. Similar nonlocal coupling topologies implemented in networks of chaotic iterated maps also yield dynamical states exhibiting coexisting spatial domains of coherence and incoherence7,8. In these discrete-time systems, the phase is not a continuous variable, so these states are generalized chimeras with respect to a broader notion of incoherence. Chimeras continue to be the subject of intense theoretical investigation, but have yet to be realized experimentally6,9,10,11,12,13,14,15,16. Here we show that these chimeras can be realized in experiments using a liquid-crystal spatial light modulator to achieve optical nonlinearity in a spatially extended iterated map system. We study the coherence–incoherence transition that gives rise to these chimera states through experiment, theory and simulation.
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Publication: IEEE J. Quantum Electron. 48, 797 (2012)Doi: 10.1109/JQE.2012.2190586
We review high-energy, broadband terahertz (THz) generation in two-color laser-produced gaseous plasma. We first describe our microscopic plasma current model for directional plasma current and far-field THz radiation generation. Experimental results for THz yield dependence on laser energy, optical phase difference, gas species, and gas pressure are presented. We also describe ultrabroadband THz generation and detection in our experiments and numerical simulations. Finally, we discuss 2-D plasma currents for THz polarization control and macroscopic phase-matched THz generation.
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Publication: Phys. Rev. Lett. 109, 085701 (2012)Doi: 10.1103/PhysRevLett.109.085701
Boolean networks, widely used to model gene regulation, exhibit a phase transition between regimes in which small perturbations either die out or grow exponentially. We show and numerically verify that this phase transition in the dynamics can be mapped onto a static percolation problem which predicts the long-time average Hamming distance between perturbed and unperturbed orbits.
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Publication: J. Solar Energy Engineering-Trans. ASME 134, 011002 (2012)Doi: 10.1115/1.4005088
This study describes the thermal modeling of a novel algal biofilm photobioreactor aimed at cultivating algae for biofuel production. The thermal model is developed to assess the photobioreactor’s thermal profile and evaporative water loss rate for a range of environmental parameters, including ambient air temperature, solar irradiation, relative humidity, and wind speed. First, a week-long simulation of the system has been performed using environmental data for Memphis, TN, on a typical week during the spring, summer, fall, and winter. Then, a sensitivity analysis was performed to assess the effect of each weather parameter on the temperature and evaporative loss rate of the photobioreactor. The range of the daily algae temperature variation was observed to be 12.2 °C, 13.2 °C, 11.7 °C, and 8.2 °C in the spring, summer, fall, and winter, respectively. Furthermore, without active cooling, the characteristic evaporative water loss from the system is approximately 6.0 L/m2 day, 7.3 L/m2 day, 3.4 L/m2 day, and 1.0 L/m2 day in the spring, summer, fall, and winter, respectively.
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Publication: Space Sci. Rev. 172, 227 (2012)Doi: 10.1007/s11214-012-9903-3
The heating and acceleration of ions during magnetic reconnection relevant to coronal heating and flares is explored via particle-in-cell (PIC) simulations and analytic modeling. We show that the dominant heating mechanism of sub-Alvénic ions during reconnection with a guide field, the case of greatest relevance to the corona, results from pickup behavior during the entry into reconnection exhausts, which produces effective thermal speeds of the order of the Alfvén velocity based on the reconnecting magnetic field. There is a mass-to-charge (M/Q) threshold for pickup behavior that favors the heating of high-M/Q ions. Ions below the threshold gain little energy beyond that associated with convective flow. PIC simulations with protons and alphas confirm the pickup threshold. The enhanced heating of high M/Q ions is consistent with observations of abundance enhancements of such ions in impulsive flares. In contrast to anti-parallel reconnection, the temperature increment during ion pickup is dominantly transverse, rather than parallel, to the local magnetic field. The simulations reveal the dominance of perpendicular heating, which is also consistent with observations.
We suggest that the acceleration of ions to energies well above that associated with the Alfvén speed takes place during the interaction with many magnetic islands, which spontaneously develop during 3-D guide-field reconnection. The exploration of particle acceleration in a full 3-D multi-island system remains computationally intractable. Instead we explore ion acceleration in a multi-current layer system with low initial β. Ion energy gain takes place due to Fermi reflection in contracting and merging magnetic islands. Particle acceleration continues until the available magnetic free-energy is significantly depleted so that the pressure of energetic ions approaches that of the reconnecting field. Depending on the strength of the ambient guide field and in spite of the low initial plasma β, the dominance of parallel heating can cause significant regions of the plasma to exceed the marginal firehose condition.
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Publication: Chaos 22, 013102 (2012)Doi: 10.1063/1.3672513
We investigate the dynamics of systems of many coupled phase oscillators with heterogeneous frequencies. We suppose that the oscillators occur in M groups. Each oscillator is connected to other oscillators in its group with “attractive” coupling, such that the coupling promotes synchronization within the group. The coupling between oscillators in different groups is “repulsive,” i.e., their oscillation phases repel. To address this problem, we reduce the governing equations to a lower-dimensional form via the ansatz of Ott and Antonsen, Chaos 18, 037113 (2008). We first consider the symmetric case where all group parameters are the same, and the attractive and repulsive coupling are also the same for each of the M groups. We find a manifold L of neutrally stable equilibria, and we show that all other equilibria are unstable. For M ≥ 3, L has dimension M − 2, and for M = 2, it has dimension 1. To address the general asymmetric case, we then introduce small deviations from symmetry in the group and coupling parameters. Doing a slow/fast timescale analysis, we obtain slow time evolution equations for the motion of the M groups on the manifold L. We use these equations to study the dynamics of the groups and compare the results with numerical simulations.
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Publication: IEEE Control Systems Magazine 32, 26 (2012)Doi: 10.1109/MCS.2011.2181584
This article is on microscale flow control, on dynamically shaping flow fields in microfluidic devices to precisely manipulate cells, quantum dots (QDs), and nanowires (Figure 1). Compared to prior methods (Table 1), manipulating microscopic and nanoscopic objects by flow control can be achieved with simpler and easy-to-fabricate devices, can steer a wider variety of objects, and enables entirely new capabilities such as placement and immobilization of specific quantum dots to desired on-chip locations with nanoscale precision. A companion article [267] investigates flow control in the body and develops methods to shape magnetic fi elds to direct ferrofluids of therapeutic magnetic nanoparticles to disease locations in patients.
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Publication: Phys. Rev. B 85, 115319 (2012)Doi: 10.1103/PhysRevB.85.115319
We present a feasible scheme for performing an optically controlled phase gate between two conduction electron spin qubits in adjacent self-assembled quantum dots. Interaction between the dots is mediated by the tunneling of the valence hole state, which is activated only by applying a laser pulse of the right polarization and frequency. Combining the hole tunneling with the Pauli blocking effect, we obtain conditional dynamics for the two quantum dots, which is the essence of our gating operations. Our results are of explicit relevance to the recent generation of vertically stacked self-assembled InAs quantum dots, and show that by a design which avoids unintended dynamics the gate could be implemented in theory in the 10-ps range and with a fidelity over 90%. Our proposal therefore offers an accessible path to the demonstration of ultrafast quantum logic in quantum dots.
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Publication: Feedback Controls of MEMS to Atoms, Book Chapter, Page 269 (2012)Doi: 10.1007/978-1-4419-5832-7_9
This chapter gives an overview of methods we have developed and experimental results we have achieved for precision feedback control of flows and objects inside microfluidic systems. Essentially, we are doing flow control, but flow control on the microscale, and further even to nanoscale accuracy, to precisely and robustly manipulate liquid packets, particles (e.g., cells and quantum dots), and micro- and nanoobjects (e.g., nanowires). Target applications include methods to miniaturize the operations of a biological laboratory (lab-on-a-chip), e.g., presenting pathogens to on-chip sensing cells or extracting cells from messy biosamples such as saliva, urine, or blood; as well as nonbiological applications such as deterministically placing quantum dots on photonic crystals to make multidot quantum information systems.
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Publication: IEEE Trans. Electromagnetic Compatibility 54, 1017 (2012)Doi: 10.1109/TEMC.2012.2188720
The potential of high-power microwave radiation to couple into and generate malfunction in microelectronic systems has become a serious concern; however, the underlying electronic mechanisms are not well understood. We present results of experiments on the response of a typical CMOS integrated circuit to pulsed microwave excitation. Our results show that electrostatic discharge protection devices detect the pulse envelope of the microwave carrier via its nonlinear conductance. The device characteristics are analyzed using device physics to describe the quasi-static and non-quasi-static behavior of the protection circuits and define the regime of operation in terms of excitation frequency. The results of experiments and analysis lead to the development of an improved effects model based on Berkeley Short-channel IGFET Model using a body resistor network. Good agreement between experiments and transient and harmonic-balance simulations is demonstrated. The results show that a deterministic method of evaluating electromagnetic effects using physical, scalable device parameters is feasible.
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Publication: Phys. Rev. ST - Accelerators and Beams 15, 072803 (2012)Doi: 10.1103/PhysRevSTAB.15.072803
Beam halo is an important factor in any high intensity accelerator. It can cause difficulties in the control of the beam, emittance growth, particle loss, and even damage to the accelerator. It is therefore essential to understand the mechanisms of halo formation and its dynamics. Experimental measurement of the halo distribution is a fundamental tool for such studies. In this paper, we present a new high dynamic range, adaptive masking method to image beam halo, which uses a digital micromirror-array device. This method has been thoroughly tested in the laboratory using standard optical techniques, and with an actual beam produced by the University of Maryland Electron Ring (UMER). A high dynamic range (DR∼105) has been demonstrated with this new method at UMER and recent studies, with more intense beams, indicate that this DR can be exceeded by more than an order of magnitude. The method is flexible, easy to implement, low cost, and can be used at any accelerator or light source. We present the results of our measurements of the performance of the method and illustrative images of beam halos produced under various experimental conditions.
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Publication: Geophys. Res. Lett. 39, L08103 (2012)Doi: 10.1029/2012GL051372
Magnetic reconnection is an important process that occurs at the magnetopause boundary of Earth's magnetosphere because it leads to transport of solar wind energy into the system, driving magnetospheric dynamics. However, the nature of magnetopause reconnection in the case of Saturn's magnetosphere is unclear. Based on a combination of Cassini spacecraft observations and simulations we propose that plasma β conditions adjacent to Saturn's magnetopause largely restrict reconnection to regions of the boundary where the adjacent magnetic fields are close to anti-parallel, severely limiting the fraction of the magnetopause surface that can become open. Under relatively low magnetosheath β conditions we suggest that this restriction becomes less severe. Our results imply that the nature of solar wind-magnetosphere coupling via reconnection can vary between planets, and we should not assume that the nature of this coupling is always Earth-like. Studies of reconnection signatures at Saturn's magnetopause will test this hypothesis.
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Publication: J. Geophys. Res.-Space Phys. 117, A02206 (2012)Doi: 10.1029/2012JA017541
[1] In the paper “Onset of collisionless magnetic reconnection in two-dimensional current sheets and formation of dipolarization fronts” by M. I. Sitnov and M. Swisdak (Journal of Geophysical Research, 116, A12216, doi:10.1029/2011JA016920, 2011) the description of the boundary condition for the z- component of the magnetic field at top and bottom boundaries of the simulation box (“Bz = Bz(t = 0)” in paragraph [16]) was incorrect. In simulations described in the paper the z-component of the magnetic field is initially equal to its equilibrium value and it further changes following Faraday's law.
[2] The authors discovered this editorial issue during the enlightening discussions with Eugene Parker, for which they are most thankful.
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Publication: Phys. Plasmas 19, 022110 (2012)Doi: 10.1063/1.3685755
The structure of shocks that form at the exhaust boundaries during collisionless reconnection of anti-parallel fields is studied using particle-in-cell (PIC) simulations and modeling based on the anisotropic magnetohydrodynamic equations. Large-scale PIC simulations of reconnection and companion Riemann simulations of shock development demonstrate that the pressure anisotropy produced by counterstreaming ions within the exhaust prevents the development of classical Petschek switch-off-slow shocks (SSS). The shock structure that does develop is controlled by the firehose stability parameter ɛ = 1-µ0(P‖-P⊥)/B2 through its influence on the speed order of the intermediate and slow waves. Here, P‖ and P⊥ are the pressure parallel and perpendicular to the local magnetic field. The exhaust boundary is made up of a series of two shocks and a rotational wave. The first shock takes ɛ from unity upstream to a plateau of 0.25 downstream. The condition ɛ = 0.25 is special because at this value, the speeds of nonlinear slow and intermediate waves are degenerate. The second slow shock leaves ɛ = 0.25 unchanged but further reduces the amplitude of the reconnecting magnetic field. Finally, in the core of the exhaust, ɛ drops further and the transition is completed by a rotation of the reconnecting field into the out-of-plane direction. The acceleration of the exhaust takes place across the two slow shocks but not during the final rotation. The result is that the outflow speed falls below that expected from the Walén condition based on the asymptotic magnetic field. A simple analytic expression is given for the critical value of ɛɛ within the exhaust below which SSSs no longer bound the reconnection outflow.
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Publication: Appl. Phys. Lett. 100, 231107 (2012)Doi: 10.1063/1.4719065
We demonstrate a method for tuning a semiconductor quantum dot (QD) onto resonance with a cavity mode all-optically using a system comprised of two evanescently coupled cavities containing a single QD. One resonance of the coupled cavity system is utilized to generate a cavity enhanced optical Stark shift, enabling the QD to be resonantly tuned to the other cavity mode. A twenty-seven fold increase in photon emission from the QD is measured when the off-resonant QD is Stark shifted onto the cavity mode resonance, which is attributed to radiative enhancement of the QD. A maximum tuning of 0.06 nm is achieved for the QD at an incident power of 88 μW.
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Publication: Phys. Rev. Lett. 108, 227402 (2012)Doi: 10.1103/PhysRevLett.108.227402
We demonstrate fast nonlinear optical switching between two laser pulses with as few as 140 photons of pulse energy by utilizing strong coupling between a single quantum dot (QD) and a photonic crystal cavity. The cavity-QD coupling is modified by a detuned pump pulse, resulting in a modulation of the scattered and transmitted amplitude of a time synchronized probe pulse that is resonant with the QD. The temporal switching response is measured to be as fast as 120 ps, demonstrating the ability to perform optical switching on picosecond timescales.
2022
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Publication: Appl. Phys. Lett. 121, 081104 (2022)Doi: 10.1063/5.0098003
We demonstrate a low power thermally induced optical bistability at telecom wavelengths and room temperature using a nanobeam photonic crystal cavity embedded with an ensemble of quantum dots. The nanobeam photonic crystal cavity is transfer-printed onto the edge of a carrier chip for thermal isolation of the cavity with an efficient optical coupling between the nanobeam waveguide and optical setup. Reflectivity measurements performed with a tunable laser reveal the thermo-optic nature of the nonlinearity. A bistability power threshold as low as 23 μW and an on/off response contrast of 6.02 dB are achieved from a cavity with a moderately low quality factor of 2830. Our device provides optical bistability at power levels an order of magnitude lower than previous quantum-dot-based devices.
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Publication: Biosensors & Bioelectron. 197, 113803 (2022)Doi: 10.1016/j.bios.2021.113803
We report the rapid detection of SARS-CoV-2 in infected patients (mid-turbinate swabs and exhaled breath aerosol samples) in concentrations as low as 60 copies/mL of the virus in seconds by electrical transduction of the SARS-CoV-2 S1 spike protein antigen via SARS-CoV-2 S1 spike protein antibodies immobilized on bilayer quasi-freestanding epitaxial graphene without gate or signal amplification. The sensor demonstrates the spike protein antigen detection in a concentration as low as 1 ag/mL. The heterostructure of the SARS-CoV-2 antibody/graphene-based sensor is developed through a simple and low-cost fabrication technique. Furthermore, sensors integrated into a portable testing unit distinguished B.1.1.7 variant positive samples from infected patients (mid-turbinate swabs and saliva samples, 4000–8000 copies/mL) with a response time of as fast as 0.6 s. The sensor is reusable, allowing for reimmobilization of the crosslinker and antibodies on the biosensor after desorption of biomarkers by NaCl solution or heat treatment above 40 °C.
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Publication: Phys. Rev. A 106, 013709 (2022)Doi: 10.1103/PhysRevA.106.013709
We propose a quantum Fourier transform on photons in which a single atom-coupled cavity system mediates the photon-photon interactions. Our protocol utilizes time-delay feedback of photons and requires no active feedforward control. The time-delay feedback enables a single atom-cavity system to implement a quantum Fourier transform on an arbitrary number of photonic qubits on-the-fly, while rapid tuning of the atomic transition implements arbitrary controlled-phase gates. We analyze the performance of the protocol numerically and show that it can implement quantum Fourier transforms with tens of photons using state-of-the-art cavity quantum electrodynamics.
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Publication: Phys. Rev. Appl. 17, 014009 (2022)Doi: 10.1103/PhysRevApplied.17.014009
The nitrogen-vacancy (N-V) defect in diamond is a versatile quantum sensor, being able to measure physical quantities such as magnetic field, electric field, temperature, and pressure. In the present work, we demonstrate multiplexed sensing of magnetic field and temperature using a N-V ensemble in diamond. The dual-frequency-driving technique we employ is based on frequency-division multiplexing, which enables the sensing of both measurables in real time. The pair of N-V resonance frequencies for dual-frequency driving must be selected to avoid coherent population trapping of N-V spin states. With enhanced optical collection efficiency higher than 50% and a type 1b diamond crystal with a natural abundance of 13C spins, we achieve sensitivities of about 70 pT/√Hz and 25μK/√Hz simultaneously. We demonstrate a high isolation factor of 34 dB in the N-V thermometry signal against the magnetic field; and we provide a theoretical description for the isolation factor. This work paves the way for extending the application of N-V diamond sensors into more demanding conditions.
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Publication: Opt. Express 30, 45508 (2022)Doi: 10.1364/OE.475896
We analyze the effects of atmospheric turbulence on the mode power spectrum of beams carrying orbital angular momentum represented by Laguerre-Gauss (LG) modes. For an input (p,m) LG mode, i.e. pump, we calculate the power transferred to other modes (p',m') due to turbulence. Our analysis is validated against split-step beam propagation simulations and shows agreement into the strong turbulence regime. These results have applications for the design and characterization of free-space laser communication systems.
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Publication: Phys. Rev. Appl. 17, 024041 (2022)Doi: 10.1103/PhysRevApplied.17.024041
Crystal-strain variation imposes significant limitations on many quantum sensing and information applications for solid-state defect qubits in diamond. Thus, the precision measurement and control of diamond crystal strain is a key challenge. Here, we report diamond strain measurements with a unique set of capabilities, including micron-scale spatial resolution, a millimeter-scale field of view, and a 2-order-of-magnitude improvement in volume-normalized sensitivity over previous work, reaching 5(2)×10−8/√Hz μm−3 (with spin-strain coupling coefficients representing the dominant systematic uncertainty). We use strain-sensitive spin-state interferometry on ensembles of nitrogen-vacancy (N-V) color centers in single-crystal bulk diamond with low strain gradients. This quantum interferometry technique provides insensitivity to magnetic-field inhomogeneity from the electronic and nuclear spin bath, thereby enabling long N-V–ensemble electronic spin dephasing times and enhanced strain sensitivity, as well as broadening the potential applications of the technique beyond isotopically enriched or high-purity diamond. We demonstrate the strain-sensitive measurement protocol first on a confocal scanning laser microscope, providing quantitative measurement of sensitivity as well as three-dimensional strain mapping; and second on a wide-field-imaging quantum diamond microscope. Our strain-microscopy technique enables fast, sensitive characterization for diamond material engineering and nanofabrication; as well as diamond-based sensing of strains applied externally, as in diamond anvil cells or embedded diamond stress sensors, or internally, as by crystal damage due to particle-induced nuclear recoils.
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Publication: Front. Phys. 9, 752943 (2022)Doi: 10.3389/fphy.2021.752943
An optical fiber was modified at the tip with a self-assembled plasmonic metamaterial that acts as a miniature surface-enhanced Raman spectroscopy (SERS) substrate. This optical fiber-based device co-localizes the laser probe signal and the chemical analyte at a distance remote from the spectrometer, and returns the scattered light signal to the spectrometer for analysis. Remote SERS chemical detection is possible in liquids and in dried samples. Under laboratory conditions, the analyte SERS signal can be separated from the background signal of the fiber itself and the solvent. An enhancement factor greater than 35,000 is achieved with a monolayer of the SERS marker 4-aminothiophenol.
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Publication: Nucl. Fusion 62, 042012 (2022)Doi: 10.1088/1741-4326/ac29d0
Improvements to the stellarator concept can be realized through advancements in theoretical and computational plasma physics. Herein, recent advances are reported in the topical areas of: (1) improved energetic ion confinement, (2) the impact of three-dimensional (3D) shaping on turbulent transport, (3) reducing coil complexity, (4) novel optimization and design methods, and (5) computational magnetohydrodynamic tools. These advances enable the development of new stellarator configurations with improved confinement properties.
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Publication: ACS Appl. Mater. Intefaces 2023, 15, 1010 (2022)Doi: 10.1021/acsami.2c17264
Mg shows great potential as a metal hydride for switchable optical response and hydrogen detection due to its ability to stably incorporate significant amounts of hydrogen into its lattice. However, this thermodynamic stability makes hydrogen removal difficult. By alloying Mg with secondary elements, the hydrogenation kinetics can be increased. Here, we report the dynamic optical, loading, and stress properties of three Mg alloy systems (Mg–Al, Mg–Ti, and Mg–Ni) and present several novel phenomena and three distinct device designs that can be achieved with them. We find that these materials all have large deviations in refractive index when exposed to H2 gas, with a wide range of potential properties in the hydride state. The magnitude and sign of the optical property change for each of the alloys are similar, but the differences have dramatic effects on device design. We show that Mg–Ti alloys perform well as both switchable windows and broadband switchable light absorbers, where Mg0.87Ti0.13 and Mg0.85Ti0.15 can achieve a 40% transmission change as a switchable window and a 55% absorption change as a switchable solar absorber. We also show how different alloys can be used for dynamically tunable color filters, where both the reflected and transmitted colors depend on the hydrogenation state. We demonstrate how small changes in the alloy composition (e.g., with Mg–Ni) can lead to dramatically different color responses upon hydrogenation (red-shifting vs blue-shifting of the resonance). Our results establish the potential for these Mg alloys in a variety of applications relating to hydrogen storage, detection, and optical devices, which are necessary for a future hydrogen economy.
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Publication: IEEE Trans. Electron Devices 69, 785 (2022)Doi: 10.1109/TED.2021.3137760
Gyrotrons operating in high-order modes often suffer from severe mode competition. For the selective excitation of the desired mode, triode-type electron guns are often used. In the case of triode-type guns, where the beam voltage and the mod-anode voltage can be varied independently, the mode excitation during the startup process is studied in this article by using a generalized approach. The term “generalized approach” means that the obtained results can be valid for gyrotrons operating at arbitrary voltages and in any mode. The conditions for excitation of the modes are analyzed for different types of startup scenarios. The goal of the study is to find such relations between the beam and mod-anode voltages during the gyrotron startup that only the desired mode will be excited. It is shown that by using a specific triode-type startup scenario with a proper timing for the rise of the mod-anode voltage with respect to the beam voltage, the initial excitation of the desired mode can be realized in the cases of practically any mode density. The mode interaction in such a case is studied; also, the dependences of the results on the timing relation between the two voltages and on the voltage rise speed are considered. This article also contains the numerical analysis of a specific megawatt (MW) gyrotron with a triode-type gun.
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Publication: J. Plasma Phys. 88, 905880616 (2022)Doi: 10.1017/S0022377822001258
A method is demonstrated to rapidly calculate the shapes and properties of quasi-axisymmetric and quasi-helically symmetric stellarators. In this approach, optimization is applied to the equations of magnetohydrodynamic equilibrium and quasisymmetry, expanded in the small distance from the magnetic axis, as formulated by Garren & Boozer [Phys. Fluids B, vol. 3, 1991, p. 2805]. Due to the reduction of the equations by the expansion, the computational cost is significantly reduced, to times of the order of 1 cpu second, enabling wide and high-resolution scans over parameter space. In contrast to traditional stellarator optimization, here, the cost function serves to maximize the volume in which the expansion is accurate. A key term in the cost function is ∥∇B∥, the norm of the magnetic field gradient, to maximize scale lengths in the field. Using this method, a database of 5×105 optimized configurations is calculated and presented. Quasisymmetric configurations are observed to exist in continuous bands, varying in the ratio of the magnetic axis length to average major radius. Several qualitatively new types of configuration are found, including quasi-helically symmetric fields in which the number of field periods is two or more than six.
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Publication: The Institute of Physics (IOP) Publishing Ltd.Doi: 10.1088/978-0-7503-4039-7
This book provides a brief exposition of the principles of beam physics and particle accelerators with an emphasis on numerical examples employing readily available computer tools. The new edition covers, as the first two editions, basic accelerator lenses and deflectors, lattice and beam functions, synchrotron radiation, beam envelope matching, betatron resonances with and without space charge, transverse and longitudinal emittance and space charge. Two new chapters cover special lattice configurations known as coupled optics, and small machines employed for physics research in scaled experiments, which cannot be easily tested in large accelerators. In addition, the general theory of accelerator magnets is presented in a new appendix. The key audiences for this book include physics and engineering graduates and senior undergraduate students, instructors in accelerator/beam physics and particle accelerator science and engineering professionals.
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Publication: Phys. Plasmas 29, 073101 (2022)Doi: 10.1063/5.0097214
We present results from two new techniques for the generation of meter-scale, low density ([Formula: see text] on axis) plasma waveguides, the “two-Bessel” technique, and the “self-waveguiding” technique. Plasma waveguides of this density and length range are needed for demonstration of a ∼10 GeV laser wakefield accelerator module, key for future staging for a ∼TeV lepton collider. Both techniques require the use of high quality ultrashort pulse Bessel beams to efficiently and uniformly ionize hydrogen gas in meter-scale supersonic gas jets via optical field ionization. We review these two techniques, describe our meter-scale gas jets, and present a new method for correction of optical aberrations in Bessel beams. Finally, we briefly present results from recent experiments employing one of our techniques, demonstrating quasi-monoenergetic acceleration of ∼5 GeV electron bunches in 20 cm long, low density plasma waveguides.
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Publication: Optics Lett. 47, 3447 (2022)Doi: 10.1364/OL.457709
We analyze the effects of atmospheric turbulence on the mode power spectrum of beams carrying orbital angular momentum represented by Laguerre-Gauss (LG) modes. For an input (0, m) LG mode, we calculate the power transferred to other modes (0, m') due to turbulence. The analysis is validated against split-step beam propagation simulations and shows agreement into the strong turbulence regime. These results have applications for the design and characterization of free-space laser communication systems.
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Publication: J. Appl. Phys. 132, 224305 (2022)Doi: 10.1063/5.0124105
Understanding and improving the contact resistance of two-dimensional materials for the fabrication of next-generation devices is of vital importance to be able to fully utilize the new physics available in these materials. In this work, eight different contact metals (Ag, Au, Cr, Cu, In, Mo, Ni, and Ti) have been investigated using the same sample of monolayer MoS2. Through the fabrication and testing of multiple, identically sized field-effect transistor devices per contact metal, we compensate for large variability in electrical properties of as-grown chemical vapor deposition MoS2 and deduce the relative performance of each metal. The general trend of lower work function metals having lower contact resistance holds with In, Ag, and Ti performing the best of the metals tested. Our results are compatible with recent research suggesting that the contact resistance in undoped, monolayer MoS2 is dominated by a lateral junction resistance, and we provide context for how this manifests in device-to-device variation. Multiple orders of magnitude differences in contact resistance are observed between metals and can be explained by this lateral barrier operating in the thermionic-field emission regime.
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Publication: Nanophoton. 11, 3823 (2022)Doi: 10.1515/nanoph-2022-0089
The exponential growth of information stored in data centers and computational power required for various data-intensive applications, such as deep learning and AI, call for new strategies to improve or move beyond the traditional von Neumann architecture. Recent achievements in information storage and computation in the optical domain, enabling energy-efficient, fast, and high-bandwidth data processing, show great potential for photonics to overcome the von Neumann bottleneck and reduce the energy wasted to Joule heating. Optically readable memories are fundamental in this process, and while light-based storage has traditionally (and commercially) employed free-space optics, recent developments in photonic integrated circuits (PICs) and optical nano-materials have opened the doors to new opportunities on-chip. Photonic memories have yet to rival their electronic digital counterparts in storage density; however, their inherent analog nature and ultrahigh bandwidth make them ideal for unconventional computing strategies. Here, we review emerging nanophotonic devices that possess memory capabilities by elaborating on their tunable mechanisms and evaluating them in terms of scalability and device performance. Moreover, we discuss the progress on large-scale architectures for photonic memory arrays and optical computing primarily based on memory performance.
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Publication: Nanophoton. 11, 4011 (2022)Doi: 10.1515/nanoph-2021-0797
With the increasing need for large volumes of data processing, transport, and storage, optimizing the trade-off between high-speed and energy consumption in today’s optoelectronic devices is getting increasingly difficult. Heterogeneous material integration into silicon- and nitride-based photonics has showed high-speed promise, albeit at the expense of millimeter-to centimeter-scale footprints. The hunt for an electro-optic modulator that combines high speed, energy efficiency, and compactness to support high component density on-chip continues. Using a double-layer graphene optical modulator integrated on a Silicon photonics platform, we are able to achieve 60 GHz speed (3 dB roll-off), micrometer compactness, and efficiency of 2.25 fJ/bit in this paper. The electro-optic response is boosted further by a vertical distributed-Bragg-reflector cavity, which reduces the driving voltage by about 40 times while maintaining a sufficient modulation depth (5.2 dB/V). Modulators that are small, efficient, and quick allow high photonic chip density and performance, which is critical for signal processing, sensor platforms, and analog- and neuromorphic photonic processors.
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Publication: Opt. Express 30, 21977 (2022)Doi: 10.1364/OE.450724
Optical switchability is an important functionality for photonic devices, which allows them to accommodate a wide range of applications. One way to achieve this switchability is to utilize the reversible and tunable optical changes of metal hydrides. When exposed to H2 gas, certain metals go through dramatic changes in optical properties as hydrogen atoms expand the lattice spacing. In this paper, we propose a switchable absorption device consisting of a Pd-capped Mg thin film deposited onto a near-zero-index substrate. By utilizing Mg's extreme optical changes upon hydrogenation and combining it with the high optical contrast of the near-zero-index substrate, we can create a device that is fully switchable from a highly reflective state to a broadband absorbing state. When modeling the substrate as a Drude material with a plasma wavelength of 600 nm, we calculate an absorption change of > 70% from 650-1230 nm, with a peak total absorption of 78% at 905 nm. We experimentally demonstrate this effect using 25 nm of Mg with a 3 nm Pd capping layer deposited onto an ITO-coated glass substrate. This device achieves an absorption change of 76% at 1335 nm illumination, with a maximum absorption of 93% in the hydride state, utilizing ITO's near-zero-index region in the near-infrared. By tuning the near-zero-index region of the substrate, this effect can be extended from the visible through the infrared.
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Publication: Opt. Photon. News 33 [12] (2022)Doi: http://optica-opn.org/home/articles/volume_33/december_2022/extras/programmable_integrated_photonics_via_phase-change/
Programmable photonic integrated circuits (PICs)1 composed of arrays of tunable phase shifters and beam splitters are expanding their application from traditional optical communications to optical signal processing, computing and even quantum information processing. However, traditional tuning methods, such as thermo-optic and electro-optic effects, are weak, power hungry and volatile. The resulting devices usually feature a large footprint (greater than 100 µm) and require a constant power supply of around 10 mW. This significantly limits the integration density and energy efficiency of the PIC.
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Publication: Phys. Rev. Lett. 128, 035001 (2022)Doi: 10.1103/PhysRevLett.128.035001
Quasisymmetry is an unusual symmetry that can be present in toroidal magnetic fields, enabling the confinement of charged particles and plasma. Here it is shown that both quasiaxisymmetry and quasihelical symmetry can be achieved to a much higher precision than previously thought over a significant volume, resulting in exceptional confinement. For a 1 Tesla mean field far from axisymmetry (vacuum rotational transform >0.4), symmetry-breaking mode amplitudes throughout a volume of aspect ratio 6 can be made as small as the typical ∼50 μT geomagnetic field.
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Publication: Nano Lett. 22, 7959 (2022)Doi: 10.1021/acs.nanolett.2c03098
Optically active spin systems coupled to photonic cavities with high cooperativity can generate strong light–matter interactions, a key ingredient in quantum networks. However, obtaining high cooperativities for quantum information processing often involves the use of photonic crystal cavities that feature a poor optical access from the free space, especially to circularly polarized light required for the coherent control of the spin. Here, we demonstrate coupling with a cooperativity as high as 8 of an InAs/GaAs quantum dot to a fabricated bullseye cavity that provides nearly degenerate and Gaussian polarization modes for efficient optical accessing. We observe spontaneous emission lifetimes of the quantum dot as short as 80 ps (an ∼15 Purcell enhancement) and a ∼80% transparency of light reflected from the cavity. Leveraging the induced transparency for photon switching while coherently controlling the quantum dot spin could contribute to ongoing efforts of establishing quantum networks.
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Publication: Astrophys J. 933, 181 (2022)Doi: 10.3847/1538-4357/ac73f6
Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal width dcs distribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 au. The dcs distribution displays a nearly exponential decay from a peak at dcs = 25 di to a median at dcs = 85 di and a 95th percentile at dcs = 905 di with a maximum exhaust width at dcs = 8077 di. A magnetic field θ-rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35° < θ < 65°. The azimuthal ϕ angles of the CS normal directions of 430 thick dcs ≥ 500 di exhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scale dcs < 25 di exhausts are isotropic in contrast, and likely associated with Alfvénic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrow dcs ∼ 15–25 di widths to well beyond dcs ∼ 500 di with an ortho-Parker azimuthal direction of a large-scale heliospheric current sheet (HCS) is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more or less aligned, secondary CSs due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 au as other HCSs.
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Publication: Phys. Rev. X 12, 031038 (2022)Doi: 10.1103/PhysRevX.12.031038
We present the first demonstration of multi-GeV laser wakefield acceleration in a fully optically formed plasma waveguide, with an acceleration gradient as high as 25 GeV/m. The guide was formed via self-waveguiding of <15 J, 45 fs (< ∼300 TW) pulses over 20 cm in a low-density hydrogen gas jet, with accelerated electron bunches driven up to 5 GeV in quasimonoenergetic peaks of relative energy width as narrow as ∼15%, with divergence down to ∼ 1 mrad and charge up to tens of picocoulombs. Energy gain is inversely correlated with on-axis waveguide density in the range Ne0 = (1.3–3.2)×1017cm−3. We find that shot-to-shot stability of bunch spectra and charge are strongly dependent on the pointing of the injected laser pulse and gas jet uniformity. We also observe evidence of pump depletion-induced dephasing, a consequence of the long optical guiding distance.
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Publication: J. Phys. Chem. Lett. 13, 2254 (2022)Doi: 10.1021/acs.jpclett.2c00131
Metal halide perovskite (MHP) photovoltaics may become a viable alternative to standard Si-based technologies, but the current lack of long-term stability precludes their commercial adoption. Exposure to standard operational stressors (light, temperature, bias, oxygen, and water) often instigate optical and electronic dynamics, calling for a systematic investigation into MHP photophysical processes and the development of quantitative models for their prediction. We resolve the moisture-driven light emission dynamics for both methylammonium lead tribromide and triiodide thin films as a function of relative humidity (rH). With the humidity and photoluminescence time series, we train recurrent neural networks and establish their ability to quantitatively predict the path of future light emission with 18% error over 4 h. Together, our in situ rH-PL measurements and machine learning forecasting models provide a framework for the rational design of future stable perovskite devices and, thus, a faster transition toward commercial applications.
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Publication: Adv. Photonics Res. 3, 2100218 (2022)Doi: 10.1002/adpr.202100218
The discovery of graphene and its unique optical and electronic properties has triggered intense developments in a vast number of optoelectronic applications, especially in spectral regions that are not easily accessible with conventional semiconductors. Particularly in the THz regime, where the free-carrier interaction with low-energetic photons usually dominates, detectors and modulators based on graphene often feature an improved response time. Nevertheless, the light−matter interaction suffers from the small interaction volume. One way to enhance the efficiency of such devices at elevated frequencies is by patterning graphene into plasmonic structures like disks. In addition to the increased linear absorption, the plasmon resonance also creates a strong, surface-localized field that enhances the nonlinear optical response. While experimental studies so far have focused on hot carrier effects, theoretical studies also suggest an increase in the nonlinearity beyond thermal effects. Herein, polarization-dependent pump-probe measurements on graphene disks that disentangle the contributions of thermal and plasmonic nonlinearity are presented. An increase in the pump-induced transmission is observed when pump and probe radiation are copolarized. To further elucidate the interplay of thermal and plasmonic effects, a model that supports the origin of the polarization-dependent enhancement of the observed THz nonlinearities is developed.
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Publication: Nucl. Fusion 62, 126054 (2022)Doi: 10.1088/1741-4326/ac9b07
Collisionless physics primarily determines the transport of fusion-born alpha particles in 3D equilibria. Several transport mechanisms have been implicated in stellarator configurations, including stochastic diffusion due to class transitions, ripple trapping, and banana drift-convective orbits. Given the guiding center dynamics in a set of six quasihelical and quasiaxisymmetric equilibria, we perform a classification of trapping states and transport mechanisms. In addition to banana drift convection and ripple transport, we observe substantial non-conservation of the parallel adiabatic invariant which can cause losses through diffusive banana tip motion. Furthermore, many lost trajectories undergo transitions between trapping classes on longer time scales, either with periodic or irregular behavior. We discuss possible optimization strategies for each of the relevant transport mechanisms. We perform a comparison between fast ion losses and metrics for the prevalence of mechanisms such as banana-drift convection (Velasco et al 2021 Nucl. Fusion 61 116059), transitioning orbits, and wide orbit widths. Quasihelical configurations are found to have natural protection against ripple-trapping and diffusive banana tip motion leading to a reduction in prompt losses.
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Publication: J. Phys. D: Appl. Phys. 55, 373001 (2022)Doi: 10.1088/1361-6463/ac5e1c
The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5–10 years.
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Publication: Phys. Rev. Lett. 129, 205101 (2022)Doi: 10.1103/PhysRevLett.129.205101
Accreting supermassive black holes can now be observed at the event-horizon scale at millimeter wavelengths. Current predictions for the image rely on hypotheses (fluid modeling, thermal electrons) which might not always hold in the vicinity of the black hole, so that a full kinetic treatment is in order. In this Letter, we describe the first 3D global general-relativistic particle-in-cell simulation of a black-hole magnetosphere. The system displays a persistent equatorial current sheet. Synthetic radio images are computed by ray-tracing synchrotron emission from nonthermal particles accelerated in this current sheet by magnetic reconnection. We identify several time-dependent features of the image at moderate viewing angles: a variable radius of the ring, and hot spots moving along it. In this regime, our model predicts that most of the flux of the image lies inside the critical curve. These results could help promote understanding of future observations of black-hole magnetospheres at improved temporal and spatial resolution.
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Publication: Phys. Plasmas 29, 082501 (2022)Doi: 10.1063/5.0098166
Quasi-symmetry can greatly improve the confinement of energetic particles and thermal plasma in a stellarator. The magnetic field of a quasi-symmetric stellarator at high plasma pressure is significantly affected by the bootstrap current, but the computational cost of accurate stellarator bootstrap calculations has precluded use inside optimization. Here, a new efficient method is demonstrated for optimization of quasi-symmetric stellarator configurations such that the bootstrap current profile is consistent with the geometry. The approach is based on the fact that all neoclassical phenomena in quasi-symmetry are isomorphic to those in axisymmetry. Therefore, accurate formulas for the bootstrap current in tokamaks, which can be evaluated rapidly, can be applied also in stellarators. The deviation between this predicted parallel current and the actual parallel current in the magnetohydrodynamic equilibrium is penalized in the objective function, and the current profile of the equilibrium is included in the parameter space. Quasi-symmetric configurations with significant pressure are thereby obtained with self-consistent bootstrap current and excellent confinement. In a comparison of fusion-produced alpha particle confinement across many stellarators, the new configurations have significantly lower alpha energy losses than many previous designs.
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Publication: Phys. Rev. Lett. 128, 164101 (2022)Doi: 10.1103/PhysRevLett.128.164101
Forecasting the dynamics of large, complex, sparse networks from previous time series data is important in a wide range of contexts. Here we present a machine learning scheme for this task using a parallel architecture that mimics the topology of the network of interest. We demonstrate the utility and scalability of our method implemented using reservoir computing on a chaotic network of oscillators. Two levels of prior knowledge are considered: (i) the network links are known, and (ii) the network links are unknown and inferred via a data-driven approach to approximately optimize prediction.
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Publication: J. Mater. Chem. A 10, 12518 (2022)Doi: 10.1039/d2ta01021f
Advanced solid-state batteries most likely will entail aggressive structures or architectures with constraints that typically limit processing temperatures. Considering this, we have identified the importance of providing lithiated electrode materials at a modest processing temperature. Here we describe a pathway to meet this by the development of a co-sputtering process using lithium oxide and vanadium oxide targets which enables the growth of lithiated vanadium oxide (LVO) thin films for application in solid-state batteries. Analysis of the structure and film composition of samples deposited with different co-sputtering rate ratios and post-annealing shows that multiple phases of LixV2O5 likely coexist (i.e., α-, ε-, δ-, and γ-V2O5), and that this is unchanged after electrochemical cycling. The co-sputtering process can tune the lithium content up to a highly lithiated state of at least LixV2O. Electrochemical half-cells showed a significant amount of lithium available on the first charge (delithiation of LVO). LVO samples post-annealed at 300 °C showed typical redox peaks for LixV2O for both one and two lithium insertion reactions, which were highly reversible in most cases. A thin-film solid-state battery prototype using LVO as a cathode had 20% of the expected capacity, although the coulombic efficiency is near 100% at a fast rate (22C). This co-sputtering technique represents an opportunity for low temperature synthesis of pre-lithiated cathodes for thin film batteries, and introduces a broader methodology of depositing metal oxides with different alkali metal contents.
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Publication: Opt. Express 30, 11360 (2022)Doi: 10.1364/OE.454796
Bessel beams generated with non-ideal axicons are affected by aberrations. We introduce a method to retrieve the complex amplitude of a Bessel beam from intensity measurements alone, and then use this information to correct the wavefront and intensity profile using a deformable mirror.
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Publication: Lap Chip 22, 3203 (2022)Doi: 10.1039/D2LC00481J
Chemotaxis is a fundamental bacterial response mechanism to changes in chemical gradients of specific molecules known as chemoattractant or chemorepellent. The advancement of biological platforms for bacterial chemotaxis research is of significant interest for a wide range of biological and environmental studies. Many microfluidic devices have been developed for its study, but challenges still remain that can obscure analysis. For example, cell migration can be compromised by flow-induced shear stress, and bacterial motility can be impaired by nonspecific cell adhesion to microchannels. Also, devices can be complicated, expensive, and hard to assemble. We address these issues with a three-channel microfluidic platform integrated with natural biopolymer membranes that are assembled in situ. This provides several unique attributes. First, a static, steady and robust chemoattractant gradient was generated and maintained. Second, because the assembly incorporates assembly pillars, the assembled membrane arrays connecting nearby pillars can be created longer than the viewing window, enabling a wide 2D area for study. Third, the in situ assembled biopolymer membranes minimize pressure and/or chemiosmotic gradients that could induce flow and obscure chemotaxis study. Finally, nonspecific cell adhesion is avoided by priming the polydimethylsiloxane (PDMS) microchannel surfaces with Pluronic F-127. We demonstrated chemotactic migration of Escherichia coli as well as Pseudomonas aeruginosa under well-controlled easy-to-assemble glucose gradients. We characterized motility using the chemotaxis partition coefficient (CPC) and chemotaxis migration coefficient (CMC) and found our results consistent with other reports. Further, random walk trajectories of individual cells in simple bright field images were conveniently tracked and presented in rose plots. Velocities were calculated, again in agreement with previous literature. We believe the biopolymer membrane-integrated platform represents a facile and convenient system for robust quantitative assessment of cellular motility in response to various chemical cues.
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Publication: Nano Lett. 22, 9457 (2022)Doi: 10.1021/acs.nanolett.2c03527
Radiative cascades emit correlated photon pairs, providing a pathway for the generation of entangled photons. The realization of a radiative cascade with impurity atoms in semiconductors, a leading platform for the generation of quantum light, would therefore provide a new avenue for the development of entangled photon pair sources. Here we demonstrate a radiative cascade from the decay of a biexciton at an impurity–atom complex in a ZnSe quantum well. The emitted photons show clear temporal correlations revealing the time-ordering of the cascade. Our result establishes impurity atoms in ZnSe as a potential platform for photonic quantum technologies using radiative cascades.
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Publication: Astrophys. J. 925, 213 (2022)Doi: 10.3847/1538-4357/ac4016
A major discovery of Parker Solar Probe (PSP) was the presence of large numbers of localized increases in the radial solar wind speed and associated sharp deflections of the magnetic field—switchbacks (SBs). A possible generation mechanism of SBs is through magnetic reconnection between open and closed magnetic flux near the solar surface, termed interchange reconnection, that leads to the ejection of flux ropes (FRs) into the solar wind. Observations also suggest that SBs undergo merging, consistent with an FR picture of these structures. The role of FR merging in controlling the structure of SBs in the solar wind is explored through direct observations, analytic analysis, and numerical simulations. Analytic analysis reveals key features of the structure of FRs and their scaling with heliocentric distance R, which are consistent with observations and demonstrate the critical role of merging in controlling the structure of SBs. FR merging is shown to energetically favor reductions in the strength of the wrapping magnetic field and the elongation of SBs. A further consequence is the resulting dominance of the axial magnetic field within SBs that leads to the observed characteristic sharp rotation of the magnetic field into the axial direction at the SB boundary. Finally, the radial scaling of the SB area in the FR model suggests that the observational probability of SB identification should be insensitive to R, which is consistent with the most recent statistical analysis of SB observations from PSP.
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Publication: Opt. Express 30, 18481 (2022)Doi: 10.1364/OE.453944
We experimentally demonstrate that when two spin-orbit coupled orbital angular momentum (OAM) modes of opposite topological charge co-propagate in the Kerr nonlinear regime in a hollow ring-core optical fiber, the vectorial mode superposition exhibits a unique power-dependent rotation effect. This effect is analogous to nonlinear polarization rotation in single-mode fibers, however, the added spatial dimension produces a visually observable rotation of the spatial pattern emerging from the fiber when imaged through a linear polarizer. A dielectric metasurface q-plate was designed and fabricated to excite the desired mode combination in a hollow ring-core fiber that supports stable propagation of OAM modes. The observed spatial patterns show strong agreement with numerical simulations of the vector coupled nonlinear Schrödinger equations. These results constitute the first measurements of what can be described as the spin-orbit coupled generalization of the nonlinear polarization rotation effect.
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Publication: Surveys Geophys. 43, 229Doi: 10.1007/s10712-021-09681-1
Understanding fluid flows in planetary cores and subsurface oceans, as well as their signatures in available observational data (gravity, magnetism, rotation, etc.), is a tremendous interdisciplinary challenge. In particular, it requires understanding the fundamental fluid dynamics involving turbulence and rotation at typical scales well beyond our day-to-day experience. To do so, laboratory experiments are fully complementary to numerical simulations, especially in systematically exploring extreme flow regimes for long duration. In this review article, we present some illustrative examples where experimental approaches, complemented by theoretical and numerical studies, have been key for a better understanding of planetary interior flows driven by some type of mechanical forcing. We successively address the dynamics of flows driven by precession, by libration, by differential rotation, and by boundary topography.
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Publication: Astrophys. J. Lett. 933, L37Doi: 10.3847/2041-8213/ac7c71
Pulsar radio emission may be generated in pair discharges that fill the pulsar magnetosphere with plasma as an accelerating electric field is screened by freshly created pairs. In this Letter, we develop a simplified analytic theory for the screening of the electric field in these pair discharges and use it to estimate total radio luminosity and spectrum. The discharge has three stages. First, the electric field is screened for the first time and starts to oscillate. Next, a nonlinear phase occurs. In this phase, the amplitude of the electric field experiences strong damping because the field dramatically changes the momenta of newly created pairs. This strong damping ceases, and the system enters a final linear phase, when the electric field can no longer dramatically change pair momenta. Applied to pulsars, this theory may explain several aspects of radio emission, including the observed luminosity, Lrad ∼ 1028 erg s−1, and the observed spectrum, Sω ∼ ω−1.4±1.0.
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Publication: J. Vac. Sci. Technol. A 40, 032403 (2022)Doi: 10.1116/6.0001752
Atomic layer deposition (ALD) is a key technique in processing new materials compatible with complex architectures. While the processing space for Li-containing ALD thin films has been relatively well explored recently, the space for other alkali metal thin films (e.g., Na) is more limited. Thermal ALD and plasma-enhanced ALD (PEALD) lithium phosphorus oxynitride [Kozen et al., Chem. Mater. 27, 5324 (2015); Pearse et al., Chem. Mater. 29, 3740 (2017)] processes as well as analogous thermal sodium phosphorus oxynitride (NaPON) (Ref. 13) have been previously developed as conformal ALD solid state electrolytes. The main difference between the Na and Li processes is the alkali tert-butoxide precursor (AOtBu, A = Li, Na). One would expect such an isoelectronic substitution with precursors that have similar structure and properties to correlate with a similarly behaved ALD process. However, this work demonstrates that the PEALD NaPON process unexpectedly behaves quite differently from its Li counterpart, introducing some insight into the development of Na-containing thin films. In this work, we demonstrate process development and characterization of an analogous low temperature (250 °C) PEALD of NaPON. This process demonstrates significant tunability of N coordination states by varying plasma nitrogen exposure time. Electrochemical characterization showed an ionic conductivity of 8.2 × 10−9 S/cm at 80 °C and activation energy of 1.03 eV. This first instance of low temperature NaPON deposition by PEALD shows promise for further development and understanding of more versatile processing of Na thin film materials.
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Publication: Mon. Not. R. Astron. Soc. 514, 4989Doi: 10.1093/mnras/stac1435
Relativistic plasmas are central to the study of black hole accretion, jet physics, neutron star mergers, and compact object magnetospheres. Despite the need to accurately capture the dynamics of these plasmas and the implications for relativistic transients, their fluid modelling is typically done using a number of (overly) simplifying assumptions, which do not hold in general. This is especially true when the mean free path in the plasma is large compared to the system size, and kinetic effects start to become important. Going beyond common approaches used in the literature, we describe a fully relativistic covariant 14-moment based two-fluid system appropriate for the study of electron–ion or electron–positron plasmas. This generalized Israel-Stewart-like system of equations of motion is obtained directly from the relativistic Boltzmann–Vlasov equation. This new formulation can account for non-ideal effects, such as anisotropic pressures and heat fluxes, not present in previous formulations of two-fluid magnetohydrodynamics. We show that a relativistic two-fluid plasma can be recast as a single fluid coupled to electromagnetic fields with (potentially large) out-of-equilibrium corrections. We keep all electron degrees of freedom, which provide self-consistent evolution equations for electron temperature and momentum. The out-of-equilibrium corrections take the form of a collisional 14-moment closure previously described in the context of viscous single fluids. The equations outlined in this paper are able to capture the full two-fluid character of collisionless plasmas found in black hole accretion and flaring processes around compact objects, as well Braginskii-like two-fluid magnetohydrodynamics applicable to weakly collisional plasmas inside accretion discs.
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Publication: Adv. Quantum Technol. 5, 2200022Doi: 10.1002/qute.202200022
Incorporating solid-state quantum emitters into optical fiber networks enables the long-distance transmission of quantum information and the remote connection of distributed quantum nodes. However, interfacing quantum emitters with fiber optics encounters several challenges, including low coupling efficiency and delicate configuration. In this study, a highly efficient fiber-interfacing photonic device that directly launches single photons from quantum dots into a standard FC/PC-connectorized single-mode fiber is demonstrated. Optimally designed photonic structures based on hole gratings produce an ultra-narrow directional beam that matches the small numerical aperture of a single-mode fiber. A pick-and-place technique precisely integrates a single miniaturized device into the core of the fiber. This approach realizes a plug-and-play single-photon device that does not require optical alignment and thus guarantees long-term stability. The results represent a major step toward practical and reliable transmission of quantum light across a fiber network.
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Publication: Ann. Rev. Astron. Astrophys. 60, 495 (2022)Doi: 10.1146/annurev-astro-052920-112338
The discovery of pulsars opened a new research field that allows studying a wide range of physics under extreme conditions. More than 3,000 pulsars are currently known, including especially more than 200 of them studied at gamma-ray frequencies. By putting recent insights into the pulsar magnetosphere in a historical context and by comparing them to key observational features at radio and high-energy frequencies, we show the following: ▪ Magnetospheric structure of young energetic pulsars is now understood. Limitations still exist for old nonrecycled and millisecond pulsars. ▪ The observed high-energy radiation is likely produced in the magnetospheric current sheet beyond the light cylinder. ▪ There are at least two different radio emission mechanisms. One operates in the inner magnetosphere, whereas the other one works near the light cylinder and is specific to pulsars with the high magnetic field strength in that region. ▪ Radio emission from the inner magnetosphere is intrinsically connected to the process of pair production, and its observed properties contain the imprint of both the geometry and propagation effects through the magnetospheric plasma. We discuss the limitations of our understanding and identify a range of observed phenomena and physical processes that still await explanation in the future. This includes connecting the magnetospheric processes to spin-down properties to explain braking and possible evolution of spin orientation, building a first-principles model of radio emission and quantitative connections with observations.
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Publication: J. Adv. Modeling Earth Systems 15, e2021MS002712 (2022)Doi: 10.1029/2021MS002712
This paper describes an implementation of the combined hybrid-parallel prediction (CHyPP) approach of Wikner et al. (2020), https://doi.org/10.1063/5.0005541 on a low-resolution atmospheric global circulation model (AGCM). The CHyPP approach combines a physics-based numerical model of a dynamical system (e.g., the atmosphere) with a computationally efficient type of machine learning (ML) called reservoir computing to construct a hybrid model. This hybrid atmospheric model produces more accurate forecasts of most atmospheric state variables than the host AGCM for the first 7–8 forecast days, and for even longer times for the temperature and humidity near the earth's surface. It also produces more accurate forecasts than a model based only on ML, or a model that combines linear regression, rather than ML, with the AGCM. The potential of the CHyPP approach for climate research is demonstrated by a 10-year long hybrid model simulation of the atmospheric general circulation, which shows that the hybrid model can simulate the general circulation with substantially smaller systematic errors and more realistic variability than the host AGCM.
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Publication: Astrophys. J. 927, 62Doi: 10.3847/1538-4357/ac4961
We present observations of ≳10–100 keV nucleon−1 suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances of <0.1 au from the Sun. Our key findings are as follows: (1) very few heavy ions are detected during the first full crossing, the heavy-ion intensities are reduced during the second partial crossing and peak just after the second crossing; (2) ion arrival times exhibit no velocity dispersion; (3) He pitch-angle distributions track the magnetic field polarity reversal and show up to ∼10:1 anti-sunward, field-aligned flows and beams closer to the HCS that become nearly isotropic farther from the HCS; (4) the He spectrum steepens either side of the HCS, and the He, O, and Fe spectra exhibit power laws of the form ∼E−4–E6; and (5) maximum energies EX increase with the ion's charge-to-mass (Q/M) ratio as EX/EH ∝ (QX/MX)δ, where δ ∼ 0.65–0.76, assuming that the average Q states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. The absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-Sun coronal-mass-ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to E/Q but also for local diffusive and magnetic-reconnection-driven acceleration models. Reevaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions.
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Phonon Assisted Electron Emission from Quasi-Freestanding Bilayer Epitaxial Graphene Microstructures
Publication: Nanotechnol. 33, 375202 (2022)Doi: 10.1088/1361-6528/ac7653Electron emission from quasi-freestanding bilayer epitaxial graphene (QFEG) on a silicon carbide substrate is reported, demonstrating emission currents as high as 8.5μA, at ∼200 °C, under 0.3 Torr vacuum. Given the significantly low turn-on temperature of these QFEG devices, ∼150°C, the electron emission is explained by phonon-assisted electron emission, where the acoustic and optical phonons of QFEG causes carrier acceleration and emission. Devices of differing dimensions and shapes are fabricated via a simple and scalable fabrication procedure and tested. Variations in device morphology increase the density of dangling bonds, which can act as electron emission sites. Devices exhibit emission enhancement at increased temperatures, attributed to greater phonon densities. Devices exhibit emission under various test conditions, and a superior design and operating methodology are identified.
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Publication: Adv. Photon. Res. 3, 2200202 (2022)Doi: 10.1002/adpr.202200202
Despite their importance in applications such as nonvolatile memory, integrated photonics, and compact optics, the crystalline-to-amorphous transition in chalcogenide phase-change materials (PCMs) is not understood. Herein, this transition in a technologically relevant infrared (IR) transparent chalcogenide material, Ge2Sb2Se4Te1 (GSST), is examined. Thin films of GSST using fully depleted silicon on insulator (FDSOI) microheaters are discussed and the phase transitions by polarized and unpolarized Raman spectroscopy is studied. It is confirmed that the crystalline-to-amorphous transition is driven by conversion of Ge–6Se octahedra to Ge–4Se tetrahedra with the extra Se being incorporated into an Se—Se network. This is similar to the mechanism reported in earlier work for Ge2Sb2Te5 (GST). Recrystallization requires disrupting the Se—Se network and the crystallization activation energy is consistent with the Se—Se bond energy. Across 1000 crystallization–amorphization cycles, GSST exhibits no qualitative change in the Raman spectrum, suggesting limited film oxidation or chemical decomposition. After several hundred cycles, recrystallization is less complete, likely due to dewetting of GSST during the high-temperature amorphization step leading to compromise of the capping layer and loss of GSST. The utility of GSST as a photonic material through fabrication and testing of a GSST-coated, integrated silicon photonic Mach–Zender interferometer, is discussed.
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Publication: Phys. Rev. Appl. 17, 024027Doi: 10.1103/PhysRevApplied.17.024027
Electromagnetic environments are becoming increasingly complex and congested, creating a growing challenge for systems that rely on electromagnetic waves for communication, sensing, or imaging, particularly in reverberating environments. The use of programmable metasurfaces provides a potential means of directing waves to optimize wireless channels on demand, ensuring reliable operation and protecting sensitive electronic components. Here we introduce a technique that combines a deep-learning network with a binary programmable metasurface to shape waves in complex reverberant electromagnetic environments, in particular ones where there is no direct line of sight. We apply this technique for wavefront reconstruction and control, and accurately determine metasurface configurations based on measured system scattering responses in a chaotic microwave cavity. The state of the metasurface that realizes desired electromagnetic wave field distribution properties was successfully determined even in cases previously unseen by the deep-learning algorithm. Our technique is enabled by the reverberant nature of the cavity, and is effective with a metasurface that covers only approximately 1.5% of the total cavity surface area.
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Publication: Mon. Not. R. Astron. Soc. 515, 2710 (2022)Doi: 10.1093/mnras/stac1909
The coalescence of two neutron stars is accompanied by the emission of gravitational waves, and can also feature electromagnetic counterparts powered by mass ejecta and the formation of a relativistic jet after the merger. Since neutron stars can feature strong magnetic fields, the non-trivial interaction of the neutron star magnetospheres might fuel potentially powerful electromagnetic transients prior to merger. A key process powering those precursor transients is relativistic reconnection in strong current sheets formed between the two stars. In this work, we provide a detailed analysis of how the twisting of the common magnetosphere of the binary leads to an emission of electromagnetic flares, akin to those produced in the solar corona. By means of relativistic force-free electrodynamics simulations, we clarify the role of different magnetic field topologies in the process. We conclude that flaring will always occur for suitable magnetic field alignments, unless one of the neutron stars has a magnetic field significantly weaker than the other.
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Publication: J. Plasma Phys. 88, 175880504 (2022)Doi: 10.1017/S0022377822000873
A single-field-period quasi-isodynamic stellarator configuration is presented. This configuration, which resembles a twisted strip, is obtained by the method of direct construction, that is, it is found via an expansion in the distance from the magnetic axis. Its discovery, however, relied on an additional step involving numerical optimization, performed within the space of near-axis configurations defined by a set of adjustable magnetic field parameters. This optimization, completed in 30 s on a single CPU core using the SIMSOPT code, yields a solution with excellent confinement, as measured by the conventional figure of merit for neoclassical transport, effective ripple, at a modest aspect ratio of eight. The optimization parameters that led to this configuration are described, its confinement properties are assessed and a set of magnetic field coils is found. The resulting transport at low collisionality is much smaller than that of W7-X, and the device needs significantly fewer coils because of the reduced number of field periods.
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Publication: IEEE Trans. Quantum Eng. 3, 4100310 (2022)Doi: 10.1109/TQE.2022.3152170
The Clauser–Horne–Shimony–Holt (CHSH) experiment is an essential test of nonlocality in quantum mechanics and can be used to validate the principle of entanglement. In addition to verifying entanglement, the measurable CHSH parameter can also be used to gauge the quality of the entanglement present in a system. The measurement of Hong–Ou–Mandel (HOM) interference is another important fundamental experiment in quantum optics that measures the indistinguishability of a pair of photons. In this article, we demonstrate how the results of a HOM interference experiment, a relatively simple experiment, can be used to obtain an estimate for the value of the CHSH S parameter, which is a more complicated measurement. We experimentally demonstrate that the HOM interference technique is capable of providing an estimate of the value of the CHSH parameter that is within one standard deviation of measurement error when spectral impairments are present. We expect that this technique will aid in the calibration of quantum optical systems.
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Publication: Opt. Mater. Express 12, 4284 (2022)Doi: 10.1364/OME.474034
We introduce the Optical Materials Express feature issue on Phase Change Materials for Optics and Photonics. This issue comprises a collection of seventeen manuscripts on the development, characterization, control, and applications of optical Phase Change Materials.
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Publication: IEEE Trans. Plasma Sci. 50, 2039m (2022)Doi: 10.1109/TPS.2022.3180634
A Cherenkov maser amplifier (CMA) for generating high-power levels over a wide frequency range is proposed, analyzed, and numerically simulated. The CMA is a wideband amplifier consisting of an annular relativistic electron beam in a cylindrical waveguide, having an inner conductor and outer layer of dielectric material all enclosed by an outer conductor. The interaction between the hybrid TEM/TM subluminal mode of the waveguide and the relativistic electron beam leads to amplification over a wide range of input frequencies in the gigahertz regime. The interaction is analyzed and simulated in the linear and nonlinear regimes. We show that conversion efficiencies can be enhanced by spatially tapering the dielectric waveguide. In addition, by premodulating the electron beam, efficiencies can be further enhanced and saturation distances reduced. Conversion efficiencies greater than 25% have been simulated by premodulating the electron beam and/or spatially tapering the dielectric waveguide over distances of a few meters. Simulation examples indicate that the ultrawideband CMA configuration operating in the gigahertz regime can generate power levels in the gigawatt range, employing electron beams in the multi-kiloampere and low megaelectronvolt range.
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Publication: Phys. Rev. A 105, 022623 (2022)Doi: 10.1103/PhysRevA.105.022623
Trapped ions form an advanced technology platform for quantum information processing with long qubit coherence times, high-fidelity quantum logic gates, optically active qubits, and a potential to scale up in size while preserving a high level of connectivity between qubits. These traits make them attractive not only for quantum computing, but also for quantum networking. Dedicated, special-purpose trapped-ion processors in conjunction with suitable interconnecting hardware can be used to form quantum repeaters that enable high-rate quantum communications between distant trapped-ion quantum computers in a network. In this regard, hybrid traps with two distinct species of ions, where one ion species can generate ion-photon entanglement that is useful for optically interfacing with the network and the other has long memory lifetimes, useful for qubit storage, have been proposed for entanglement distribution. We consider an architecture for a repeater based on such dual-species trapped-ion systems. We propose and analyze a protocol based on spatial and temporal mode multiplexing for entanglement distribution across a line network of such repeaters. Our protocol offers enhanced rates compared to rates previously reported for such repeaters. We determine the ion resources required at the repeaters to attain the enhanced rates, and the best rates attainable when constraints are placed on the number of repeaters and the number of ions per repeater. Our results bolster the case for near-term trapped-ion systems as quantum repeaters for long distance quantum communications.
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Publication: J. Phys. D: Appl. Phys. 55, 155204 (2022)Doi: 10.1088/1361-6463/ac4728
In this work, we investigated atmospheric pressure plasma jet (APPJ)-assisted methane oxidation over a Ni-SiO2/Al2O3 catalyst. We evaluated possible reaction mechanisms by analyzing the correlation of gas phase, surface and plasma-produced species. Plasma feed gas compositions, plasma powers, and catalyst temperatures were varied to expand the experimental parameters. Real-time Fourier-transform infrared spectroscopy was applied to quantify gas phase species from the reactions. The reactive incident fluxes generated by plasma were measured by molecular beam mass spectroscopy using an identical APPJ operating at the same conditions. A strong correlation of the quantified fluxes of plasma-produced atomic oxygen with that of CH4 consumption, and CO and CO2 formation implies that O atoms play an essential role in CH4 oxidation for the investigated conditions. With the integration of APPJ, the apparent activation energy was lowered and a synergistic effect of 30% was observed. We also performed in-situ diffuse reflectance infrared Fourier-transform spectroscopy to analyze the catalyst surface. The surface analysis showed that surface CO abundance mirrored the surface coverage of CHn at 25 °C. This suggests that CHn adsorbed on the catalyst surface as an intermediate species that was subsequently transformed into surface CO. We observed very little surface CHn absorbance at 500 °C, while a ten-fold increase of surface CO and stronger CO2 absorption were seen. This indicates that for a nickel catalyst at 500 °C, the dissociation of CH4 to CHn may be the rate-determining step in the plasma-assisted CH4 oxidation for our conditions. We also found the CO vibrational frequency changes from 2143 cm−1 for gas phase CO to 2196 cm−1 for CO on a 25 °C catalyst surface, whereas the frequency of CO on a 500 °C catalyst was 2188 cm−1. The change in CO vibrational frequency may be related to the oxidation of the catalyst.
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Publication: IEEE Trans. Plasma Sci. 50, 2568 (2022)Doi: 10.1109/TPS.2022.3158277
By formulating and solving the adjoint equations governing the beam–wave interaction in a traveling-wave tube (TWT), we show the partial derivatives with respect to design parameters of various TWT figures of merit (FoMs) may be efficiently calculated. FoMs include average gain, gain flatness, and gain–bandwidth product, and design parameters include beam voltage and circuit geometry. We use these derivatives in an optimization algorithm that finds parameter values that minimize or maximize the desired FoM. We show that a 1-D large-signal simulation code, such as CHRISTINE, may be easily modified to compute the adjoint solutions. We further show that only three runs of the modified code suffice to compute the partial derivatives of the output power and phase at a specified frequency with respect to an arbitrary number of parameters, resulting in potentially large savings in computing time compared with direct, finite difference calculation of the partial derivatives. We illustrate the method by optimizing the beam voltage and gap spacing of a W -band folded-waveguide (FWG) TWT. The formulation given here applies only to TWT slow wave structures, such as coupled-cavity and FWGs, and to klystrons, composed of discrete gaps followed by drift spaces; it does not apply to helix structures, which may be the subject of a future paper.
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Publication: J. Plasma Phys. 88, 905880401 (2022)Doi: 10.1017/S0022377822000563
We propose a new method to compute magnetic surfaces that are parametrized in Boozer coordinates for vacuum magnetic fields. We also propose a measure for quasisymmetry on the computed surfaces and use it to design coils that generate a magnetic field that is quasisymmetric on those surfaces. The rotational transform of the field and complexity measures for the coils are also controlled in the design problem. Using an adjoint approach, we are able to obtain analytic derivatives for this optimization problem, yielding an efficient gradient-based algorithm. Starting from an initial coil set that presents nested magnetic surfaces for a large fraction of the volume, our method converges rapidly to coil systems generating fields with excellent quasisymmetry and low particle losses. In particular for low complexity coils, we are able to significantly improve the performance compared with coils obtained from the standard two-stage approach, e.g., reduce losses of fusion-produced alpha particles born at half-radius from 17.7 % to 6.6 %. We also demonstrate 16-coil configurations with alpha loss <1 % and neoclassical transport magnitude ε3/2eff less than approximately 5 × 10−9.
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Publication: Astrophys. J. 926, 218 (2022)Doi: 10.3847/1538-4357/ac4028
We analyze the structure and evolution of ribbons from the M7.3 SOL2014-04-18T13 flare using ultraviolet images from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), magnetic data from the SDO/Helioseismic and Magnetic Imager, hard X-ray (HXR) images from the Reuven Ramaty High Energy Solar Spectroscopic Imager, and light curves from the Fermi/Gamma-ray Burst Monitor, in order to infer properties of coronal magnetic reconnection. As the event progresses, two flare ribbons spread away from the magnetic polarity inversion line. The width of the newly brightened front along the extension of the ribbon is highly intermittent in both space and time, presumably reflecting nonuniformities in the structure and/or dynamics of the flare current sheet. Furthermore, the ribbon width grows most rapidly in regions exhibiting concentrated nonthermal HXR emission, with sharp increases slightly preceding the HXR bursts. The light curve of the ultraviolet emission matches the HXR light curve at photon energies above 25 keV. In other regions the ribbon-width evolution and light curves do not temporally correlate with the HXR emission. This indicates that the production of nonthermal electrons is highly nonuniform within the flare current sheet. Our results suggest a strong connection between the production of nonthermal electrons and the locally enhanced perpendicular extent of flare ribbon fronts, which in turn reflects the inhomogeneous structure and/or reconnection dynamics of the current sheet. Despite this variability, the ribbon fronts remain nearly continuous, quasi-one-dimensional features. Thus, although the reconnecting coronal current sheets are highly structured, they remain quasi-two-dimensional and the magnetic energy release occurs systematically, rather than stochastically, through the volume of the reconnecting magnetic flux.
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Publication: Phys. Rev. Appl. 18, 044006 (2022)Doi: 10.1103/PhysRevApplied.18.044006
A common scientific inverse problem is the placement of magnets that produce a desired magnetic field inside a prescribed volume. This is a key component of stellarator design and recently permanent magnets have been proposed as a potentially useful tool for magnetic field shaping. Here, we take a closer look at possible objective functions for permanent-magnet optimization, reformulate the problem as sparse regression, and propose an algorithm that can efficiently solve many convex and nonconvex variants. The algorithm generates sparse solutions that are independent of the initial guess, explicitly enforces maximum strengths for the permanent magnets, and accurately produces the desired magnetic field. The algorithm is flexible, and our implementation is open source and computationally fast. We conclude with two permanent-magnet configurations for the NCSX and MUSE stellarators. Our methodology can be additionally applied for effectively solving permanent-magnet optimizations in other scientific fields, as well as for solving quite general high-dimensional constrained sparse-regression problems, even if a binary solution is required.
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Publication: J. Lightwave Technol. 40, 7060 (2022)Doi: 10.1109/JLT.2022.3198967
Radiofrequency (RF) fingerprinting refers to a range of technologies that recognize transmitters by their intrinsic hardware-level characteristics. These characteristics are often introduced during the fabrication process and form a unique fingerprint of the transmitter that is very hard to counterfeit. RF fingerprinting often serves as a security measure at the physical-layer of communication networks against potentials attacks. In recent years, neuromorphic computing techniques such as convolutional neural networks (CNNs) have been explored as classifiers for RF fingerprinting. However, in radiofrequency communication networks, the transmitted signals are I/Q modulated on multi-GHz carriers while most conventional machine learning algorithms operate at the baseband. Therefore, the I/Q modulated signals have to be demodulated and converted into compatible formats before applying to these platforms – a procedure that inevitably slows down the processing speed. Moreover, the deep learning technologies often require a large amount of data to train the artificial neural networks (ANNs) while in practice, the available amount of data for a new transmitter is limited. Reservoir computing (RC) provides a relatively simple yet powerful structure that is capable of reaching state-of-the-art performance on several benchmarks. However, traditional digital RC also operates at baseband, which is not suitable for directly processing the I/Q modulated signals. In this article, we propose a reservoir computer based on narrowband optoelectronic oscillator (OEO) that can be utilized to directly classify I/Q modulated signals without the need for demodulation. We successfully train and test our narrowband OEO-based RC on three publicly available benchmarks, namely the FIT/CorteXlab RF fingerprinting dataset, the ORACLE RF fingerprinting dataset, and the AirID RF fingerprinting dataset. We show that for all three datasets, the narrowband OEO-based RC demonstrates competing accuracy with much less training data comparing to CNNs, and achieves an accuracy as high as 97%.
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Publication: PhotoniX 3, 26 (2022)Doi: 10.1186/x43074-022-00070-4
Optical phase shifters constitute the fundamental building blocks that enable programmable photonic integrated circuits (PICs)—the cornerstone of on-chip classical and quantum optical technologies. Thus far, carrier modulation and thermo-optical effect are the chosen phenomena for ultrafast and low-loss phase shifters, respectively; however, the state and information they carry are lost once the power is turned off—they are volatile. The volatility not only compromises energy efficiency due to their demand for constant power supply, but also precludes them from emerging applications such as in-memory computing. To circumvent this limitation, we introduce a phase shifting mechanism that exploits the nonvolatile refractive index modulation upon structural phase transition of Sb2Se3, a bi-state transparent phase change material (PCM). A zero-static power and electrically-driven phase shifter is realized on a CMOS-backend silicon-on-insulator platform, featuring record phase modulation up to 0.09 π/µm and a low insertion loss of 0.3 dB/π, which can be further improved upon streamlined design. Furthermore, we demonstrate phase and extinction ratio trimming of ring resonators and pioneer a one-step partial amorphization scheme to enhance speed and energy efficiency of PCM devices. A diverse cohort of programmable photonic devices is demonstrated based on the ultra-compact PCM phase shifter.
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Publication: Astrophys. J. 926, 1 (2022)Doi: 10.3847/1538-4357/ac423b
The formation, development, and impact of slow shocks in the upstream regions of reconnecting current layers are explored. Slow shocks have been documented in the upstream regions of magnetohydrodynamic (MHD) simulations of magnetic reconnection as well as in similar simulations with the kglobal kinetic macroscale simulation model. They are therefore a candidate mechanism for preheating the plasma that is injected into the current layers that facilitate magnetic energy release in solar flares. Of particular interest is their potential role in producing the hot thermal component of electrons in flares. During multi-island reconnection, the formation and merging of flux ropes in the reconnecting current layer drives plasma flows and pressure disturbances in the upstream region. These pressure disturbances steepen into slow shocks that propagate along the reconnecting component of the magnetic field and satisfy the expected Rankine–Hugoniot jump conditions. Plasma heating arises from both compression across the shock and the parallel electric field that develops to maintain charge neutrality in a kinetic system. Shocks are weaker at lower plasma β, where shock steepening is slow. While these upstream slow shocks are intrinsic to the dynamics of multi-island reconnection, their contribution to electron heating remains relatively minor compared with that from Fermi reflection and the parallel electric fields that bound the reconnection outflow.
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Publication: IEEE Photon. J. 14, 3042904 (2022)Doi: 10.1109/JPHOT.2022.3192490
Kerr optical frequency combs have found various applications in science and technology, and minimizing their pump power has become an important area of research. These combs are generated using a wide variety of platforms, with a size ranging from micrometers to millimeters, and quality factors ranging from millions to billions. It is therefore not trivial to assess the pump power requirements for comb generation when they have such a large diversity in terms of resonator properties and pump configurations. We propose a suitably normalized threshold pump power as a metric to optimize Kerr comb generation independently of the platform. This method allows one to evaluate the minimum threshold power solely based on the properties of the bare resonator, and independently of dispersion, detuning or coupling considerations. In order to confirm the validity of this approach, we experimentally demonstrate Kerr comb generation in a millimeter-size magnesium fluoride whispering-gallery mode resonator with a threshold pump power of only 1.2 mW, which is one of the lowest pump powers reported to date for a mm-size resonator.
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Publication: J. Phys. D: Appl. Phys. 55, 155202 (2022)Doi: 10.1088/1361-6463/ac4724
Atmospheric pressure plasma has shown promise in improving thermally activated catalytic reactions through a process termed plasma-catalysis synergy. In this work, we investigated atmospheric pressure plasma jet (APPJ)-assisted CH4 oxidation over a Ni/SiO2.Al2O3 catalyst. Downstream gas-phase products from CH4 conversion were quantified by Fourier transform infrared spectroscopy. The catalyst near-surface region was characterized by in-situ diffuse reflectance infrared Fourier transform spectroscopy. The catalyst was observed to be activated at elevated temperature (500 °C) if it was exposed to the APPJ operated at large plasma power. 'Catalyst activation' signifies that the purely thermal conversion of CH4 using catalysts which had been pre-exposed to plasma became more intense and produced consistently CO product, even if the plasma was extinguished. Without the application of the APPJ to the Ni catalyst surface this was not observed at 500 °C. The study of different exposure conditions of the activated catalyst indicates that the reduction of the catalyst by the APPJ is likely the cause of the catalyst activation. We also observed a systematic shift of the vibrational frequency of adsorbed CO on Ni catalyst when plasma operating conditions and catalyst temperatures were varied and discussed possible explanations for the observed changes. This work provides insights into the plasma-catalyst interaction, especially catalyst modification in the plasma catalysis process, and potentially demonstrates the possibility of utilizing the surface CO as a local probe to understand the plasma-catalyst interaction and shed light on the complexity of plasma catalysis.
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Publication: Mater. Adv. 3, 8332 (2022)Doi: 10.1039/d2ma00776b
Li10GeP2S12 (LGPS) is a superionic conductor that has an ionic conductivity equivalent to conventional liquid electrolytes (∼10−2 S cm−1) and thus shows exceptional potential to fulfill the promise of solid-state batteries. Nonetheless, LGPS is chemically and electrochemically unstable against Li metal, decomposing into the thermodynamically favorable byproducts of Li3P, Li2S, and alloyed LixGe. Contact between Li metal and LGPS results in formation of high impedance interphase layers due to lithium diffusion into and subsequent reaction with the LGPS structure. Artificial solid electrolyte interphase (ASEI) layers are a promising route to mitigate and reduce the chemical reactivity of the LGPS surface. Here, we differentiate between static chemical degradation induced by LGPS-Li contact, from electrochemical degradation induced via galvanostatic cycling of Li/LGPS/Li cells as critical to rational ASEI evaluation. From this perspective, we utilize a thin ASEI coating of lithium phosphorous oxynitride (LiPON), deposited by atomic layer deposition (ALD), to mitigate both chemical and electrochemical degradation at the Li/LGPS interface.
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Publication: J. Comp. Phys. 459, 111147 (2022)Doi: 10.1016/j.jcp.2022.111147
We present a new coil design paradigm for magnetic confinement in stellarators. Our approach directly optimizes coil shapes and coil currents to produce a vacuum quasi-symmetric magnetic field with a target rotational transform on the magnetic axis. This approach differs from the traditional two-stage approach in which first a magnetic configuration with desirable physics properties is found, and then coils to approximately realize this magnetic configuration are designed. The proposed single-stage approach allows us to find a compromise between confinement and engineering requirements, i.e., find easy-to-build coils with good confinement properties. Using forward and adjoint sensitivities, we derive derivatives of the physical quantities in the objective, which is constrained by a nonlinear periodic differential equation. In two numerical examples, we compare different gradient-based descent algorithms and find that incorporating approximate second-order derivative information through a quasi-Newton method is crucial for convergence. We also explore the optimization landscape in the neighborhood of a minimizer and find many directions in which the objective is mostly flat, indicating ample freedom to find simple and thus easy-to-build coils.
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Publication: arXiv (2022)Doi: 10.48550/arXiv.2210.08142
Optical phase-change materials are highly promising for emerging applications such as tunable metasurfaces, reconfigurable photonic circuits, and non-von Neumann computing. However, these materials typically require both high melting temperatures and fast quenching rates to reversibly switch between their crystalline and amorphous phases, a significant challenge for large-scale integration. Here, we present an experimental technique which leverages the thermo-optic effect in GST to enable both spatial and temporal thermal measurements of two common electro-thermal microheater designs currently used by the phase-change community. Our approach shows excellent agreement between experimental results and numerical simulations and provides a non-invasive method for rapid characterization of electrically programmable phase-change devices.
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Publication: ACM J. Emerg. Technol. Comput. Syst. 18, 67 (2022)Doi: 10.1145/3531010
This article presents a method for hardware trojan detection in integrated circuits. Unsupervised deep learning is used to classify wide field-of-view (4 × 4 mm2), high spatial resolution magnetic field images taken using a Quantum Diamond Microscope (QDM). QDM magnetic imaging is enhanced using quantum control techniques and improved diamond material to increase magnetic field sensitivity by a factor of 4 and measurement speed by a factor of 16 over previous demonstrations. These upgrades facilitate the first demonstration of QDM magnetic field measurement for hardware trojan detection. Unsupervised convolutional neural networks and clustering are used to infer trojan presence from unlabeled data sets of 600 × 600 pixel magnetic field images without human bias. This analysis is shown to be more accurate than principal component analysis for distinguishing between field programmable gate arrays configured with trojan-free and trojan-inserted logic. This framework is tested on a set of scalable trojans that we developed and measured with the QDM. Scalable and TrustHub trojans are detectable down to a minimum trojan trigger size of 0.5% of the total logic. The trojan detection framework can be used for golden-chip-free detection, since knowledge of the chips’ identities is only used to evaluate detection accuracy.
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Publication: Phys. Rev. A 106, L030402 (2022)Doi: 10.1103/PhysRevA.106.L030402
Defects in wide-band-gap semiconductors are promising qubit candidates for quantum communication and computation. Epitaxially grown II-VI semiconductors are particularly promising host materials due to their direct band gap and potential for isotopic purification to a spin-zero nuclear background. Here, we show an alternative type of single photon emitter with potential electron spin qubits based on Cl impurities in ZnSe. We utilize a quantum well to increase the binding energies of donor emission and confirm single photon emission with short radiative lifetimes of 192 ps. Furthermore, we verify that the ground state of the Cl donor complex contains a single electron by observing two-electron-satellite emission, leaving the electron in higher orbital states. We also characterize the Zeeman splitting of the exciton transition by performing polarization-resolved magnetic spectroscopy on single emitters. Our results suggest single Cl impurities are suitable as a single photon source with a potential photonic interface.
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Publication: J. Lightwave Technol. 40, 6131 (2022)Doi: 10.1109/JLT.2022.3190695
Narrowband optoelectronic oscillators are time-delayed system that can generate ultrapure microwave signals. In order to study these oscillators, one approach is to derive a time-delayed envelope equation. In this article, we show that this approach is universal and applies to a wide variety of OEO families. By studying different oscillators, it is possible to show that their dynamics is governed by an envelope equation that can be rewritten under a generalized universal form. In this article, this universal microwave envelope equation is proposed and investigated analytically for optoelectronic microwave oscillators, and the bifurcation analysis of the deterministic envelope equation is performed. The universal model is further generalized and written in a stochastic form in order to predict the phase noise performance of the oscillators.
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Publication: J. Infrared Millim. Terahertz Waves 43, 1 (2022)Doi: 10.1007/s10762-022-00845-7
The studies on mode interaction in gyrotrons had always been an active field of research due to their theoretical and practical importance for better understanding of the underlying physical principles and the development and optimization of various gyro-devices. However, lately, the problems that stem from the mode interaction have become more pronounced and severe due to the recently demonstrated advancement of gyrotrons towards higher (terahertz) frequencies at which, for keeping the same power level, the gyrotrons should operate in higher order modes. So the mode spectrum of the gyrotron cavity is significantly denser, and, hence, the mode competition is inevitable. In this overview, we present both the evolution and the progress of these investigations that assist the further development of high-performance sub-THz and THz gyrotrons for numerous novel and emerging applications in the broad fields of science and technologies. The targeted readership of this paper includes not only the experts in gyrotron development but rather a wider community of specialists working on other vacuum microwave devices seeking a synergy between different research fields.
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Publication: Nanoscale Horiz. 7, 267 (2022)Doi: 10.1039/d1nh00497b
Developments in the field of nanoplasmonics have the potential to advance applications from information processing and telecommunications to light-based sensing. Traditionally, nanoscale noble metals such as gold and silver have been used to achieve the targeted enhancements in light-matter interactions that result from the presence of localized surface plasmons (LSPs). However, interest has recently shifted to intrinsically doped semiconductor nanocrystals (NCs) for their ability to display LSP resonances (LSPRs) over a much broader spectral range, including the infrared (IR). Among semiconducting plasmonic NCs, spinel metal oxides (sp-MOs) are an emerging class of materials with distinct advantages in accessing the telecommunications bands in the IR and affording useful environmental stability. Here, we report the plasmonic properties of Fe3O4 sp-MO NCs, known previously only for their magnetic functionality, and demonstrate their ability to modify the light-emission properties of telecom-emitting quantum dots (QDs). We establish the synthetic conditions for tuning sp-MO NC size, composition and doping characteristics, resulting in unprecedented tunability of electronic behavior and plasmonic response over 450 nm. In particular, with diameter-dependent variations in free-electron concentration across the Fe3O4 NC series, we introduce a strong NC size dependency onto the optical response. In addition, our observation of plasmonics-enhanced decay rates from telecom-emitting QDs reveals Purcell enhancement factors for simple plasmonic-spacer-emitter sandwich structures up to 51-fold, which are comparable to values achieved previously only for emitters in the visible range coupled with conventional noble metal NCs.
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SiO2 Etching and Surface Evolution Using Combined Exposure to CF4/O2 Remote Plasma and Electron Beam
Publication: J. Vac. Sci. Technol. A: Vacuum, Surfaces, and Films 40, 063004 (2022)Doi: 10.1116/6.0002038Electron-based surface activation of surfaces functionalized by remote plasma appears like a flexible and novel approach to atomic scale etching and deposition. Relative to plasma-based dry etching that uses ion bombardment of a substrate to achieve controlled material removal, electron beam-induced etching (EBIE) is expected to reduce surface damage, including atom displacement, surface roughness, and undesired material removal. One of the issues with EBIE is the limited number of chemical precursors that can be used to functionalize material surfaces. In this work, we demonstrate a new configuration that was designed to leverage flexible surface functionalization using a remote plasma source, and, by combining with electron beam bombardment to remove the chemically reacted surface layer through plasma-assisted electron beam-induced etching, achieve highly controlled etching. This article describes the experimental configuration used for this demonstration that consists of a remote plasma source and an electron flood gun for enabling electron beam-induced etching of SiO2 with Ar/CF4/O2 precursors. We evaluated the parametric dependence of SiO2 etching rate on processing parameters of the flood gun, including electron energy and emission current, and of the remote plasma source, including radiofrequency source power and flow rate of CF4/O2, respectively. Additionally, two prototypical processing cases were demonstrated by temporally combining or separating remote plasma treatment and electron beam irradiation. The results validate the performance of this approach for etching applications, including photomask repair and atomic layer etching of SiO2. Surface characterization results that provide mechanistic insights into these processes are also presented and discussed.
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Stochastic and a posteriori Optimization to Mitigate Coil Manufacturing Errors in Stellarator Design
Publication: Nucl. Fusion 62, 076034 (2022)Doi: 10.1088/1741-4326/ac45f3It was recently shown in Wechsung et al (2022 Proc. Natl Acad. Sci. USA 119 e2202084119) that there exist electromagnetic coils that generate magnetic fields, which are excellent approximations to quasi-symmetric fields and have very good particle confinement properties. Using a Gaussian process-based model for coil perturbations, we investigate the impact of manufacturing errors on the performance of these coils. We show that even fairly small errors result in noticeable performance degradation. While stochastic optimization yields minor improvements, it is not possible to mitigate these errors significantly. As an alternative to stochastic optimization, we then formulate a new optimization problem for computing optimal adjustments of the coil positions and currents without changing the shapes of the coil. These a-posteriori adjustments are able to reduce the impact of coil errors by an order of magnitude, providing a new perspective for dealing with manufacturing tolerances in stellarator design.
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Publication: Phys. Rev. Appl. 18, 014107 (2022)Doi: 10.1103/PhysRevApplied.18.014017
Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments have been shown to clear foggy regions, improving the transmission of subsequent pulses. However, the detailed fog-clearing mechanism had yet to be determined. Here, we directly measure and simulate the dynamics of water droplets with a radius of about 5 μm, typical of fog, under the influence of optical and acoustic interactions that are characteristic of femtosecond filaments. We find that, for filaments generated by the collapse of collimated near-infrared femtosecond pulses, the main droplet-clearing mechanism is optical shattering by laser light. For such filaments, the single-cycle acoustic wave launched by filament-energy deposition in air leaves droplets intact and drives negligible transverse displacement, and therefore, negligible fog clearing. Only for tightly focused nonfilamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.
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Publication: Dalton Trans. 51, 2068 (2022)Doi: 10.1039/D1DT03736f
A key trailblazer in the development of thin-film solid-state electrolytes has been lithium phosphorous oxynitride (LiPON), the success of which has led to recent progress in thin-film ion conductors. Here we compare the structural, electrochemical, and processing parameters between previously published LiPON and NaPON ALD processes with a novel ALD process for the K analogue potassium phosphorous oxynitride (KPON). In each ALD process, alkali tert-butoxides and diethylphosphoramidate are used as precursors. To understand the ALD surface reactions, this work proposes a reaction mechanism determined by in-operando mass spectrometry for the LiPON process as key to understanding the characteristics of the APON (A = Li, Na, K) family. As expected, NaPON and LiPON share similar reaction mechanisms as their structures are strikingly similar. KPON, however, exhibits similar ALD process parameters but the resulting film composition is quite different, showing little nitrogen incorporation and more closely resembling a phosphate glass. Due to the profound difference in structure, KPON likely undergoes an entirely different reaction mechanism. This paper presents a comprehensive summary of ALD ion conducting APON films as well as a perspective that highlights the versatility of ALD chemistries as a tool for the development of novel thin film ion-conductors.
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Publication: Adv. Energy Mater. 13, 2202209 (2022)Doi: 10.1002/aenm.202202209
Metal halide perovskite solar cells (PSCs) have emerged as an important direction for photovoltaic research. Although the power conversion efficiency (PCE) of lead-based PSCs has reached 25.7%, still the toxicity of Pb remains one main obstacle for commercial adoption. Thus, to address this issue, Pb-free perovskites have been proposed. Among them, tin-based perovskites have emerged as promising candidates. Unfortunately, the fast oxidation of Sn2+ to Sn4+ leads to low stability and efficiency. Many strategies have been implemented to address these challenges in Sn-based PSCs. This work introduces stability and efficiency improvement strategies for pure Sn-based PSCs by optimization of the crystal structure, processing and interfaces as well as, implementation of low-dimension structures. Finally, new perspectives for further developing Sn-based PSCs are provided.
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Publication: Phys. Plasma 29, 042505 (2022)Doi: 10.1063/5.0080809
Good magnetic surfaces, as opposed to magnetic islands and chaotic field lines, are generally desirable for stellarators. In previous work, Landreman et al. [Phys. of Plasmas 28, 092505 (2021)] showed that equilibria computed by the Stepped-Pressure Equilibrium Code (SPEC) [Hudson et al., Phys. Plasmas 19, 112502 (2012)] could be optimized for good magnetic surfaces in vacuum. In this paper, we build upon their work to show the first finite-β, fixed-, and free-boundary optimization of SPEC equilibria for good magnetic surfaces. The objective function is constructed with the Greene's residue of selected rational surfaces, and the optimization is driven by the SIMSOPT framework [Landreman et al., J. Open Source Software 6, 3525 (2021)]. We show that the size of magnetic islands and the consequent regions occupied by chaotic field lines can be minimized in a classical stellarator geometry (rotating ellipse) by optimizing either the injected toroidal current profile, the shape of a perfectly conducting wall surrounding the plasma (fixed-boundary case), or the vacuum field produced by the coils (free-boundary case). This work shows that SPEC can be used as an equilibrium code both in a two-step or single-step stellarator optimization loop.
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Publication: Nanophoton. 11, 289 (2022)Doi: 10.1515/nanoph-2021-0365
Miniaturized, low-cost wavelength detectors are gaining enormous interest as we step into the new age of photonics. Incompatibility with integrated circuits or complex fabrication requirement in most of the conventionally used filters necessitates the development of a simple, on-chip platform for easy-to-use wavelength detection system. Also, intensity fluctuations hinder precise, noise free detection of spectral information. Here we propose a novel approach of utilizing wavelength sensitive photocurrent across semiconductor heterojunctions to experimentally validate broadband wavelength detection on an on-chip platform with simple fabrication process. The proposed device utilizes linear frequency response of internal photoemission via 2-D electron gas in a ZnO based heterojunction along with a reference junction for coherent common mode rejection. We report sensitivity of 0.96 μA/nm for a broad wavelength-range of 280 nm from 660 to 940 nm. Simple fabrication process, efficient intensity noise cancelation along with heat resistance and radiation hardness of ZnO makes the proposed platform simple, low-cost and efficient alternative for several applications such as optical spectrometers, sensing, and Internet of Things (IOTs).
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Publication: IEEE Photon. J. 14, 7607309 (2022)Doi: 10.1109/JPHOT.2021.3136502
Trapped ions are excellent candidates for quantum computing and quantum networks because of their long coherence times, ability to generate entangled photons as well as high fidelity single- and two-qubit gates. To scale up trapped ion quantum computing, we need a Bell-state analyzer on a reconfigurable platform that can herald high fidelity entanglement between ions. In this work, we design a photonic Bell-state analyzer on a reconfigurable thin-film lithium niobate platform for polarization-encoded qubits. We optimize the device to achieve high fidelity entanglement between two trapped ions and find >99% fidelity. Apart from that, the directional coupler used in our design can achieve any polarization-independent power splitting ratio which can have a rich variety of applications in the integrated photonic technology. The proposed device can scale up trapped ion quantum computing as well as other optically active spin qubits, such as color centers in diamond, quantum dots, and rare-earth ions.
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Publication: Phys. Rev. Accel. Beams 25, 044002 (2022)Doi: 10.1103/PhysRevAccelBeams.25.044002
A continuous system of moment equations is introduced that models the transverse dynamics of a beam of charged particles as it passes through an arbitrary lattice of quadrupoles and solenoids in the presence of self-fields. Then, figures of merit are introduced specifying system characteristics to be optimized. The resulting model is used to optimize the parameters of the lattice elements of a flat to round transformer with self-fields, as could be applied in electron cooling. Results are shown for a case of no self-fields and two cases with self-fields. The optimization is based on a gradient descent algorithm in which the gradient is calculated using adjoint methods that prove to be very computationally efficient. Two figures of merit are studied and compared: one emphasizing radial force balance in the solenoid, the other emphasizing minimization of transverse beam energy in the solenoid.
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Publication: Phys. Rev. Res. 4, 023167 (2022)Doi: 10.1103/PhysRevResearch.4.023167
Machine learning (ML) has found widespread application over a broad range of important tasks. To enhance ML performance, researchers have investigated computational architectures whose physical implementations promise compactness, high-speed execution, physical robustness, and low-energy cost. Here, we experimentally demonstrate an approach that uses the high sensitivity of reverberant short-wavelength waves for physical realization and enhancement of computational power of a type of ML known as reservoir computing (RC). The potential computation power of RC systems increases with their effective size. We here exploit the intrinsic property of short-wavelength reverberant wave sensitivity to perturbations to expand the effective size of the RC system by means of spatial and spectral perturbations. Working in the microwave regime, this scheme is tested experimentally on different ML tasks. Our results indicate the general applicability of reverberant wave based implementations of RC and of our effective reservoir size expansion techniques.
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Publication: Phys. Today 75, 62 (2022)Doi: 10.1063/PT.3.4948
After Theodore Maiman’s demonstration of the laser in 1960, researchers quickly discovered that tightly focused laser pulses generated a bright spark of ionized air. The initial reports caught the physics community off guard; in the words of an early researcher, C. Grey Morgan, a “flash of laser light can set the air on fire!” Because each laser photon didn’t have enough energy to knock an electron off an air molecule, it should have been impossible for the laser to ionize the air directly. Eventually, researchers realized that the extremely high electric fields at the laser’s focus were driving an electron avalanche breakdown, an already well-known process using high static fields and high-power microwaves. An initial population of free electrons gains energy by acceleration in the laser field, ionizing other molecules in a cascading, exponential process. The source of the initial population of electrons was a mystery, however, and it spurred pioneering theoretical work by Leonid Keldysh. In the quantum or multiphoton ionization (MPI) limit—at moderate intensity and short laser wavelength—of the theory, an electron is liberated when an atom absorbs many photons simultaneously. In the semiclassical limit (at high intensity and long wavelength), the laser’s large electric field pulls electrons out of atoms by tunneling ionization. With the basic process understood, researchers rushed to apply laser-driven avalanche breakdowns to such varied fields as breakdown spectroscopy, fast switching of high voltages, laser surgery, and laser machining. In this Quick Study, we recount the physics governing the laser-driven sparks and show how revisiting early experiments with new technology has uncovered the ability to pinpoint individual electrons in ambient gases.
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Publication: Phys. Rev. Appl. 18, 014039 (2022)Doi: 10.1103/PhysRevApplied.18.014039
The increasing scale of neural networks and their growing application space have produced demand for more energy- and memory-efficient artificial-intelligence-specific hardware. Avenues to mitigate the main issue, the von Neumann bottleneck, include in-memory and near-memory architectures, as well as algorithmic approaches. Here we leverage the low-power and the inherently binary operation of magnetic tunnel junctions (MTJs) to demonstrate neural network hardware inference based on passive arrays of MTJs. In general, transferring a trained network model to hardware for inference is confronted by degradation in performance due to device-to-device variations, write errors, parasitic resistance, and nonidealities in the substrate. To quantify the effect of these hardware realities, we benchmark 300 unique weight matrix solutions of a two-layer perceptron to classify the Wine dataset for both classification accuracy and write fidelity. Despite device imperfections, we achieve software-equivalent accuracy of up to 95.3% with proper tuning of network parameters in 15 × 15 MTJ arrays having a range of device sizes. The success of this tuning process shows that new metrics are needed to characterize the performance and quality of networks reproduced in mixed signal hardware.
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Publication: Phys. Plasmas 29, 052904 (2022)Doi: 10.1063/5.0085647
Electrons in earth's magnetotail are energized significantly both in the form of heating and in the form of acceleration to non-thermal energies. While magnetic reconnection is considered to play an important role in this energization, it still remains unclear how electrons are energized and how energy is partitioned between thermal and non-thermal components. Here, we show, based on in situ observations by NASA's Magnetospheric Multiscale mission combined with multi-component spectral fitting methods, that the average electron energy ε¯ (or equivalently temperature) is substantially higher when the locally averaged electric field magnitude |E| is also higher. While this result is consistent with the classification of “plasma-sheet” and “tail-lobe” reconnection during which reconnection is considered to occur on closed and open magnetic field lines, respectively, it further suggests that a stochastic Fermi acceleration in 3D, reconnection-driven turbulence is essential for the production and confinement of energetic electrons in the reconnection region. The puzzle is that the non-thermal power-law component can be quite small even when the electric field is large and the bulk population is significantly heated. The fraction of non-thermal electron energies varies from sample to sample between ∼20% and ∼60%, regardless of the electric field magnitude. Interestingly, these values of non-thermal fractions are similar to those obtained for the above-the-looptop hard x-ray coronal sources for solar flares.
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Publication: J. Geophys. Res. 127, e2022JA030610 (2022)Doi: 10.1029/2022JA030610
Electron ring velocity space distributions have previously been seen in numerical simulations of magnetic reconnection exhausts and have been suggested to be caused by the magnetization of the electron outflow jet by the compressed reconnected magnetic fields (Shuster et al., 2014, https://doi.org/10.1002/2014GL060608). We present a theory of the dependence of the major and minor radii of the ring distributions solely in terms of upstream (lobe) plasma conditions, thereby allowing a prediction of the associated temperature and temperature anisotropy of the rings in terms of upstream parameters. We test the validity of the prediction using 2.5-dimensional particle-in-cell (PIC) simulations with varying upstream plasma density and temperature, finding excellent agreement between the predicted and simulated values. We confirm the Shuster et al. suggestion for the cause of the ring distributions, and also find that the ring distributions are located in a region marked by a plateau, or shoulder, in the reconnected magnetic field profile. The predictions of the temperature are consistent with observed electron temperatures in dipolarization fronts, and may provide an explanation for the generation of plasma with temperatures in the 10s of MK in super-hot solar flares. A possible extension of the model to dayside reconnection is discussed. Since ring distributions are known to excite whistler waves, the present results should be useful for quantifying the generation of whistler waves in reconnection exhausts.
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Publication: Sensors Actuators A-Physical 342, 113659 (2022)Doi: 10.1016/j.sna.2022.113659
We have developed a highly stretchable and flexible strain sensor based on eutectic gallium indium (EGaIn) and EcoFlex through a simple and low-cost fabrication process. The sensor has two different sensing channels in a single device with novel architecture that can allow fast and effective detections simultaneously from the different joint movements. The sensor is tested with several different angles up to 90° (strain: 250 %) using a 3D printed test setup to imitate joint movements, and it shows superior performance with an exceptional signal-to-noise ratio (69 dB), gauge factor (~3), measurement resolution (0.43 %), and response/recovery times (0.4 s/0.2 s), leveraging the high conductivity of EGaIn and excellent deformability of EcoFlex. Furthermore, the sensor successfully demonstrates the motions of a human finger as a practical application.
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Publication: Opt. Express 30, 39431 (2022)Doi: 10.1364/OE.466330
Single photon detectors have dark count rates that depend strongly on the bias level for detector operation. In the case of weak light sources such as novel lasers or single-photon emitters, the rate of counts due to the light source can be comparable to that of the detector dark counts. In such cases, a characterization of the statistical properties of the dark counts is necessary. The dark counts are often assumed to follow a Poisson process that is statistically independent of the incident photon counts. This assumption must be validated for specific types of photodetectors. In this work, we focus on single-photon avalanche photodiodes (SPADs) made for 1550 nm. For the InGaAs detectors used, we find the measured distributions often differ significantly from Poisson due to the presence of dead time and afterpulsing with the difference increasing with the bias level used for obtaining higher quantum efficiencies. We find that when the dead time is increased to remove the effects of afterpulsing, it is necessary to correct the measured distributions for the effects of the dead time. To this end, we apply an iterative algorithm to remove dead time effects from the probability distribution for dark counts as well as for the case where light from an external weak laser source (known to be Poisson) is detected together with the dark counts. We believe this to be the first instance of the comprehensive application of this algorithm to real data and find that the dead time corrected probability distributions are Poisson distributions in both cases. We additionally use the Grassberger-Procaccia algorithm to estimate the entropy production rates of the dark count processes, which provides a single metric that characterizes the temporal correlations between dark counts as well as the shape of the distribution. We have thus developed a systematic procedure for taking data with 1550 nm SPADs and obtaining accurate photocount statistics to examine novel light sources.
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Publication: Phys. Plasmas 29, 032101 (2022)Doi: 10.1063/5.0082633
Using incoherent Thomson scattering, electron heating and acceleration at the electron velocity distribution function (EVDF) level are investigated during electron-only reconnection in the PHAse Space MApping (PHASMA) facility. Reconnection arises during the merger of two kink-free flux ropes. Both push and pull type reconnection occur in a single discharge. Electron heating is localized around the separatrix, and the electron temperature increases continuously along the separatrix with distance from the X-line. The local measured gain in enthalpy flux is up to 70% of the incoming Poynting flux. Notably, non-Maxwellian EVDFs comprised of a warm bulk population and a cold beam are directly measured during the electron-only reconnection. The electron beam velocity is comparable to, and scales with, electron Alfvén speed, revealing the signature of electron acceleration caused by electron-only reconnection. The observation of oppositely directed electron beams on either side of the X-point provides “smoking-gun” evidence of the occurrence of electron-only reconnection in PHASMA. 2D particle-in-cell simulations agree well with the laboratory measurements. The measured conversion of Poynting flux into electron enthalpy is consistent with recent observations of electron-only reconnection in the magnetosheath [Phan et al., Nature 557, 202 (2018)] at similar dimensionless parameters as in the experiments. The laboratory measurements go beyond the magnetosheath observations by directly resolving the electron temperature gain.
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Publication: AVS Quantum Sci. 4, 044701 (2022)Doi: 10.1116/5.0117301
Next-generation dark matter (DM) detectors searching for weakly interacting massive particles (WIMPs) will be sensitive to coherent scattering from solar neutrinos, demanding an efficient background-signal discrimination tool. Directional detectors improve sensitivity to WIMP DM despite the irreducible neutrino background. Wide-bandgap semiconductors offer a path to directional detection in a high-density target material. A detector of this type operates in a hybrid mode. The WIMP or neutrino-induced nuclear recoil is detected using real-time charge, phonon, or photon collection. The directional signal, however, is imprinted as a durable sub-micron damage track in the lattice structure. This directional signal can be read out by a variety of atomic physics techniques, from point defect quantum sensing to x-ray microscopy. In this Review, we present the detector principle as well as the status of the experimental techniques required for directional readout of nuclear recoil tracks. Specifically, we focus on diamond as a target material; it is both a leading platform for emerging quantum technologies and a promising component of next-generation semiconductor electronics. Based on the development and demonstration of directional readout in diamond over the next decade, a future WIMP detector will leverage or motivate advances in multiple disciplines toward precision dark matter and neutrino physics.
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Electromagnetic Fireworks: Fast Radio Bursts from Rapid Reconnection in the Compressed Magnetar Wind
Publication: Astrophys. J. Lett. 932, L20 (2022)Doi: 10.3847/2041-8213/ac7156One scenario for the generation of fast radio bursts (FRBs) is magnetic reconnection in a current sheet of the magnetar wind. Compressed by a strong magnetic pulse induced by a magnetar flare, the current sheet fragments into a self-similar chain of magnetic islands. Time-dependent plasma currents at their interfaces produce coherent radiation during their hierarchical coalescence. We investigate this scenario using 2D radiative relativistic particle-in-cell simulations to compute the efficiency of the coherent emission and to obtain frequency scalings. Consistent with expectations, a fraction of the reconnected magnetic field energy, f ∼ 0.002, is converted to packets of high-frequency fast magnetosonic waves, which can escape from the magnetar wind as radio emission. In agreement with analytical estimates, we find that magnetic pulses of 1047 erg s−1 can trigger relatively narrowband GHz emission with luminosities of approximately 1042 erg s−1, sufficient to explain bright extragalactic FRBs. The mechanism provides a natural explanation for a downward frequency drift of burst signals, as well as the ∼100 ns substructure recently detected in FRB 20200120E .
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Publication: Astrophys. J. 933, 174 (2022)Doi: 10.3847/1538-4357/ac7529
The most common form of magnetar activity is short X-ray bursts, with durations from milliseconds to seconds, and luminosities ranging from 1036–1043 erg s−1. Recently, an X-ray burst from the galactic magnetar SGR 1935+2154 was detected to be coincident with two fast radio burst (FRB) like events from the same source, providing evidence that FRBs may be linked to magnetar bursts. Using fully 3D force-free electrodynamics simulations, we show that such magnetar bursts may be produced by Alfvén waves launched from localized magnetar quakes: a wave packet propagates to the outer magnetosphere, becomes nonlinear, and escapes the magnetosphere, forming an ultra-relativistic ejecta. The ejecta pushes open the magnetospheric field lines, creating current sheets behind it. Magnetic reconnection can happen at these current sheets, leading to plasma energization and X-ray emission. The angular size of the ejecta can be compact, ≲1 sr if the quake launching region is small, ≲0.01 sr at the stellar surface. We discuss implications for the FRBs and the coincident X-ray burst from SGR 1935+2154.
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Publication: Nature Commun. 13, 2954 (2022)Doi: 10.1038/s41467-022-30561-8
Coulomb collisions provide plasma resistivity and diffusion but in many low-density astrophysical plasmas such collisions between particles are extremely rare. Scattering of particles by electromagnetic waves can lower the plasma conductivity. Such anomalous resistivity due to wave-particle interactions could be crucial to many processes, including magnetic reconnection. It has been suggested that waves provide both diffusion and resistivity, which can support the reconnection electric field, but this requires direct observation to confirm. Here, we directly quantify anomalous resistivity, viscosity, and cross-field electron diffusion associated with lower hybrid waves using measurements from the four Magnetospheric Multiscale (MMS) spacecraft. We show that anomalous resistivity is approximately balanced by anomalous viscosity, and thus the waves do not contribute to the reconnection electric field. However, the waves do produce an anomalous electron drift and diffusion across the current layer associated with magnetic reconnection. This leads to relaxation of density gradients at timescales of order the ion cyclotron period, and hence modifies the reconnection process.
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Publication: Phys. Rev. B 106, 054110 (2022)Doi: 10.1103/PhysRevB.106.054110
Nitrogen-vacancy (NV) spin ensembles in diamond provide an advanced magnetic sensing platform, with applications in both the physical and life sciences. The development of isotopically engineered 15NV diamond offers advantages over naturally occurring 14NV for magnetometry, due to its simpler hyperfine structure. However, for sensing modalities requiring a bias magnetic field not aligned with the sensing NV axis, the absence of a quadrupole moment in the 15N nuclear spin leads to pronounced envelope modulation effects in time-dependent measurements of 15NV spin evolution. While such behavior in spin echo experiments are well studied, analogous effects in Ramsey measurements and the implications for magnetometry remain under-explored. Here, we derive the modulated 15NV Ramsey response to a misaligned bias field, using a simple vector description of the effective magnetic field on the nuclear spin. The predicted modulation properties are then compared to experimental results, revealing significant magnetic sensitivity loss if unaddressed. We demonstrate that double-quantum coherences of the NV S=1 electronic spin states dramatically suppress these envelope modulations, while additionally proving resilient to other parasitic effects such as strain heterogeneity and temperature shifts.
2021
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Publication: Opt. Lett. 46, 5449 (2021)Doi: 10.1364/OL.439232
We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 μm-wavelength Yb:YAG laser at repetition rates up to f=1kHz. Interferograms of the wake generated show that while pulses in a train of repetition rate f=0.1kHz encounter a nearly unperturbed environment, at f=1kHz, a channel with an axial air density hole of ∼20% is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at f=1kHz show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.
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Publication: J. Plasma Phys. 87, 905870214; PII S0022377821000283 (2021)Doi: 10.1017/S0022377821000283
Using recently developed adjoint methods for computing the shape derivatives of functions that depend on magnetohydrodynamic (MHD) equilibria (Antonsen et al., J. Plasma Phys., vol. 85, issue 2, 2019; Paul et al., J. Plasma Phys., vol. 86, issue 1, 2020), we present the first example of analytic gradient-based optimization of fixed-boundary stellarator equilibria. We take advantage of gradient information to optimize figures of merit of relevance for stellarator design, including the rotational transform, magnetic well and quasi-symmetry near the axis. With the application of the adjoint method, we reduce the number of equilibrium evaluations by the dimension of the optimization space (∼50--500∼50--500) in comparison with a finite-difference gradient-based method. We discuss regularization objectives of relevance for fixed-boundary optimization, including a novel method that prevents self-intersection of the plasma boundary. We present several optimized equilibria, including a vacuum field with very low magnetic shear throughout the volume.
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Publication: Phys. Rev. A 103, 023521 (2021)Doi: 10.1103/PhysRevA.103.023521
Kerr comb generation is usually based on the nonlinear dynamics of the intracavity field in a whispering-gallery-mode resonator pumped by a continuous-wave laser. However, using a pulsed instead of a continuous-wave pump opens an alternative research avenue from both the theoretical and experimental viewpoints, as it permits us to tailor the spectral properties of ultrashort pulse trains with a single passive nonlinear element. In this article we study the dynamics of Kerr optical frequency combs when the whispering-gallery-mode resonator is pumped by a synchronous pulse train. We propose a model that is based on an extension of the Lugiato-Lefever equation, which accounts for both the pulsed nature of the pump and the mismatch between the free-spectral range of the resonator and the repetition rate of the pulse train. We lay a particular emphasis on the effect of pump-cavity desynchronization on the spectral shape of the output combs. The numerical simulations are successfully compared with experimental measurements where the optical pulses are generated via time-lens soliton compression, and the resonator is a millimeter-size magnesium fluoride resonator with a billion quality factor at the pump wavelength.
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Publication: Phys. Rev. B 104, 054427 (2021)Doi: 10.1103/PhysRevB.104.054427
Superparamagnetic tunnel junctions (SMTJs) are promising sources for the randomness required by some compact and energy-efficient computing schemes. Coupling SMTJs gives rise to collective behavior that could be useful for cognitive computing. We use a simple linear electrical circuit to mutually couple two SMTJs through their stochastic electrical transitions. When one SMTJ makes a thermally induced transition, the voltage across both SMTJs changes, modifying the transition rates of both. This coupling leads to significant correlation between the states of the two devices. Using fits to a generalized Néel-Brown model for the individual thermally bistable magnetic devices, we can accurately reproduce the behavior of the coupled devices with a Markov model.
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Publication: Small Structures 2, 2100014 (2021)Doi: 10.1002/sstr.202100014
Lithium metal batteries bring greater promise for energy density, often relying on solid-state electrolytes to meet critical benchmarks. However, Li dendrite formation is a prevailing problem that limits the cycle life and Coulombic efficiency of solid-state Li metal batteries. For the first time, a thin (<100 nm) layer of electronically insulating, ionically conducting lithium phosphorus oxynitride (LiPON) is applied using atomic layer deposition between a Li anode and garnet Li7La3Zr2O12 (LLZO). The performance of a conformal LiPON layer as an electron barrier in symmetric Li-LLZO cells is observed through potential step chronoamperometry, galvanostatic cycling, electron microscopy, and various spectroscopic techniques. The LiPON-coated LLZO achieves 100 times lower electronic conductance than LLZO alone. Cycling carried out at 0.1 mA cm−2 for 100 cycles demonstrates that suppression of electron pathways into the bulk solid electrolyte improves the cycle life of a lithium metal cell. These findings suggest an electronic conductivity effect in solid-state electrolytes. A strategy is demonstrated to design thin-film (LiPON)-modulated bulk solid-state electrolytes (LLZO) capable of maintaining high ionic conductivity and electrochemical stability while reducing the effective electronic conductivity, which results in significantly decreased dendrite formation, improved cycle life, and greater interfacial integrity between the electrolyte and a Li anode.
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Publication: J. Microelectromechanical Systems 29, 846 (2021)Doi: 10.1109/JMEMS.2020.3007342
Real-time toxic gas mapping in complex urban environments have become increasingly possible with improvements in data analysis and network infrastructures. Hindering this is the cost and operation requirements of commercial gas sensors, requiring sensors with high sensitivity and selectivity that are robust and capable of operating at room temperature. Transition metal oxide-based sensors are of historical significance in the production of commercial gas sensors due to their low cost and high selectivity to target gases. The low inherent conductivity of metal oxides, however, requires operating temperatures higher than 150°C, limiting their operation to controlled environments. To overcome this limitation, heterostructures have been formed between graphene and transition metal oxides, seeking to couple the conductivity of graphene with the reactivity of transition metal oxides. Among these transition metal oxides, manganese dioxide exhibits unique properties that can be leveraged to improve gas sensing. Its wide variety of synthesized structural polymorphs (1 × 1 tunnel (β), 1 × 2 tunnel (α), spinel (y), and layered (δ)) allow for control over the available reactive surface area to enhance gas response. By utilizing defect rich δ-phase, the reactivity of the material can be improved. Here we present a δ-MnO2 /epitaxial graphene/silicon carbide heterostructure for use as a room temperature gas sensor. We confirm the composition through Raman spectroscopy and surface morphology through scanning electron microscopy and atomic force microscopy. We then demonstrate its room-temperature detection by testing against NO2 , NH3 , IPA, and CH3 OH at room temperature.
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Switchbacks as Signatures of Magnetic Flux Ropes Generated by Interchange Reconnection in the Corona
Publication: Astron. Astrophys. 650, A2 (2021)Doi: 10.1051/0004-6361/202039432The structure of magnetic flux ropes injected into the solar wind during reconnection in the coronal atmosphere is explored with particle-in-cell simulations and compared with in situ measurements of magnetic “switchbacks” from the Parker Solar Probe. We suggest that multi-x-line reconnection between open and closed flux in the corona injects flux ropes into the solar wind and that these flux ropes convect outward over long distances before eroding due to reconnection. Simulations that explore the magnetic structure of flux ropes in the solar wind reproduce the following key features of the switchback observations: a rapid rotation of the radial magnetic field into the transverse direction, which is a consequence of reconnection with a strong guide field; and the potential to reverse the radial field component. The potential implication of the injection of large numbers of flux ropes in the coronal atmosphere for understanding the generation of the solar wind is discussed.
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Publication: IEEE Micro 41, 71 (2021)Doi: 10.1109/MM.2021.3066377
Creating computing systems able to address our ever-increasing needs, especially as we reach the end of CMOS transistor scaling, will require truly novel methods of computing. However, the traditional logic abstractions and the digital design patterns we understand so well have coevolved with the hardware technology that has embodied them. As we look past CMOS, there is no reason to think that those same abstractions best serve to encapsulate the computational potential inherent to emerging devices. We posit that a new and radically more efficient foundation for computing lies at the intersection of superconductor electronics and delay-coded computation. Building on recent work in race logic, we show that superconducting circuits can naturally compute directly over temporal relationships between pulse arrivals; that the computational relationships between those pulse arrivals can be formalized through a functional extension to a temporal predicate logic used in the verification community; and that the resulting architectures can operate asynchronously and describe real and useful computations. We verify our hypothesis through a combination of detailed analog circuit models and layout designs, a formal analysis of our abstractions, and an evaluation of several superconducting temporal accelerators.
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Publication: Adv. Opt. Mater. 9, AI, 2100710 (2021)Doi: 10.1002/adom.202100710
Metal halide perovskites (MHP) can be made more stable through the addition of small amounts of cesium. Despite the improvement, these multication absorbers still display strong environmental sensitivity to any combination of factors, including water, oxygen, bias, temperature, and light. Here, the relationship is elucidated between light absorption, charge carrier radiative recombination, and relative humidity (rH) for the Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3 composition, revealing partially reversible reductions in the extinction coefficient and fully reversible 25× enhancements in absolute light emission registered across the same humidity cycles up to 70% rH. With in situ excitation wavelength-dependent measurements, irreversible changes are identified in the perovskite after a single cycle of humidity-dependent photoluminescence (PL) performed with 450 nm excitation. The in situ measurement platform can be extended to test the effect of other stressors on thin films’ optical behavior.
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Publication: J. Plasma Phys. 87, 94580601 (2021)Doi: 10.1017/S0022377821000945
Below we clarify the conventions used for normalization in our paper (Punk, Landreman & Herlander 2019), and correct some associated errors in the equations. The validity of the numerical solutions and the main conclusions of the paper are unaffected by these corrections.
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Publication: AMBIO 50, 1446Doi: 10.1007/s13280-020-01365-1
Nature-based solutions (NBS) find increasing attention as actions to address societal challenges through harnessing ecological processes, yet knowledge gaps exist regarding approaches to landscape planning with NBS. This paper aims to provide suggestions of how planning NBS can be conceptualized and applied in practice. We develop a framework for planning NBS by merging insights from literature and a case study in the Lahn river landscape, Germany. Our framework relates to three key criteria that define NBS, and consists of six steps of planning: Co-define setting, Understand challenges, Create visions and scenarios, Assess potential impacts, Develop solution strategies, and Realize and monitor. Its implementation is guided by five principles, namely Place-specificity, Evidence base, Integration, Equity, and Transdisciplinarity. Drawing on the empirical insights from the case study, we suggest suitable methods and a checklist of supportive procedures for applying the framework in practice. Taken together, our framework can facilitate planning NBS and provides further steps towards mainstreaming.
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Publication: Astrophys. J. 923, 245 (2021)Doi: 10.3847/1538-4357/ac1ff1
Transport equations for electron thermal energy in the high-βe intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds Vw ~ vte/βe « vte dominate the turbulent spectrum, with vte the electron thermal speed and βe » 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when Lc > L/βe, with Lc the classical electron mean free path and L the electron temperature scale length, and (2) in the whistler-dominated regime the electron thermal flux is controlled by both advection at Vw and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the field length decreases, extending the domain of thermal instability to smaller length scales, (2) the heat flux temperature dependence changes from Te7/2 / L to VwnTe ~ Te1/2, (3) the magneto-thermal- and heat-flux-driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models.
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Publication: J. Electrochem. Soc. 168, 010533 (2021)Doi: 10.1149/1945-7111/abdb44
Li10GeP2S12 (LGPS) is a superionic conductor that has an ionic conductivity matching conventional liquid electrolytes (10−3 S cm−1) and thus shows exceptional potential to fulfill the promise of solid-state Li metal batteries. Conventional mechanical die pressing of LGPS powder into pellets for electrochemical testing can result in large porosity, low density, and large grain boundary resistance at the solid-solid interface with the electrodes which greatly decrease the performance of LGPS, in addition to poor mechanical stability of such pressed pellets. We demonstrate the use of hot pressing to fabricate of LGPS pellets using commercially available powder. We obtain pellets that are the most dense, and accordingly have the highest ionic conductance, at 150°C. XPS demonstrates there is no chemical degradation of the LGPS powder during the hot pressing process.
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Publication: APL Photon. 6, 080801 (2021)Doi: 10.1063/5.0045241
The majority of photonic quantum information technologies rely on single photons that have high purity and indistinguishability. Although solid-state quantum emitters can serve such single photons on demand, their asymmetric temporal and spatial mode profiles limit the optimal efficiency and fidelity of quantum interaction. Here, we demonstrate single-photon pulses at a telecom wavelength with a Gaussian-like temporal mode profile from a cavity-coupled single quantum dot. Engineering the exciton dynamics via multi-exciton cascade recombination and cavity detuning enables us to modify the rise and decay dynamics of single excitons. Furthermore, the cascade recombination process temporally retards the single-exciton emission from the background emission, leading to possible purification of single photons at high excitation power. In addition, coupling quantum dots into a low Q cavity mode leads to a Gaussian-like spatial mode profile, which brings a high collection efficiency. This approach paves the way for producing single photons with an optimized temporal and spatial waveform.
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Publication: J. Vac. Sci. Technol. A 39, 043001 (2021)Doi: 10.1103/PhysRevFluids.6.033801
Maintaining uniform sample etching during a plasma process is a critical requirement for applications in large-scale wafer processing. The interface between the plasma and the sample surface is defined by the plasma sheath, which accelerates ions toward the sample surface. In areas where the plasma sheath is not parallel to the sample surface, such as near the sample edges, the incident ions arrive at shallower, off-normal angles that can result in a greater etch yield relative to other areas of the sample. This phenomenon leads to nonuniform etching, along with characteristic surface morphology evolution. In this work, we utilized a combination of spatial ellipsometry for etching behavior, atomic force microscopy (AFM) for surface morphology evolution, and power spectral density (PSD) analysis to quantify the extent and spatial dependence of the nonuniform etching near the sample edges. The spatial ellipsometry indicated that a region extending for about 1000 μm from the sample edge experiences approximately 10%–15% more thickness loss (∼10–15 nm) relative to areas near the center of the sample under the tested processing conditions. Within this area, the greatest rate of change in the sample thickness occurs within 5 Debye lengths or ∼300 μm from the sample edge. Via AFM analysis, we detected the presence of ripple features that are consistent with directional ion impacts caused by deflection of ions from normal incidence on the surface morphology [Merkulov et al., Appl. Phys. Lett. 80, 4816 (2002)] AFM scans performed in two different sample orientations confirmed that the ripple features are oriented perpendicular to the direction of incident ions and propagate along the direction of the incident ions. Correspondingly, the magnitude of surface roughness decreases as the distance from the sample edge increases. The ripple features were quantified via PSD analysis, which found the presence of a greater population of long-wavelength roughness closer to the sample edge. The findings of this study provide insight into the influence of the plasma sheath distortions near sample edges on the extent of nonuniform sample etching and characteristic surface morphology evolution in plasma etching applications.
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Publication: Phys. Rev. Lett. 126, 135101 (2021)Doi: 10.1103/PhysRevLett.126.135101
The first self-consistent simulations of electron acceleration during magnetic reconnection in a macroscale system are presented. Consistent with solar flare observations, the spectra of energetic electrons take the form of power laws that extend more than two decades in energy. The drive mechanism for these nonthermal electrons is Fermi reflection in growing and merging magnetic flux ropes. A strong guide field suppresses the production of nonthermal electrons by weakening the Fermi drive mechanism. For a weak guide field the total energy content of nonthermal electrons dominates that of the hot thermal electrons even though their number density remains small. Our results are benchmarked with the hard x-ray, radio, and extreme ultraviolet observations of the X8.2-class solar flare on September 10, 2017.
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Publication: Phys. Rev. A 104, 013107 (2021)Doi: 10.1103/PhysRevA.104.013107
An ultrashort laser pulse focused in air creates a plasma that radiates broadband electromagnetic waves. We experimentally compare the generation of microwaves from plasmas produced with two different laser systems that operate in the near- and mid-infrared regimes. Changing the laser wavelength increases the microwave power by 100 times and changing the input pulse energy allows for tuning of the microwave frequency spectrum, which we absolutely calibrate over a range of 2–70 GHz. The variation of the spectrum with laser pulse energy confirms the existence of a distinct mechanism that generates microwave radiation from laser-produced plasmas in gases. We propose that a radial diffusive expansion wave of the plasma electrons drives a longitudinal current along the plasma surface whose amplitude varies with the total residual electron energy imparted by the laser field and this longitudinal current produces the detected radiation.
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Publication: Chaos 31, 053114 (2021)Doi: 10.1063/5.0048050
We consider the problem of data-assisted forecasting of chaotic dynamical systems when the available data are in the form of noisy partial measurements of the past and present state of the dynamical system. Recently, there have been several promising data-driven approaches to forecasting of chaotic dynamical systems using machine learning. Particularly promising among these are hybrid approaches that combine machine learning with a knowledge-based model, where a machine-learning technique is used to correct the imperfections in the knowledge-based model. Such imperfections may be due to incomplete understanding and/or limited resolution of the physical processes in the underlying dynamical system, e.g., the atmosphere or the ocean. Previously proposed data-driven forecasting approaches tend to require, for training, measurements of all the variables that are intended to be forecast. We describe a way to relax this assumption by combining data assimilation with machine learning. We demonstrate this technique using the Ensemble Transform Kalman Filter to assimilate synthetic data for the three-variable Lorenz 1963 system and for the Kuramoto–Sivashinsky system, simulating a model error in each case by a misspecified parameter value. We show that by using partial measurements of the state of the dynamical system, we can train a machine-learning model to improve predictions made by an imperfect knowledge-based model.
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Publication: Adv. Mater. Interfaces 8, 2101598 (2021)Doi: 10.1002/admi.202101598
Magnesium intercalated “quasi-freestanding” bilayer graphene on 6H-SiC(0001) (Mg-QFSBLG) has many favorable properties (e.g., highly n-type doped, relatively stable in ambient conditions). However, intercalation of Mg underneath monolayer graphene is challenging, requiring multiple intercalation steps. Here, these challenges are overcome and the rate of Mg intercalation is significantly increased by laser patterning (ablating) the graphene to form micron-sized discontinuities. Low energy electron diffraction is then used to verify Mg-intercalation and conversion to Mg-QFSBLG, and X-ray photoelectron spectroscopy to determine the Mg intercalation rate for patterned and non-patterned samples. By modeling Mg intercalation with the Verhulst equation, it is found that the intercalation rate increase for the patterned sample is 4.5 ± 1.7. Since the edge length of the patterned sample is ≈5.2 times that of the non-patterned sample, the model implies that the increased intercalation rate is proportional to the increase in edge length. Moreover, Mg intercalation likely begins at graphene discontinuities in pristine samples (not step edges or flat terraces), where the 2D-like crystal growth of Mg-silicide proceeds. The laser patterning technique may enable the rapid intercalation of other atomic or molecular species, thereby expanding upon the library of intercalants used to modify the characteristics of graphene, or other 2D materials and heterostructures.
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Publication: Nature Commun. 12, 7187 (2021)Doi: 10.1038/s41467-021-27121-x
Integrated nanophotonics is an emerging research direction that has attracted great interests for technologies ranging from classical to quantum computing. One of the key-components in the development of nanophotonic circuits is the phase-change unit that undergoes a solid-state phase transformation upon thermal excitation. The quaternary alloy, Ge2Sb2Se4Te, is one of the most promising material candidates for application in photonic circuits due to its broadband transparency and large optical contrast in the infrared spectrum. Here, we investigate the thermal properties of Ge2Sb2Se4Te and show that upon substituting tellurium with selenium, the thermal transport transitions from an electron dominated to a phonon dominated regime. By implementing an ultrafast mid-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in these materials, we find that this reduction in thermal conductivity is a result of a drastic change in electronic lifetimes of Ge2Sb2Se4Te, leading to a transition from an electron-dominated to a phonon-dominated thermal transport mechanism upon selenium substitution. In addition to thermal conductivity measurements, we provide an extensive study on the thermophysical properties of Ge2Sb2Se4Te thin films such as thermal boundary conductance, specific heat, and sound speed from room temperature to 400 °C across varying thicknesses.
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Publication: Phys. Rev. Fluids 6, 033801 (2021)Doi: 10.1103/PhysRevFluids.6.033801
We present results of torque and velocity measurement of a 40-cm spherical Couette flow experiment with rough boundaries and compare them with previous work done for smooth boundaries. Spherical Couette flows in liquid metals are a suitable candidate for generating magnetic dynamo states in the laboratory. However, previous work in our 3-m spherical Couette flow experiment and numerical simulation have shown that an enhancement of the poloidal flows and the helicity are likely required to lower the threshold to achieve dynamo action. Finke and Tilgner [Phys. Rev. E 86, 016310 (2012)] suggested roughening the inner sphere boundary by adding baffles in order to achieve these goals. We perform hydrodynamic studies of the effect of three baffle designs: straight (symmetric) and two types of chevronlike (asymmetric) baffles. In addition, we test the effect of baffle height with two variants: 5% and 10% radius height. We observe important differences in the dimensionless torque as a function of the Reynolds and Rossby numbers for these different configurations and explore an asymmetry in the torque with asymmetric baffles. Velocity measurements in both the equatorial and the meridional planes show an effective enhancement of the equatorial jet and the poloidal flows when adding baffles and two different flow topologies for asymmetric baffles in concordance with the torque measurements. Results point to one of the chevronlike baffle designs as a promising upgrade that we will use in our 3-m experiment to effectively increase our chances of obtaining dynamo action.
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Publication: Opt. Mater. Express 11, 1555 (2021)Doi: 10.1364/OME.422707
The fixed post-manufacturing properties of metal-based photonic devices impose limitations on their adoption in dynamic photonics. Modulation approaches currently available (e.g. mechanical stressing or electrical biasing) tend to render the process cumbersome or energy-inefficient. Here we demonstrate the promise of utilizing magnesium (Mg) in achieving optical tuning in a simple and controllable manner: etching in water. We revealed an evident etch rate modulation with the control of temperature and structural dimensionality. Further, our numerical calculations demonstrate the substantial tuning range of optical resonances spanning the entire visible frequency range with the etching-induced size reduction of several archetypal plasmonic nanostructures. Our work will help to guide the rational design and fabrication of bio-degradable photonic devices with easily tunable optical responses and minimal power footprint.
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Publication: Phys. Plasmas 28, 092107 (2021)Doi: 10.1063/5.0050575
The reversibility of the transfer of energy from the magnetic field to the surrounding plasma during magnetic reconnection is examined. Trajectories of test particles in an analytic field model demonstrate that irreversibility is associated with separatrix crossings and passages through regions of weaker magnetic field. Inclusion of a guide field enhances the magnetization of particles and the extent to which forward and reverse trajectories overlap. Full kinetic simulations with a particle-in-cell code support these results and demonstrate that while time-reversed simulations at first “un-reconnect,” they eventually evolve into a reconnecting state.
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Publication: J. Plasma Phys. 87, 905870112 (2021)Doi: 10.1017/S0022377820001658
A new paradigm for rapid stellarator configuration design has been recently demonstrated, in which the shapes of quasisymmetric or omnigenous flux surfaces are computed directly using an expansion in small distance from the magnetic axis. To further develop this approach, here we derive several other quantities of interest that can be rapidly computed from this near-axis expansion. First, the ∇B and ∇∇B tensors are computed, which can be used for direct derivative-based optimization of electromagnetic coil shapes to achieve the desired magnetic configuration. Moreover, if the norm of these tensors is large compared to the field strength for a given magnetic field, the field must have a short length scale, suggesting it may be hard to produce with coils that are suitably far away. Second, we evaluate the minor radius at which the flux surface shapes would become singular, providing a lower bound on the achievable aspect ratio. This bound is also shown to be related to an equilibrium beta limit. Finally, for configurations that are constructed to achieve a desired magnetic field strength to first order in the expansion, we compute the error field that arises due to second order terms.
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Publication: PRX Quantum 2, 017002 (2021)Doi: 10.1103/PRXQuantum.2.017002
Just as “classical” information technology rests on a foundation built of interconnected informationprocessing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the “interconnect,” a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a “quantum internet” poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on “Quantum Interconnects” that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry, and national laboratories is required.
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Publication: Phys. Rev. X 11, 021055 (2021)Doi: 10.1103/PhysRevX.11.021055
We demonstrate laser wakefield acceleration of quasimonoenergetic electron bunches up to 15 MeV at 1-kHz repetition rate with 2.5-pC charge per bunch and a core with < 7−mrad beam divergence. Acceleration is driven by 5-fs, < 2.7−mJ laser incident on a thin, near-critical-density hydrogen gas jet. Low beam divergence is attributed to reduced sensitivity to laser carrier-envelope phase slip, achieved in two ways using gas jet positon control and laser polarization: (i) electron injection into the wake on the gas jet’s plasma density downramp and (ii) use of circularly polarized drive pulses. These results demonstrate the generation of high-quality electron beams from a few-cycle-pulse-driven laser plasma accelerator without the need for carrier-envelope phase stabilization.
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Publication: Appl. Surface Sci. 541, 148612 (2021)Doi: 10.1016/j.apsusc.2020.148612
We use angle-resolved photoemission spectroscopy to investigate the electronic structure of bilayer graphene at high n-doping and extreme displacement fields, created by intercalating epitaxial monolayer graphene on silicon carbide with magnesium to form quasi-freestanding bilayer graphene on magnesium-terminated silicon carbide. Angle-resolved photoemission spectroscopy reveals that upon magnesium intercalation, the single massless Dirac band of epitaxial monolayer graphene is transformed into the characteristic massive double-band Dirac spectrum of quasi-freestanding bilayer graphene. Analysis of the spectrum using a simple tight binding model indicates that magnesium intercalation results in an n-type doping of 2.1 × 1014 cm−2 and creates an extremely high displacement field of 2.6 V/nm, thus opening a considerable gap of 0.36 eV at the Dirac point. This is further confirmed by density-functional theory calculations for quasi-freestanding bilayer graphene on magnesium-terminated silicon carbide, which show a similar doping level, displacement field and bandgap. Finally, magnesium-intercalated samples are surprisingly robust to ambient conditions; no significant changes in the electronic structure are observed after 30 min exposure to air.
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Publication: Phys. Plasmas 28, 092501 (2021)Doi: 10.1063/5.0057834
A neoclassically optimized compact stellarator with simple coils has been designed. The magnetic field of the new stellarator is generated by only four planar coils including two interlocking coils of elliptical shape and two circular poloidal field coils. The interlocking coil topology is the same as that of the Columbia Non-neutral Torus (CNT) [Pedersen et al., Phys. Rev. Lett. 88, 205002 (2002)]. The new configuration was obtained by minimizing the effective helical ripple directly via the shape of the two interlocking coils. The optimized compact stellarator has very low effective ripple in the plasma core, implying excellent neoclassical confinement. This is confirmed by the results of the drift-kinetic code SFINCS [Landreman et al., Phys. Plasmas 21, 042503 (2014)], showing that the particle diffusion coefficient of the new configuration is one order of magnitude lower than CNT's.
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Publication: Phys. Plasmas 28, 092505 (2021)Doi: 10.1063/5.0061665
A method is demonstrated to optimize a stellarator's geometry to eliminate magnetic islands and achieve other desired physics properties at the same time. For many physics quantities that have been used in stellarator optimization, including quasisymmetry, neoclassical transport, and magnetohydrodynamic stability, it is convenient to use a magnetic equilibrium representation that assures the existence of magnetic surfaces. However, this representation hides the possible presence of magnetic islands, which are typically undesirable. To include both surface-based objectives and island widths in a single optimization, two fixed-boundary equilibrium calculations are run at each iteration of the optimization: one that enforces the existence of magnetic surfaces (the Variational Moments Equilibrium Code) [S. P. Hirshman and J. C. Whitson, Phys. Fluids 26, 3553 (1983)] and one that does not (the Stepped Pressure Equilibrium Code) [Hudson et al., Phys. Plasmas 19, 112502 (2012)]. By penalizing the island residues in the objective function, the two magnetic field representations are brought into agreement during the optimization. An example is presented in which, particularly on the surface where quasisymmetry was targeted, quasisymmetry is achieved more accurately than in previously published examples.
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Publication: Phys. Rev X 11, 031014 (2021)Doi: 10.1103/PhysRevX.11.031014
We devise a machine learning technique to solve the general problem of inferring network links that have time delays using only time series data of the network nodal states. This task has applications in many fields, e.g., from applied physics, data science, and engineering to neuroscience and biology. Our approach is to first train a type of machine learning system known as reservoir computing to mimic the dynamics of the unknown network. We then use the trained parameters of the reservoir system output layer to deduce an estimate of the unknown network structure. Our technique, by its nature, is noninvasive but is motivated by the widely used invasive network inference method, whereby the responses to active perturbations applied to the network are observed and employed to infer network links (e.g., knocking down genes to infer gene regulatory networks). We test this technique on experimental and simulated data from delay-coupled optoelectronic oscillator networks, with both identical and heterogeneous delays along the links. We show that the technique often yields very good results, particularly if the system does not exhibit synchrony. We also find that the presence of dynamical noise can strikingly enhance the accuracy and ability of our technique, especially in networks that exhibit synchrony.
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Publication: Phys. Rev. A 104, 013703 (2021)Doi: 10.1103/PhysRevA.104.013703
Cluster states are useful in many quantum information processing applications. In particular, universal measurement-based quantum computation (MBQC) utilizes two-dimensional cluster states [R. Raussendorf and H. J. Briegel, Phys. Rev. Lett. 86, 5188 (2001)] and topologically fault-tolerant MBQC requires cluster states of dimension 3 or higher [R. Raussendorf et al., New J. Phys. 9, 199 (2007)]. This work proposes a protocol to deterministically generate multidimensional photonic cluster states using a single atom-cavity system and time-delay feedback. The dimensionality of the cluster state increases linearly with the number of time-delay feedbacks. We first give a diagrammatic derivation of the tensor network states, which is valuable in simulating matrix product states and projected entangled pair states generated from sequential photons. Our method also provides a simple way to bridge and analyze the experimental imperfections and the logical errors of the generated states. In this method, we analyze the generated cluster states under realistic experimental conditions and address both one-qubit and two-qubit errors. Through numerical simulation, we observe an optimal atom-cavity cooperativity for the fidelity of the generated states, which is surprising given the prevailing assumption that higher-cooperativity systems are inherently better for photonic applications.
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Publication: Opt. Express 29, 14630 (2021)Doi: 10.1364/OE.421861
We present a theoretical analysis for tunable optoelectronic oscillators (OEOs) based on stimulated Brillouin scattering (SBS). A pump laser is used to generate a Brillouin gain which selectively amplifies a phase-modulated and contra-propagating laser signal. The radiofrequency beatnote generated after photodetection is filtered, amplified and fed back to the phase modulator to close the optoelectronic loop. Tunability is readily achieved by the adjustable detuning of the pump and signal lasers. OEOs based on stimulated Brillouin scattering have been successfully demonstrated at the experimental level, and they feature competitive phase noise performances along with continuous tunability for the output radiofrequency signal, up to the millimeter-wave band. However, the nonlinear dynamics of SBS-based OEOs remains largely unexplored at this date. In this article, we propose a model that describes the temporal dynamics of the microwave envelope, thereby allowing us to track the dynamics of the amplitude and phase of the radiofrequency signal. The corresponding nonlinear and time-delayed differential equation is then analyzed to reveal the underlying bifurcation behavior that emerges as the feedback gain is increased. It is shown that after the primary Hopf bifurcation that triggers the microwave oscillations, the system undergoes a secondary Neimark-Sacker bifurcation before fully developed chaos emerges for the highest gain values. We also propose a model for the chipscale version of this SBS-based OEO where the delay line is replaced by a highly nonlinear waveguide. The numerical simulations are found to be in excellent agreement with the analytical study.
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Publication: Opt. Express 29, 33144 (2021)Doi: 10.1364/OE.434005
Ultrathin optical limiters are needed to protect light sensitive components in miniaturized optical systems. However, it has proven challenging to achieve a sufficiently low optical limiting threshold. In this work, we theoretically show that an ultrathin optical limiter with low threshold intensity can be realized using a nonlinear zone plate. The zone plate is embedded with nonlinear saturable absorbing materials that allow the device to focus low intensity light, while high intensity light is transmitted as a plane wave without a focal spot. Based on this proposed mechanism, we use the finite-difference time-domain method to computationally design a zone plate embedded with InAs quantum dots as the saturable absorbing material. The device has a thickness of just 0.5 μm and exhibits good optical limiting behavior with a threshold intensity as low as 0.45 kW/cm2, which is several orders of magnitude lower than bulk limiter counterparts based on a similar mechanism, and also performs favorably compared to current ultrathin flat-optics-based optical limiters. This design can be optimized for different operating wavelengths and threshold intensities by using different saturable absorbing materials. Additionally, the diameter and focal length of the nonlinear zone plate can be easily adjusted to fit different systems and applications. Due to its flexible design, low power threshold, and ultrathin thickness, this optical limiting concept may be promising for application in miniaturized optical systems.
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Publication: ACM J. Emerging Technol. Comput. Systems, 17, 28 (2021)Doi: 10.1145/3451214
Race logic, an arrival-time-coded logic family, has demonstrated energy and performance improvements for applications ranging from dynamic programming to machine learning. However, the various ad hoc mappings of algorithms into hardware rely on researcher ingenuity and result in custom architectures that are difficult to systematize. We propose to associate race logic with the mathematical field of tropical algebra, enabling a more methodical approach toward building temporal circuits. This association between the mathematical primitives of tropical algebra and generalized race logic computations guides the design of temporally coded tropical circuits. It also serves as a framework for expressing high-level timing-based algorithms. This abstraction, when combined with temporal memory, allows for the systematic exploration of race logic–based temporal architectures by making it possible to partition feed-forward computations into stages and organize them into a state machine. We leverage analog memristor-based temporal memories to design such a state machine that operates purely on time-coded wavefronts. We implement a version of Dijkstra’s algorithm to evaluate this temporal state machine. This demonstration shows the promise of expanding the expressibility of temporal computing to enable it to deliver significant energy and throughput advantages.
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Publication: J. Plasma Phys. 87, 905870222: PII S0022377821000386 (2021)Doi: 10.1017/S0022377821000386
Coil complexity is a critical consideration in stellarator design. The traditional two-step optimization approach, in which the plasma boundary is optimized for physics properties and the coils are subsequently optimized to be consistent with this boundary, can result in plasma shapes which cannot be produced with sufficiently simple coils. To address this challenge, we propose a method to incorporate considerations of coil complexity in the optimization of the plasma boundary. Coil complexity metrics are computed from the current potential solution obtained with the REGCOIL code (Landreman, Nucl. Fusion, vol. 57, 2017, 046003). While such metrics have previously been included in derivative-free fixed-boundary optimization (Drevlak et al., Nucl. Fusion, vol. 59, 2018, 016010), we compute the local sensitivity of these metrics with respect to perturbations of the plasma boundary using the shape gradient (Landreman & Paul, Nucl. Fusion, vol. 58, 2018, 076023). We extend REGCOIL to compute derivatives of these metrics with respect to parameters describing the plasma boundary. In keeping with previous research on winding surface optimization (Paul et al., Nucl. Fusion, vol. 58, 2018, 076015), the shape derivatives are computed with a discrete adjoint method. In contrast with the previous work, derivatives are computed with respect to the plasma surface parameters rather than the winding surface parameters. To further reduce the resolution required to compute the shape gradient, we present a more efficient representation of the plasma surface which uses a single Fourier series to describe the radial distance from a coordinate axis and a spectrally condensed poloidal angle. This representation is advantageous over the standard cylindrical representation used in the VMEC code (Hirshman & Whitson, Phys. Fluids, vol. 26, 1983, pp. 3553-3568), as it provides a uniquely defined poloidal angle, eliminating a null space in the optimization of the plasma surface. In comparison with previous spectral condensation methods (Hirshman & Breslau, Phys. Plasmas, vol. 5, 1998, p. 2664), the modified poloidal angle is obtained algebraically rather than through the solution of a nonlinear optimization problem. The resulting shape gradient highlights features of the plasma boundary that are consistent with simple coils and can be used to couple coil and fixed-boundary optimization.
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Publication: Front. Phys. 9, 644884 (2021)Doi: 10.3389/fphy.2021.644884
Magnetic reconnection is a fundamental process providing topological changes of the magnetic field, reconfiguration of space plasmas and release of energy in key space weather phenomena, solar flares, coronal mass ejections and magnetospheric substorms. Its multiscale nature is difficult to study in observations because of their sparsity. Here we show how the lazy learning method, known as K nearest neighbors, helps mine data in historical space magnetometer records to provide empirical reconstructions of reconnection in the Earth’s magnetotail where the energy of solar wind-magnetosphere interaction is stored and released during substorms. Data mining reveals two reconnection regions (X-lines) with different properties. In the mid tail (∼30RE from Earth, where RE is the Earth’s radius) reconnection is steady, whereas closer to Earth (∼20RE) it is transient. It is found that a similar combination of the steady and transient reconnection processes can be reproduced in kinetic particle-in-cell simulations of the magnetotail current sheet.
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Publication: Optica 8, 594 (2021)Doi: 10.1364/OPTICA.422743
A spatiotemporal optical vortex (STOV) is an intrinsic optical orbital angular momentum (OAM) structure in which the OAM vector is orthogonal to the propagation direction [Optica 6, 1547 (2019) and the optical phase circulates in space-time. Here, we experimentally and theoretically demonstrate the generation of the second harmonic of a STOV-carrying pulse along with the conservation of STOV-based OAM. Our experiments verify that photons can have intrinsic orbital angular momentum perpendicular to their propagation direction.
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Publication: Nature Phys. 17, 1169 (2021)Doi: 10.1038/s41567-021-01302-3
Recent advances in realizing optical frequency combs using nonlinear parametric processes in integrated photonic resonators have revolutionized on-chip optical clocks, spectroscopy and multichannel optical communications. At the same time, the introduction of topological physics in photonic systems has allowed the design of photonic devices with novel functionalities and inherent robustness against fabrication disorders. Here we use topological design principles to theoretically propose the generation of optical frequency combs and temporal dissipative Kerr solitons in a two-dimensional array of coupled ring resonators that creates a synthetic magnetic field for photons and exhibits topological edge states. We show that these topological edge states constitute a travelling-wave super-ring resonator that leads to the generation of coherent nested optical frequency combs, as well as the self-formation of nested temporal solitons and Turing rolls that are remarkably phase-locked over more than 40 rings. Moreover, we show that the topological nested solitons are robust against defects in the lattice, and a single nested soliton achieves a mode efficiency of over 50%, an order of magnitude higher than single-ring frequency combs. Our topological frequency comb works in a parameter regime that can be readily accessed using existing low-loss integrated photonic platforms like silicon nitride.
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Publication: Phys. Plasmas 28, 013110 (2021)Doi: 10.1063/5.0036054
In gyrotrons operating in high-order modes, during the startup process, the shadowing of the operating mode by two sidebands may take place. By “shadowing,” we mean the situation when, during the voltage rise, one of the parasitic modes is excited first, and this excitation prevents the excitation of the desired mode. Then, the oscillations of the first parasitic mode, whose frequency is higher than the frequency of the desired operating mode, can be replaced by excitation of the second parasitic mode, whose frequency is lower than the operating one. As a result, the desired mode remains in the “shadow” of these parasitic modes and is never excited. This paper describes such effect in gyrotrons with diode-type electron guns. This paper consists of two parts. First, the problem is studied in a generalized approach, which means that the results are valid to gyrotrons operating at arbitrary voltages and in any modes. By using this approach, it is possible to determine the critical density of the mode spectrum, above which the shadowing occurs. This study is carried out for the cases when the interaction between modes is synchronous and when it is nonsynchronous. Second, this paper contains the analysis of a typical Megawatt-class gyrotron with a diode-type electron gun. It is studied whether the moving of this gyrotron to operating in higher-order modes will lead to the shadowing of the desired mode or other, more complicated, dynamic, and/or stochastic processes will take place.
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Publication: Geophys. Res Lett. 10, 32021GL093065 (2021)Doi: 10.1029/2021GL093065
Mining of substorm magnetic field data reveals the formation of two X-lines preceded by the flux accumulation at the tailward end of a thin current sheet (TCS). Three-dimensional particle-in-cell simulations guided by these pre-onset reconnection features are performed, taking also into account weak external driving, negative charging of TCS and domination of electrons as current carriers. Simulations reveal an interesting multiscale picture. On the global scale, they show the formation of two X-lines, with stronger magnetic field variations and inhomogeneous electric fields found closer to Earth. The X-line appearance is preceded by the formation of two diverging electron outflow regions embedded into a single diverging ion outflow pattern and transforming into faster electron-scale reconnection jets after the onset. Distributions of the agyrotropy parameters suggest that reconnection is provided by ion and then electron demagnetization. The bulk flow and agyrotropy distributions are consistent with MMS observations.
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Publication: Opt. Lett. 46, 1013 (2021)Doi: 10.1364/OL.417803
We present a technique for the single-shot measurement of the space- and time-resolved spatiotemporal amplitude and phase of an ultrashort laser pulse. The method, transient-grating single-shot supercontinuum spectral interferometry (TG- SSSI), is demonstrated by the space-time imaging of short pulses carrying spatiotemporal optical vortices (STOVs). TG-SSSI is well-suited for characterizing ultrashort laser pulses that contain singularities associated with spin/orbital angular momentum or polarization.
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Quantum Analysis of Polarization Entanglement Degradation Induced by Multiple-Photon-Pair Generation
Publication: Phys. Rev. A 104, 022411 (2021)Doi: 10.1103/PhysRevA.104.022411Polarization-encoded entanglement remains the simplest platform for the generation, manipulation, and visualization of entangled-photon states. While quantum dots have the potential to emit on-demand polarization entanglement, spontaneous-parametric-down-conversion (SPDC) sources remain the leading method for the generation of polarization-entangled states. SPDC sources suffer from the potential to produce multiple photon pairs in a single pass of an experiment. These multiple pairs have been shown to have negative impacts on quantum experiments involving entanglement. In this work, we now provide a rigorous theoretical model for the loss of entanglement due to additional photon pairs. This is seen as a reduction in a possible measurement of the Clauser-Horne-Shimony-Holt (CHSH) parameter. We perform these calculations for two different methods for the generation of polarization entanglement involving SPDC. The results agree with other observations presented in the literature. We also find that, even for small mean photon numbers, the CHSH parameter is reduced linearly, demonstrating that multiple photon pairs have a critical impact on the entanglement in the system.
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Publication: Nature Commun. 12, 7275 (2021)Doi: 10.1038/s41467-021-27469-0
Broad bandwidth and stable microresonator frequency combs are critical for accurate and precise optical frequency measurements in a compact and deployable format. Typically, broad bandwidths (e.g., octave spans) are achieved by tailoring the microresonator's geometric dispersion. However, geometric dispersion engineering alone may be insufficient for sustaining bandwidths well beyond an octave. Here, we introduce the novel concept of synthetic dispersion, in which a second pump laser effectively alters the dispersion landscape to create Kerr soliton microcombs that extend far beyond the anomalous geometric dispersion region. Through detailed numerical simulations, we show that the synthetic dispersion model captures the system's key physical behavior, in which the second pump enables non-degenerate four-wave mixing that produces new dispersive waves on both sides of the spectrum. We experimentally demonstrate these concepts by pumping a silicon nitride microring resonator at 1060 nm and 1550 nm to generate a single soliton microcomb whose bandwidth approaches two octaves (137 THz to 407 THz) and whose phase coherence is verified through beat note measurements. Such ultra-broadband microcombs provide new opportunities for full microcomb stabilization in optical frequency synthesis and optical atomic clocks, while the synthetic dispersion concept can extend microcomb operation to wavelengths that are hard to reach solely through geometric dispersion engineering.
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Publication: J. Phys. Photon. 3, 01LT01 (2021)Doi: 10.1088/2515-7647/abd7d0
Graphene patterned into plasmonic structures like ribbons or discs strongly increases the linear and nonlinear optical interaction at resonance. The nonlinear optical response is governed by hot carriers, leading to a red-shift of the plasmon frequency. In magnetic fields, the plasmon hybridizes with the cyclotron resonance, resulting in a splitting of the plasmonic absorption into two branches. Here we present how this splitting can be exploited to tune the nonlinear optical response of graphene discs. In the absence of a magnetic field, a strong pump-induced increase in on-resonant transmission can be observed, but fields in the range of 3T can change the characteristics completely, leading to an inverted nonlinearity. A two temperature model is provided that describes the observed behavior well.
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Publication: J. Phys. Chem. Lett.12, 7866 (2021)Doi: 10.1021/acs.jpclett.1c01961
Perovskite solar cells (PSC) are a favorable candidate for next-generation solar systems with efficiencies comparable to Si photovoltaics, but their long-term stability must be proven prior to commercialization. However, traditional trial-and-error approaches to PSC screening, development, and stability testing are slow and labor-intensive. In this Perspective, we present a survey of how machine learning (ML) and autonomous experimentation provide additional toolkits to gain physical understanding while accelerating practical device advancement. We propose a roadmap for applying ML to PSC research at all stages of design (compositional selection, perovskite material synthesis and testing, and full device evaluation). We also provide an overview of relevant concepts and baseline models that apply ML to diverse materials problems, demonstrating its broad relevance while highlighting promising research directions and associated challenges. Finally, we discuss our outlook for an integrated pipeline that encompasses all design stages and presents a path to commercialization.
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Publication: Phys. Rev. Lett. 127, 193901 (2021)Doi: 10.1103/PhysRevLett.127.193901
We identify a class of modal solutions for spatiotemporal optical vortex (STOV) electromagnetic pulses propagating in dispersive media with orbital angular momentum (OAM) orthogonal to propagation. We find that symmetric STOVs in vacuum can carry half-integer intrinsic OAM; for general asymmetric STOVs in a dispersive medium, the OAM is quantized in integer multiples of a parameter that depends on the STOV symmetry and the group velocity dispersion. Our results suggest that STOVs propagating in dispersive media are accompanied by a polaritonlike quasiparticle. The modal theory is in excellent agreement with measurements of free space propagation of STOVs.
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Publication: IEEE Photon. J. 13, 3000110 (2021)Doi: 10.1109/JPHOT.2021.3070846
We propose a theoretical analysis of the stochastic dynamics of miniature optoelectronic oscillators (OEOs) based on whispering-gallery mode resonators. The core element in this microwave photonic oscillator is a high-Q whispering-gallery mode resonator with quadratic nonlinearity, which simultaneously performs electrooptical modulation, frequency filtering and energy storage. This multi-task resonator allows the oscillator to feature improved size, weight and power metrics. In this article, we analyze how the various sources of optical and electrical noise in the oscillator are converted to output microwave signal fluctuations. We use an approach based on stochastic differential equations to characterize the dynamics of the microwave signal as a function of radiofrequency gain and laser pump power. This stochastic analysis also allows us to understand how key parameters of the resonator such as its intrinsic and extrinsic Q-factors influence the system's dynamics below and above threshold. The time-domain numerical simulations for miniature OEO stochastic dynamics provides an excellent agreement with the analytical predictions.
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Publication: IEEE J. Quantum Electron. 57, 1 (2021)Doi: 10.1109/JQE.2021.3074132
We numerically perform the classification of IQ-modulated radiofrequency signals using reservoir computing based on narrowband optoelectronic oscillators (OEOs) driven by a continuous-wave semiconductor laser. In general, the OEOs used for reservoir computing are wideband and are processing analog signals in the baseband. However, their hardware architecture is inherently inadequate to directly process radiotelecom or radar signals, which are modulated carriers. On the other hand, the high- Q OEOs that have been developed for ultra-low phase noise microwave generation have the adequate hardware architecture to process such multi-GHz modulated signals, but they have never been investigated as possible reservoir computing platforms. In this article, we show that these high-Q OEOs are indeed suitable for reservoir computing with modulated carriers. Our dataset (DeepSig RadioML) is composed with 11 analog and digital formats of IQ-modulated radio signals (BPSK, QAM64, WBFM, etc.), and the task of the high-Q OEO reservoir computer is to recognize and classify them. Our numerical simulations show that with a simpler architecture, a smaller training set, fewer nodes and fewer layers than their neural network counterparts, high-Q OEO-based reservoir computers perform this classification task with an accuracy better than the state-of-the-art, for a wide range of parameters. We also investigate in detail the effects of reducing the size of the training sets on the classification performance.
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Publication: Opt. Lett. 46, 4867 (2021)Doi: 10.1364/OL.440804
The theoretical framework for a novel, to the best of our knowledge, stimulated Raman spectroscopy process using a UV probe laser pulse train is formulated and simulated. The laser pulse train consists of multi-femtosecond micro-pulses separated by a varying time duration, having a fixed carrier frequency. The comb-like probe spectrum undergoes self-beating. By appropriately varying the separation time between the micro-pulses, the full Raman spectrum can be excited. We also show that a Raman wakefield, containing the entire Raman signatures of complex molecules, is induced behind the probe pulse train and can be used for additional classification. Kerr and non-resonant effects are included in our model. As an illustration, simulations of the Raman spectrum of a particular pathogen are presented and discussed.
2020
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Publication: Phys. Rev. E 101, 062304 (2020)Doi: 10.1103/PhysRevE.101.062304
Network science is a rapidly expanding field, with a large and growing body of work on network-based dynamical processes. Most theoretical results in this area rely on the so-called locally treelike approximation. This is, however, usually an "uncontrolled" approximation, in the sense that the magnitudes of the error are typically unknown, although numerical results show that this error is often surprisingly small. In this paper we place this approximation on more rigorous footing by calculating the magnitude of deviations away from tree-based theories in the context of discrete-time critical network cascades with re-excitable nodes. We discuss the conditions under which tree-like approximations give good results for calculating network criticality, and also explain the reasons for deviation from this approximation, in terms of the density of certain kinds of network motifs. Using this understanding, we derive results for network criticality that apply to general networks that explicitly do not satisfy the locally treelike approximation. In particular, we focus on the biparallel motif, the smallest motif relevant to the failure of a tree-based theory in this context, and we derive the corrections due to such motifs on the conditions for criticality. We verify our claims on computer-generated networks, and we confirm that our theory accurately predicts the observed deviations from criticality. Using our theory, we explain why numerical simulations often show that deviations from a tree-based theory are surprisingly small. More specifically, we show that these deviations are negligible for networks whose average degree is even modestly large compared to one, justifying why tree-based theories appear to work well for most real-world networks.
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Publication: IEEE Trans. Antennas Propag. 68, 3919 (2020)Doi: 10.1109/TAP.2019.2963568
We present a statistical, mathematical, and computational model for prediction and analysis of wave propagation through complex, wave-chaotic environments. These are generally enclosed environments that are many wavelengths in extent and are such that, in the geometric optics limit, ray trajectories diverge from each other exponentially with distance traveled. The wave equation solution is expressed in terms of a novel stochastic Green's function that includes both coherent coupling due to direct path propagation and incoherent coupling due to propagation through multiple paths of the scattering environment. The statistically fluctuating portion of the Green's function is characterized by random wave model and random matrix theory. Built upon the stochastic Green's function, we have derived a stochastic integral equation method, and a hybrid formulation to incorporate the component-specific attributes. The proposed model is evaluated and validated through representative experiments.
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Publication: ACS Appl. Mater. Intefaces 12, 21641 (2020)Doi: 10.1021/acsami.0c03578
The development of novel materials that are compatible with nanostructured architectures is required to meet the demands of next-generation energy-storage technologies. Atomic layer deposition (ALD) allows for the precise synthesis of new materials that can conformally coat complex 3D structures. In this work, we demonstrate a thermal ALD process for sodium phosphorus oxynitride (NaPON), a thin-film solid-state electrolyte (SSE), for sodium-ion batteries (SIBs). NaPON is analogous to the commonly used lithium phosphorus oxynitride SSE in lithium-ion batteries. The ALD process produces a conformal film with a stoichiometry of Na4PO3N, corresponding to a sodium polyphosphazene structure. The electrochemical properties of NaPON are characterized to evaluate its potential in SIBs. The NaPON film exhibited a high ionic conductivity of 1.0 × 10–7 S/cm at 25 °C and up to 2.5 × 10–6 S/cm at 80 °C, with an activation energy of 0.53 eV. In addition, the ionic conductivity is comparable and even higher than the ionic conductivities of ALD-fabricated Li+ conductors. This promising result makes NaPON a viable SSE or passivation layer in solid-state SIBs.
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Publication: Phys. Rev. Res. 2, 023355 (2020)Doi: 10.1103/PhysRevResearch.2.023355
The Casimir force is a consequence of quantum electrodynamic fluctuations, which induce interactions between materials. Patch potentials (i.e., spatial variations of electrostatic potentials across a surface) are a concern in measurements of the Casimir force because they can cause an additional force with a similar separation dependence. Previously, Kelvin probe force microscopy has been used to show that patch potentials on a flat surface cause an additional force that can reach over 1% of the value of the predicted Casimir force. Although nearly all Casimir force measurements are performed in a sphere-plate geometry, there has been little investigation into how the patches are distributed on the sphere. Here we present a measurement of the Casimir force between a sphere and a plate, where the electrostatic patch potentials are mapped on both surfaces and their effects are determined. Large patches are detected for gold deposited onto glass, but an ion-blocking layer is shown to reduce the voltage contrast and spatial extent of the patches. We find that the patch potential force is at least an order of magnitude less than the Casimir force when the sphere contains an ion-blocking layer; however, without this ion-blocking layer, the measured force can contain a significant electrostatic contribution, hence masking the Casimir force. Our results show the importance of measuring the electrostatic patches for individual Casimir force experiments.
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Publication: Chaos 30, 063151 (2020)Doi: 10.1063/5.0016505
Machine learning (ML), a subset of artificial intelligence, refers to methods that have the ability to “learn” from experience, enabling them to carry out designated tasks. Examples of machine learning tasks are detection, recognition, diagnosis, optimization, and prediction. Machine learning can also often be used in different areas of complex systems research involving identification of the basic system structure (e.g., network nodes and links) and study of the dynamic behavior of nonlinear systems (e.g., determining Lyapunov exponents, prediction of future evolution, and inferring causality of interactions). Conversely, machine learning procedures, such as “reservoir computing” and “long short-term memory”, are often dynamical in nature, and the understanding of when, how, and why they are able to function so well can potentially be addressed using tools from dynamical systems theory. For example, a recent consequence of this has been the realization of new optics-based physical realizations of reservoir computers. In the area of the application of machine learning to complex physical problems, it has been successfully used to construct and recover the complex structures and dynamics of climate networks, genetic regulatory systems, spatiotemporal chaotic systems, and neuronal networks. On the other hand, complex systems occur in a wide variety of practical settings, including engineering, neuroscience, social networks, geoscience, economics, etc. Since complex systems research and machine learning have a close relationship between each other, they provide a common basis for a wide range of cross-disciplinary interactions. Hence, exploring how machine learning works for issues involving complex systems has been a subject of significant research interest. With the advent of machine learning, it has become possible to develop new algorithms and strategies for identification, control, and data analytics of complex systems, thereby promoting the application of machine learning in many fields.
The main focus of this Focus Issue is on the new algorithms, strategies, and techniques with machine learning applied to complex systems and on applying complex system techniques to leverage the performance of machine learning techniques with high-efficiency. This Focus Issue provides a platform to facilitate interdisciplinary research and to share the most recent developments in various related fields. The specific areas represented include reservoir computing, modeling of complex systems, prediction and manipulations of complex systems, data-driven research, control and optimization, and applications.
For the Focus Issue, 58 papers were accepted for publication. In the following, we will divide the editorial into the following five parts, including reservoir computing, model of complex systems, prediction and manipulations of complex systems, data-driven research, control and optimization, and applications.
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Publication: Chem. Mater. 32, 6464 (2020)Doi: 10.1021/acs.chemmater.0c01729
The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied extensively, yet precisely where the Ca resides remains elusive. Furthermore, the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been reported. Here, we use low energy electron diffraction, X-ray photoelectron spectroscopy, secondary electron cutoff photoemission, and scanning tunneling microscopy to elucidate the physical and electronic structures of both Ca- and Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca intercalates underneath the buffer layer and bonds to the Si-terminated SiC surface, breaking the C–Si bonds of the buffer layer, i.e., “freestanding” the buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene (Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC surface is formed and Mg bonds to the Si-terminated SiC surface are found, resulting in Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG). Ca-intercalation underneath the buffer layer has not been considered in previous studies of Ca-intercalated epitaxial graphene. Furthermore, we find no evidence that either Ca or Mg intercalates between graphene layers. However, we do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low work functions of 3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG remains remarkably stable.
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Publication: Chaos 30, 013111 (2020)Doi: 10.1063/1.5120788
The concept of reservoir computing emerged from a specific machine learning paradigm characterized by a three-layered architecture (input, reservoir, and output), where only the output layer is trained and optimized for a particular task. In recent years, this approach has been successfully implemented using various hardware platforms based on optoelectronic and photonic systems with time-delayed feedback. In this review, we provide a survey of the latest advances in this field, with some perspectives related to the relationship between reservoir computing, nonlinear dynamics, and network theory.
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Publication: Phys. Rev. E 101, 022201 (2020)Doi: 10.1103/PhysRevE.101.022201
The statistics of the scattering of waves inside single ray-chaotic enclosures have been successfully described by the random coupling model (RCM). We expand the RCM to systems consisting of multiple complex ray-chaotic enclosures with various coupling scenarios. The statistical properties of the model-generated quantities are tested against measured data of electrically large multicavity systems of various designs. The statistics of model-generated transimpedance and induced voltages on a load impedance agree well with the experimental results. The RCM coupled chaotic enclosure model is general and can be applied to other physical systems, including coupled quantum dots, disordered nanowires, and short-wavelength electromagnetic and acoustic propagation through rooms in buildings, aircraft, and ships.
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Publication: Nucl. Fusion 60, 126045 (2020)Doi: 10.1088/1741-4326/abb891
Local linear gyrokinetic simulations show that electron temperature gradient (ETG) instabilities are the fastest growing modes for
in the steep gradient region for a JET pedestal discharge (92174) where the electron temperature gradient is steeper than the ion temperature gradient. Here, ky is the wavenumber in the direction perpendicular to both the magnetic field and the radial direction, and ρi is the ion gyroradius. At
, the fastest growing mode is often a novel type of toroidal ETG instability. This toroidal ETG mode is driven at scales as large as
and at a sufficiently large radial wavenumber that electron finite Larmor radius effects become important; that is,
where Kx is the effective radial wavenumber. Here, ρe is the electron gyroradius, R0 is the major radius of the last closed flux surface, and 1/LTe is an inverse length proportional to the logarithmic gradient of the equilibrium electron temperature. The fastest growing toroidal ETG modes are often driven far away from the outboard midplane. In this equilibrium, ion temperature gradient instability is subdominant at all scales and kinetic ballooning modes are shown to be suppressed by
shear. ETG modes are very resilient to
shear. Heuristic quasilinear arguments suggest that the novel toroidal ETG instability is important for transport.
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Publication: Adv. Opt. Mater. 8, 2001082 (2020)Doi: 10.1002/adom.202001082
Metallic nanostructures and thin films are fundamental building blocks for next-generation nanophotonics. Yet, the fixed permittivity of pure metals often imposes limitations on the materials employed and/or on device performance. Alternatively, metallic mixtures, or alloys, represent a promising pathway to tailor the optical and electrical properties of devices, enabling further control of the electromagnetic spectrum. In this Review, a survey of recent advances in photonics and plasmonics achieved using metal alloys is presented. An overview of the primary fabrication methods to obtain subwavelength alloyed nanostructures is provided, followed by an in-depth analysis of experimental and theoretical studies of their optical properties, including their correlation with band structure. The broad landscape of optical devices that can benefit from metallic materials with engineered permittivity is also discussed, spanning from superabsorbers and hydrogen sensors to photovoltaics and hot electron devices. This Review concludes with an outlook of potential research directions that would benefit from the on demand optical properties of metallic mixtures, leading to new optoelectronic materials and device opportunities.
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Publication: Adv. Mater. Interfaces 7, 2000515 (2020)Doi: 10.1002/admi.202000515
Perovskite semiconductors are an exciting class of materials due to their promising performance outputs in photovoltaic devices. To boost their efficiency further, researchers introduce additives during sample synthesis, such as KI. However, it is not well understood how KI changes the material and, often, leaves precipitants. To fully resolve the role of KI, multiple microscopy techniques are applied and the electrical and chemical behavior of a Reference (untreated) and a KI-treated perovskite are compared. Upon correlation between electrical and chemical nanoimaging techniques, it is discovered that these local properties are linked to the macroscopic voltage enhancement of the KI-treated perovskite. The heterogeneity revealed in both the local electrical and chemical responses indicates that the additive partially migrates to the surface, yet surprisingly does not deteriorate the performance locally, rather, the voltage response homogeneously increases. The research presented within provides a diagnostic methodology, which connects the nanoscale electrical and chemical properties of materials, relevant to other perovskites, including multication and Pb-free alternatives.
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Publication: ACS Photonics 7, 1689 (2020)Doi: 10.1021/acsphotonics.0c00140
Alloying of metals provides a vast parameter space for tuning of material, chemical, and mechanical properties, impacting disciplines ranging from photonics and catalysis to aerospace. From an optical point-of-view, pure thin metal films yield enhanced light absorption due to their cavity effects. However, an ideal metal–semiconductor photodetector requires not only high absorption, but also long hot carrier attenuation lengths in order to efficiently collect excited carriers. Here we demonstrate that Ag-Au alloys provide an ideal model system for controlling the optical and electrical responses in nanoscale thin metal films for hot carrier photodetectors with improved performance. While pure Ag and Au have long hot carrier attenuation lengths >20 nm, their optical absorption is insufficient for high efficiency devices. Instead, we find that alloying Ag and Au enhances the absorption by ∼50% while maintaining attenuation lengths >15 nm, currently limited by grain boundary scattering, although the electron attenuation length of pure Au outperforms pure Ag as well as all of the alloys investigated here. Further, our density functional theory analysis shows that the addition of small amounts of Au to the Ag lattice significantly enhances the hot hole generation rate. Combined, these findings suggest a route to high efficiency hot carrier devices based on metallic alloying with potential applications ranging from photodetectors and sensors to improved catalytic materials.
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Publication: Phys. Rev. Appl. 14, 014022 (2020)Doi: 10.1103/PhysRevApplied.14.014022
The random coupling model (RCM) has been successfully applied to predicting the statistics of currents and voltages at ports in complex electromagnetic (EM) enclosures operating in the short-wavelength limit. Recent studies have extended the RCM to systems of multimode aperture-coupled enclosures. However, as the size (as measured in wavelengths) of a coupling aperture grows, the coupling matrix used in the RCM increases as well, and the computation becomes more complex and time consuming. A simple power balance (PWB) model can provide fast predictions for the averaged power density of waves inside electrically large systems for a wide range of cavity and coupling scenarios. However, the important interference-induced fluctuations of the wave field retained in the RCM are absent in the PWB model. Here we aim to combine the best aspects of each model to create a hybrid treatment and study the EM fields in coupled enclosure systems. The proposed hybrid approach provides both mean and fluctuation information of the EM fields without the full computational complexity of the coupled-cavity RCM. We compare the hybrid model predictions with experiments on linear cascades of over-moded cavities. We find good agreement over a set of different loss parameters and for different coupling strengths between cavities. The range of validity and applicability of the hybrid method are tested and discussed.
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Publication: Chaos 30, 083146 (2020)Doi: 10.1063/5.0006303
We investigate the effects of environmental stochastic fluctuations on Kerr optical frequency combs. This spatially extended dynamical system can be accurately studied using the Lugiato–Lefever equation, and we show that when additive noise is accounted for, the correlations of the modal field fluctuations can be determined theoretically. We propose a general theory for the computation of these field fluctuations and correlations, which is successfully compared to numerical simulations.
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Publication: Opt. Exp. 28, 11023 (2020)Doi: 10.1364/OE.386631
We have experimentally demonstrated a simplified method for performing single-shot supercontinuum spectral interferometry (SSSI) that does not require pre-characterization of the probe pulse. The method, originally proposed by D. T. Vu, D. Jang, and K. Y. Kim, uses a genetic algorithm (GA) and as few as two time-delayed pump-probe shots to retrieve the pump-induced phase shift on the probe [Opt. Express26, 20572 (2018)]. We show that the GA is able to successfully retrieve the transient modulations on the probe, and that the error in the retrieved modulation decreases dramatically with the number of shots used. In addition, we propose and demonstrate a practical method that allows SSSI to be done with a single pump-probe shot (again, without the need for pre-characterization of the probe). This simplified method can prove to be immensely useful when performing SSSI with a low-repetition-rate laser source.
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Publication: Appl. Opt. 59, 5870 (2020)Doi: 10.1364/AO.395864
Using laser excitation, expression microdissection (xMD) can selectively heat cancer cells targeted via immunohistochemical staining to enable their selective retrieval from tumor tissue samples, thus reducing misdiagnoses caused by contamination of noncancerous cells. Several theoretical models have been validated for the photothermal effect in highly light absorbing and scattering media. However, these models are not generally applicable to the physics behind the process of xMD. In this study, we propose a thermal model that can analyze the transient temperature distribution and heat melt zone in an xMD sample medium composed of a thermoplastic film and a tumor tissue sample sandwiched between two glass slides. Furthermore, we experimentally examined the model using an ink layer with controllable optical properties to serve as a microscale-thin, tissue-mimicking phantom and found the experimentally measured film temperature is in good agreement with the model predictions. The validated model can help researchers to optimize cell retrieval by xMD for improved diagnostics of cancer and other diseases.
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Publication: Phys. Rev. E 101, 022303 (2020)Doi: 10.1103/PhysRevE.101.022303
Various functions of a network of excitable units can be enhanced if the network is in the “critical regime,” where excitations are, on average, neither damped nor amplified. An important question is how can such networks self-organize to operate in the critical regime. Previously, it was shown that regulation via resource transport on a secondary network can robustly maintain the primary network dynamics in a balanced state where activity doesn't grow or decay. Here we show that this internetwork regulation process robustly produces a power-law distribution of activity avalanches, as observed in experiments, over ranges of model parameters spanning orders of magnitude. We also show that the resource transport over the secondary network protects the system against the destabilizing effect of local variations in parameters and heterogeneity in network structure. For homogeneous networks, we derive a reduced three-dimensional map which reproduces the behavior of the full system.
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Publication: Chaos 30, 023123 (2020)Doi: 10.1063/1.5132766
We demonstrate the utility of machine learning in the separation of superimposed chaotic signals using a technique called reservoir computing. We assume no knowledge of the dynamical equations that produce the signals and require only training data consisting of finite-time samples of the component signals. We test our method on signals that are formed as linear combinations of signals from two Lorenz systems with different parameters. Comparing our nonlinear method with the optimal linear solution to the separation problem, the Wiener filter, we find that our method significantly outperforms the Wiener filter in all the scenarios we study. Furthermore, this difference is particularly striking when the component signals have similar frequency spectra. Indeed, our method works well when the component frequency spectra are indistinguishable—a case where a Wiener filter performs essentially no separation.
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Publication: J. Plasma Phys. 86, 905860317, PII S002237782000574 (2020)Doi: 10.1017/S0022377820000574
Impurity temperature screening is a favourable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields lead to intrinsically ambipolar neoclassical fluxes that give rise to temperature screening for low enough η−1 ≡ dlnn/dlnT. In contrast, neoclassical fluxes in generic stellarators will depend on the radial electric field, which is predicted to be inward for ion-root plasmas, potentially leading to impurity accumulation. Here, we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry breaking in the magnetic field. For the majority of this work, neoclassical fluxes have been obtained using the SFINCS drift-kinetic equation solver with electrostatic potential Φ=Φ(r), where r is a flux-surface label. Results indicate that achieving temperature screening is possible, but unlikely, at low-collisionality reactor-relevant conditions in the core. Thus, the small departures from symmetry in nominally quasisymmetric stellarators are large enough to significantly alter the neoclassical impurity transport. A further look at the magnitude of these fluxes when compared to a gyro-Bohm turbulence estimate suggests that neoclassical fluxes are small in configurations optimized for quasisymmetry when compared to turbulent fluxes.
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Publication: Phys. Rev. E 102, 042217 (2020)Doi: 10.1103/PhysRevE.102.042217
We investigate the nonlinear dynamics of a recent architecture of an optoelectronic oscillator, where the emitting laser and the receiving diode are connected in a head-to-tail configuration via an optical fiber delay line. The resulting nonlinear transfer function is a piecewise linear profile, and its interplay with the delay leads to many complex behaviors such as relaxation oscillations and deterministic chaos. This system belongs to a recent class of optoelectronic oscillators where the nonlinearity does not originate from the sinusoidal transfer function of an imbalanced interferometer, and, in particular, it is a simple optoelectronic oscillator configuration that is capable of displaying a chaotic behavior. The results of the analytic study are confirmed by numerical simulations and experimental measurements.
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Publication: J. Plasma Phys. 86, 905860103 (2020)Doi: 10.1017/S0022377819000916
The shape gradient is a local sensitivity function defined on the surface of an object which provides the change in a characteristic quantity, or figure of merit, associated with a perturbation to the shape of the object. The shape gradient can be used for gradient-based optimization, sensitivity analysis and tolerance calculations. However, it is generally expensive to compute from finite-difference derivatives for shapes that are described by many parameters, as is the case for typical stellarator geometry. In an accompanying work (Antonsen, Paul & Landreman J. Plasma Phys., vol. 85 (2), 2019), generalized self-adjointness relations are obtained for magnetohydrodynamic (MHD) equilibria. These describe the relation between perturbed equilibria due to changes in the rotational transform or toroidal current profiles, displacements of the plasma boundary, modifications of currents in the vacuum region or the addition of bulk forces. These are applied to efficiently compute the shape gradient of functions of MHD equilibria with an adjoint approach. In this way, the shape derivative with respect to any perturbation applied to the plasma boundary or coil shapes can be computed with only one additional MHD equilibrium solution. We demonstrate that this approach is applicable for several figures of merit of interest for stellarator configuration optimization: the magnetic well, the magnetic ripple on axis, the departure from quasisymmetry, the effective ripple in the low-collisionality 1/ν regime (ɛeff3/2) (Nemov et al. Phys. Plasmas, vol. 6 (12), 1999, pp. 4622-4632) and several finite-collisionality neoclassical quantities. Numerical verification of this method is demonstrated for the magnetic well figure of merit with the VMEC code (Hirshman & Whitson Phys. Fluids, vol. 26 (12), 1983, p. 3553) and for the magnetic ripple with modification of the ANIMEC code (Cooper et al. Comput. Phys. Commun., vol. 72 (1), 1992, pp. 1-13). Comparisons with the direct approach demonstrate that, in order to obtain agreement within several per cent, the adjoint approach provides a factor of O(103) in computational savings.
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Publication: Adv. Energy Mater. 19, SI, 1903161 (2020)Doi: 10.1002/aenm.201903161
Metal halide perovskites exhibit optimal properties for optoelectronic devices, ranging from photovoltaics to light-emitting diodes, utilizing simple fabrication routes that produce impressive electrical and optical tunability. As perovskite technologies continue to mature, an understanding of their fundamental properties at length scales relevant to their morphology is critical. In this review, an overview is presented of the key insights into perovskite material properties provided by measurement methods based on the atomic force microscopy (AFM). Specifically, the manner in which AFM-based techniques supply valuable information regarding electrical and chemical heterogeneity, ferroelectricity and ferroelasticity, surface passivation and chemical modification, ionic migration, and material/device stability is discussed. Continued advances in perovskite materials will require multimodal approaches and machine learning, where the output of these scanning probe measurements is combined with high spatial resolution structural and chemical information to provide a complete nanoscale description of materials behavior and device performance.
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Nanoscale Depth and Lithiation Dependence of V34O5 Band Structure by Cathodoluminescence Spectrocopy
Publication: J. Mater. Chem. A 8, 11800 (2020)Doi: 10.1039/d0ta03204bVanadium pentoxide (V2O5) is a very well-known cathode material that has attracted considerable interest for its potential use in solid-state lithium-ion batteries. We pioneer the use of depth-resolved cathodoluminescence spectroscopy (DRCLS) to monitor the changes in the electronic structure of lithiated V2O5 from the free surface to the thin film bulk several hundred nm below as a function of lithiation. DRCLS measurements of V2O5 interband transitions are in excellent agreement with density functional theory (DFT) calculations. The direct measure of V2O5's electronic band structure as a function of lithiation level provided by DRCLS can help inform solid-state battery designs to further withstand degradation and increase efficiency. In particular, these unique electrode measurements may reveal physical mechanisms of lithiation that change V2O5 irreversibly, as well as methods to mitigate them in solid-state batteries.
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Publication: Opt. Exp. 28, 34111 (2020)Doi: 10.1364/OE.405188
The property of self-imaging combined with the polarization birefringence of the angled multimode waveguide is used to design a silicon nitride (SiN) polarization splitter (PS) at λ ∼ 1550 nm. The demonstrated PS on a 450 nm thick SiN device layer (with 2.5 µm cladding oxide) has a footprint of 80 µm×13 µm and exhibits nearly wavelength independent performance over the C+L bands. Also, the device can be configured as a polarization combiner (PC) in reverse direction with similar bandwidth and performance. The measured crosstalk (CT) and insertion loss (IL) are respectively <−18 dB (<−20 dB) and ∼0.7 dB (∼0.8 dB) for TE (TM) polarization over the measurement wavelength range of 1525 nm ≤λ ≤ 1625 nm. The measured device parameter variations suggest some tolerance to fabrication variations. Such a device is a good candidate for a photonics integrated chip (PIC) foundry-compatible, SiN PS.
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Publication: J. Opt. Soc. Amer. B-Opt. Phys. 37, A69 (2020)Doi: 10.1364/JOSAB.396610
We propose a theoretical study to analyze how both dark and bright Kerr solitons can be generated in whispering-gallery mode resonators with various regimes of the group-velocity dispersion, namely normal, anomalous, and null. The coexistence of these solitonic structures in each regime is shown to appear around a critical value of the laser pump. We also evidence that these solitons build up owing to a mechanism related to oscillation locking of switching waves, which connect the upper and the lower homogenous steady states.
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Publication: M. Microelectromechan. Systems 29, 846 (2020)Doi: 10.1109/JMEMS.2020.3007342
Real-time toxic gas mapping in complex urban environments have become increasingly possible with improvements in data analysis and network infrastructures. Hindering this is the cost and operation requirements of commercial gas sensors, requiring sensors with high sensitivity and selectivity that are robust and capable of operating at room temperature. Transition metal oxide-based sensors are of historical significance in the production of commercial gas sensors due to their low cost and high selectivity to target gases. The low inherent conductivity of metal oxides, however, requires operating temperatures higher than 150°C, limiting their operation to controlled environments. To overcome this limitation, heterostructures have been formed between graphene and transition metal oxides, seeking to couple the conductivity of graphene with the reactivity of transition metal oxides. Among these transition metal oxides, manganese dioxide exhibits unique properties that can be leveraged to improve gas sensing. Its wide variety of synthesized structural polymorphs (1 × 1 tunnel (β), 1 × 2 tunnel (α), spinel (y), and layered (δ)) allow for control over the available reactive surface area to enhance gas response. By utilizing defect rich δ-phase, the reactivity of the material can be improved. Here we present a δ-MnO2 /epitaxial graphene/silicon carbide heterostructure for use as a room temperature gas sensor. We confirm the composition through Raman spectroscopy and surface morphology through scanning electron microscopy and atomic force microscopy. We then demonstrate its room-temperature detection by testing against NO2 , NH3 , IPA, and CH3 OH at room temperature.
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Publication: Phys. Rev. B 102, 245123 (2020)Doi: 10.1103/PhysRevB.102.245123
Solids with topologically robust electronic states exhibit unusual electronic and optical properties that do not exist in other materials. A particularly interesting example is chiral charge pumping, the so-called chiral anomaly, in recently discovered topological Weyl semimetals, where simultaneous application of parallel DC electric and magnetic fields creates an imbalance in the number of carriers of opposite topological charge (chirality). Here, using time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic field, we optically interrogate the chiral anomaly by dynamically pumping the chiral charges and monitoring their subsequent relaxation of the nonequilibrium state. Theory based on Boltzmann transport shows that the observed effects originate from an optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the nonequilibrium chiral excitation relaxation time is much greater than 1 ns. The observation of terahertz-controlled chiral carriers with long coherence times and topological protection suggests the application of Weyl semimetals for quantum optoelectronic technology.
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Publication: J. Mater. Chem. A 8, 14921 (2020)Doi: 10.1039/d0ta05568a
Aqueous lithium ion batteries (ALIBs) have attracted increasing attention due to their excellent safety profile. The water-in-salt electrolyte (WiSE) has enabled a wider voltage window (3.0 V) through the formation of an solid–electrolyte–interphase (SEI) on the anode. However, the cathodic limit of the WiSE and its derivatives cannot effectively support the desired energy-dense anodes, such as Li4Ti5O12 (LTO). At the anode, the hydrogen evolution reaction (HER) is the main parasitic process that competes with the desired lithiation process therein. We investigated the catalytic activity of different coating layers and postulated the selection criterion for the surface layers. We demonstrated that Al2O3 had a surface that effectively suppressed the HER and enabled the cycling of the LTO anode in the WiSE, thereby delivering a capacity of 145 mA h g−1. Such understanding provides important guidelines for designing electrolytes and interphases for aqueous battery chemistries.
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Publication: J. Plasma Phys. 86, 905860510, PII S002237782000121X (2020)Doi: 10.1017/S002237782000121X
We have recently demonstrated that by expanding in small distance from the magnetic axis compared with the major radius, stellarator shapes with low neoclassical transport can be generated efficiently. To extend the utility of this new design approach, here we evaluate measures of magnetohydrodynamic interchange stability within the same expansion. In particular, we evaluate the magnetic well, Mercier's criterion, and resistive interchange stability near a magnetic axis of arbitrary shape. In contrast to previous work on interchange stability near the magnetic axis, which used an expansion of the flux coordinates, here we use the `inverse expansion' in which the flux coordinates are the independent variables. Reduced expressions are presented for the magnetic well and stability criterion in the case of quasisymmetry. The analytic results are shown to agree with calculations from the VMEC equilibrium code. Finally, we show that near the axis, Glasser, Greene and Johnson's stability criterion for resistive modes approximately coincides with Mercier's ideal condition.
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Publication: Optical 6, 708 (2020)Doi: 10.1364/OPTICA.385959
Surface plasmon mediated hot-carrier generation is utilized widely for the manipulation of electron–photon interactions in many types of optoelectronic devices including solar cells, photodiodes, and optical modulators. A diversity of plasmonic systems such as nanoparticles, resonators, and waveguides has been introduced to enhance hot-carrier generation; however, the impact of propagating surface plasmons on hot-carrier lifetime has not been clearly demonstrated. Here, we systematically study the hot-carrier relaxation in thin film gold (Au) samples under surface plasmon coupling with the Kretschmann configuration. We observe that the locally confined electric field at the surface of the metal significantly affects the hot-carrier distribution and electron temperature, which results in a slowing of the hot electrons’ relaxation time, regardless of the average value of the absorbed power in the Au thin film. This result could be extended to other plasmonic nanostructures, enabling the control of hot-carrier lifetimes throughout the optical frequency range.
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Publication: Optica 7, 1796 (2020)Doi: 10.1364/OPTICA.409060
Nonlinear propagation of signals in single-mode fibers is well understood, and is typically observed by measuring the temporal profile or optical spectrum of an emerging signal. In multimode fibers, the nonlinearity has both a spatial and a temporal element, and a complete investigation of the interactions between propagating modes requires resolving the output in both space and time. We report here spatiotemporal measurements of a time-dependent mode interference effect, arising from the Kerr nonlinearity, of two selectively excited Lp0m modes of a step-index few-mode fiber. We describe a method to selectively excite two propagating modes through the use of a phase mask directly patterned on the entrance face of the fiber. The output is resolved by raster-scanning a near-field tapered single-mode optical fiber probe that is connected to a high-speed detector. The results show that in the presence of nonlinearity, the output exhibits a spatiotemporal character that cannot be adequately characterized by a camera image or pulse shape alone.
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Publication: J. Vac. Sci. Technol. A 38, 052601 (2020)Doi: 10.1116/6.0000289
Extreme ultraviolet (EUV) lithography has emerged as the next generational step in advancing the manufacturing of increasingly complex semiconductor devices. The commercial viability of this new lithographic technique requires compatible photoresist (PR) materials that satisfy both the lithographic and etch requirements of good feature resolution, chemical sensitivity, a low line edge roughness, and good critical dimension uniformity. Achieving the decreased feature pitches of modern processing nodes via EUV lithography places a limit on the available photoresist thickness for a pattern transfer process. Therefore, etch processes are required to maximize the etching selectivity of a hard mask material, such as SiO2, to an EUV photoresist. In this work, the authors evaluated the ability of an atomic layer etching (ALE) process to maximize the SiO2/EUV PR etching selectivity. Through the flexible parameter space available in an ALE process, the authors evaluated the etching behaviors as a function of the ALE parameters of ion energy, etch step length, fluorocarbon (FC) deposition thickness, and precursor gas type. The authors found that the interaction between the energetic argon ion bombardment and a deposited FC layer produces a modified surface layer on the PR material that can strongly control the PR etch rate and even produce an etch stop under some conditions. Under the same processing conditions, the etching behavior of SiO2 continues unimpeded, thus resulting in a high overall SiO2/PR etching selectivity. Secondary characterization using x-ray photoelectron spectroscopy and atomic force microscopy was used to support the conclusions derived from the ellipsometric modeling based on the surface chemistry evolution and determine the impact of the ALE process on the surface roughness of the EUV PR, respectively. Additionally, attenuated total reflection Fourier-transform infrared spectroscopy was used to track the impact on specific functional groups within the PR composition from both the argon ion bombardment and FC deposition components of the ALE process. The ALE-based PR etching concept established in this work serves as a foundation for both the understanding of the impacts of an ALE process on an EUV PR material and for future works, employing an ALE process for PR-based pattern transfer.
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Publication: Appl. Phys. Lett. 117, 091105 (2020)Doi: 10.1063/5.0016509
We demonstrate a single-shot terahertz spectrometer consisting of a modified Mach–Zehnder interferometer and a microbolometer focal plane array. The spectrometer is simple to use and can measure terahertz field autocorrelations and spectral power with no moving parts and no ultrashort-pulsed laser. It can effectively detect radiation at 10–40 THz when tested with a thermal source. It can also be used to measure the complex refractive index of a sample material. In principle, it can characterize both laser-based and non-laser-based terahertz sources and potentially cover 1–10 THz with specially designed terahertz microbolometers.
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Publication: J. Phys. Chem. C 124, 6830 (2020)Doi: 10.1021/acs.jpcc.9b11868
In combining organometallic with organic precursors, molecular layer deposition (MLD) offers not only an expanded portfolio of molecular combinations but specifically the possibility of tuning mechanical properties for more robust functionality. This is appealing for applications in energy storage, where ion transport in and out of electrodes causes significant stress/strain cycling. It is particularly opportune for Li-ion solid state batteries (LISSBs), where electrode and solid electrolyte structures are usually arranged densely for high power and energy. Despite diverse MLD applications to date, little prior research has been aimed at Li-containing MLD materials and processes. Here, we report the MLD growth and process for a lithium-containing organic thin film using lithium tert-butoxide (LiOtBu) and 1,3-propanediol, leading to an MLD film of lithium propane dioxide, Li2O2C3H6 (LPDO), identified through X-ray photoelectron spectroscopy (XPS) and ab initio calculations. The growth showed self-limiting behavior for both precursors, with significant nucleation delay before linear growth at 0.23 Å/cycle at 150 °C, and 0.15 Å/cycle at 200 °C. XPS-determined stoichiometry was Li1.6O2.2C3H6 at both 150 and 200 °C, while additional species, presumably from incomplete reaction, were found at 100 °C, leading to a notably higher (0.84 Å/cycle) growth rate. The LPDO film showed crystallinity and high surface roughness when grown on the crystalline substrate, while on the amorphous substrate, an amorphous LPDO film with low surface roughness was observed. In addition, high air sensitivity of LPDO film was observed, with Li propyl carbonate and Li carbonate formation under air exposure. Further modification strategies were proposed in order to achieve a MLD or atomic layer deposition-/MLD-based solid electrolyte material.
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Publication: Molecular Biology Cell 31, 1753 (2020)Doi: 10.1091/mbc.E19-11-0614
The dynamic rearrangement of the actin cytoskeleton is an essential component of many mechanotransduction and cellular force generation pathways. Here we use periodic surface topographies with feature sizes comparable to those of in vivo collagen fibers to measure and compare actin dynamics for two representative cell types that have markedly different migratory modes and physiological purposes: slowly migrating epithelial MCF10A cells and polarizing, fast-migrating, neutrophil-like HL60 cells. Both cell types exhibit reproducible guidance of actin waves (esotaxis) on these topographies, enabling quantitative comparisons of actin dynamics. We adapt a computer-vision algorithm, optical flow, to measure the directions of actin waves at the submicron scale. Clustering the optical flow into regions that move in similar directions enables micron-scale measurements of actin-wave speed and direction. Although the speed and morphology of actin waves differ between MCF10A and HL60 cells, the underlying actin guidance by nanotopography is similar in both cell types at the micron and submicron scales.
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Publication: Opt. Exp. 28, 33528 (2020)Doi: 10.1364/OE.406093
The plasmon resonance of a structure is primarily dictated by its optical properties and geometry, which can be modified to enable hot-carrier photodetectors with superior performance. Recently, metal alloys have played a prominent role in tuning the resonance of plasmonic structures through chemical composition engineering. However, it has been unclear how alloying modifies the time dynamics of the generated hot-carriers. In this work, we elucidate the role of chemical composition on the relaxation time of hot-carriers for the archetypal AuxAg1-x thin film system. Through time-resolved optical spectroscopy measurements in the visible wavelength range, we measure composition-dependent relaxation times that vary up to 8× for constant pump fluency. Surprisingly, we find that the addition of 2% of Ag into Au films can increase the hot-carrier lifetime by approximately 35% under fixed fluence, as a result of a decrease in optical loss. Further, the relaxation time is found to be inversely proportional to the imaginary part of the permittivity. Our results indicate that alloying is a promising approach to effectively control hot-carrier relaxation time in metals.
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Publication: Phys. Rev. Appl. 13, 034016 (2020)Doi: 10.1103/PhysRevApplied.13.034016
Superparamagnetic tunnel junctions (SMTJs) have emerged as a competitive, realistic nanotechnology to support novel forms of stochastic computation in CMOS-compatible platforms. One of their applications is to generate random bitstreams suitable for use in stochastic computing implementations. We describe a method for digitally programmable bitstream generation based on pre-charge sense amplifiers. This generator is significantly more energy efficient than SMTJ-based bitstream generators that tune probabilities with spin currents and a factor of two more efficient than related CMOS-based implementations. The true randomness of this bitstream generator allows us to use them as the fundamental units of a novel neural network architecture. To take advantage of the potential savings, we codesign the algorithm with the circuit, rather than directly transcribing a classical neural network into hardware. The flexibility of the neural network mathematics allows us to adapt the network to the explicitly energy efficient choices we make at the device level. The result is a convolutional neural network design operating at ≈ 150 nJ per inference with 97 % performance on MNIST-a factor of 1.4 to 7.7 improvement in energy efficiency over comparable proposals in the recent literature.
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Publication: Plasma Process. Polym. 17, e1900103 (2020)Doi: 10.1002/ppap.201900103
Cold atmospheric plasma (CAP) produces reactive oxygen species and reactive nitrogen species, which may disproportionally damage tumor cells, resulting in potentially selective cancer therapy. Here, we compare the effects of two CAP sources, that is, the atmospheric pressure plasma jet and the surface micro discharge, on the selectivity of CAP-treated cell-culture media. CAP-treated media were applied to metastatic breast tumor cells and their normal breast epithelial cell counterparts to assess treatment selectivity, while systematically varying common cell-culture media and cell-matrix binding moieties. We show that media compositions are crucial in a CAP-treated media selectivity, while binding moieties (specifically, collagen I, fibronectin, and poly-d-lysine) play a lesser role. These data have further implications in the translation of CAP to in vivo use.
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Publication: Opt. Exp. 28, 21220 (2020)Doi: 10.1364/OE.398268
We report multicycle, narrowband, terahertz radiation at 14.8 THz produced by phase-matched optical rectification of femtosecond laser pulses in bulk lithium niobate (LiNbO3) crystals. Our experiment and simulation show that the output terahertz energy greatly enhances when the input laser pulse is highly chirped, contrary to a common optical rectification process. We find this abnormal behavior is attributed to a linear electro-optic (EO) effect, in which the laser pulse propagating in LiNbO3 is modulated by the terahertz field it produces, and this in turn drives optical rectification more effectively to produce the terahertz field. This resonant cascading effect can greatly increase terahertz conversion efficiencies when the input laser pulse is properly pre-chirped with additional third order dispersion. We also observe similar multicycle terahertz emission from lithium tantalate (LiTaO3) at 14 THz and barium borate (BBO) at 7 THz, 10.6 THz, and 14.6 THz, all produced by narrowband phase-matched optical rectification.
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Publication: Chaos 30, 053111 (2020)Doi: 10.1063/5.0005541
We consider the commonly encountered situation (e.g., in weather forecast) where the goal is to predict the time evolution of a large, spatiotemporally chaotic dynamical system when we have access to both time series data of previous system states and an imperfect model of the full system dynamics. Specifically, we attempt to utilize machine learning as the essential tool for integrating the use of past data into predictions. In order to facilitate scalability to the common scenario of interest where the spatiotemporally chaotic system is very large and complex, we propose combining two approaches: (i) a parallel machine learning prediction scheme and (ii) a hybrid technique for a composite prediction system composed of a knowledge-based component and a machine learning-based component. We demonstrate that not only can this method combining (i) and (ii) be scaled to give excellent performance for very large systems but also that the length of time series data needed to train our multiple, parallel machine learning components is dramatically less than that necessary without parallelization. Furthermore, considering cases where computational realization of the knowledge-based component does not resolve subgrid-scale processes, our scheme is able to use training data to incorporate the effect of the unresolved short-scale dynamics upon the resolved longer-scale dynamics (subgrid-scale closure).
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Publication: NPG Asia Mater. 12, 28 (2020)Doi: 10.1038/s41427-020-0204-0
Hybrid supercapacitors have recently emerged as next-generation energy storage devices that bridge the gap between supercapacitors and lithium-ion batteries. However, developing high energy cathodes that maintain long-term cycle stability and a high rate capability for real applications remains a significantly challenging issue. Herein, we report a facile synthesis method for a laser-scribed graphene/LiNi1/3Mn1/3Co1/3O2 (LSG/NMC) composite for high energy cathode materials for use in hybrid supercapacitors. LSG/NMC composites exhibit not only a high capacitance of up to 141.5 F/g but also an excellent capacitance retention of 98.1% after 1000 cycles at a high current density of 5.0 A/g. The introduction of an NMC spacer between the LSG layers provides an enlarged interspace that can act as an efficient channel for additional storage sites and rapid access. In addition, we further confirmed that hybrid supercapacitors using LSG/NMC cathodes and H2T12O25 anodes with an AlPO4/carbon hybrid coating layer (H-HTO) deliver a remarkable energy density of ~123.5 Wh/kg, power density of ~14074.8 W/kg, and a long-term cycle stability of 94.6% after 20,000 cycles. This work demonstrates that our proposed material can be considered a strong cathode candidate for next-generation hybrid supercapacitors.
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Publication: Nature 578, 219 (2020)Doi: 10.1038/d41586-020-00323-x
Devices known as quantum cascade lasers produce useful terahertz radiation, but are typically highly sensitive to fabrication defects. This limitation has now been overcome using a property called topological robustness.
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Publication: Photonics 7, 1 (2020)Doi: 10.1021/acsphotonics.9b00679
Photovoltaics possess significant potential due to the abundance of solar power incident on earth; however, they can only generate electricity during daylight hours. In order to produce electrical power after the sun has set, we consider an alternative photovoltaic concept that uses the earth as a heat source and the night sky as a heat sink, resulting in a “nighttime photovoltaic cell” that employs thermoradiative photovoltaics and concepts from the advancing field of radiative cooling. In this Perspective, we discuss the principles of thermoradiative photovoltaics, the theoretical limits of applying this concept to coupling with deep space, the potential of advanced radiative cooling techniques to enhance their performance, and a discussion of the practical limits, scalability, and integrability of this nighttime photovoltaic concept.
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Publication: J. Infrared Millim. Terahertz Waves 41, 141 (2020)Doi: 10.1007/s10762-019-00646-5
The main trends in gyrotron development are escalation of the radiated power and increasing the frequency of coherent radiation. For both trends, it is beneficial to develop gyrotrons with wide emitters because this allows one to use cryomagnets with smaller inner bore sizes. For analyzing and optimizing the operation of gyrotrons with wide emitters, it is proposed to represent such emitters as a superposition of thin rings and analyze the properties of electron beams emitted by each of these rings. The present paper consists of two parts. In the first part, the peak values of the orbital velocities and their spread are determined in all fractions of an electron beam in a gyrotron with the standard and widened emitters; also, the effect of profiling the anode on characteristics of these electron beam fractions is considered. In the second part, the interaction efficiency of electron beams produced by thin emitter rings is described and the relationship between these efficiencies and orbital-to-axial velocity ratios in these beams is discussed.
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Publication: Opt. Lett. 45, 3617 (2020)Doi: 10.1364/OL.393913
We demonstrate efficient multicycle terahertz pulse generation at 14.6 THz from large-area lithium niobate crystals by using high-energy (up to 2 J) femtosecond Ti:sapphire laser pulses. Such terahertz radiation is produced by phase-matched optical rectification in lithium niobate. Experimentally, we achieve maximal terahertz energy of 0.71 mJ with conversion efficiency of ∼0.04%. Our experimental setup is simple and easily upscalable to produce multi-millijoule, multicycle terahertz radiation with proper lithium niobate crystals.
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Publication: Opt. Exp. 28, 30889 (2020)Doi: 10.1364/Oe.403728
The tunability of the longitudinal localized surface plasmon resonances (LSPRs) of metallic nanoarcs is demonstrated with key relationships identified between geometric parameters of the arcs and their resonances in the infrared. The wavelength of the LSPRs is tuned by the mid-arc length of the nanoarc. The ratio between the attenuation of the fundamental and second order LSPRs is governed by the nanoarc central angle. Beneficial for plasmonic enhancement of harmonic generation, these two resonances can be tuned independently to obtain octave intervals through the design of a non-uniform arc-width profile. Because the character of the fundamental LSPR mode in nanoarcs combines an electric and a magnetic dipole, plasmonic nanoarcs with tunable resonances can serve as versatile building blocks for chiroptical and nonlinear optical devices.
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Publication: Phys. Rev. B 101, 205303 (2020)Doi: 10.1103/PhysRevB.101.205303
Chiral nanophotonic components, such as waveguides and resonators coupled to quantum emitters, provide a fundamentally new approach to manipulate light-matter interactions. The recent emergence of topological photonics has provided a new paradigm to realize helical/chiral nanophotonic structures that are flexible in design and, at the same time, robust against sharp bends and disorder. Here we demonstrate such a topologically protected chiral nanophotonic resonator that is strongly coupled to a solid-state quantum emitter. Specifically, we employ the valley-Hall effect in a photonic crystal to achieve topological edge states at an interface between two topologically distinct regions. Our helical resonator supports two counterpropagating edge modes with opposite polarizations. We first show chiral coupling between the topological resonator and the quantum emitter such that the emitter emits preferably into one of the counterpropagating edge modes depending upon its spin. Subsequently, we demonstrate strong coupling between the resonator and the quantum emitter using resonant Purcell enhancement in the emission intensity by a factor of 3.4. Such chiral resonators could enable designing complex nanophotonic circuits for quantum information processing and studying novel quantum many-body dynamics.
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Publication: J. Vac. Sci. Technol. A 38, 033011 (2020)Doi: 10.1116/1.5143537
A material etching system was developed by combining beam electron injection from a direct current hollow cathode (HC) electron source with the downstream reactive environment of a remote CF4/O2 low temperature plasma. The energy of the injected beam electrons is controlled using an acceleration electrode biased positively relative to the HC argon discharge. For an acceleration voltage greater than the ionization potential of Ar, the extracted primary electrons can produce a secondary plasma in the process chamber. The authors characterized the properties of the secondary plasma by performing Langmuir probe measurements of the electron energy probability function (EEPF) 2.5 cm below the extraction ring. The data indicate the existence of two major groups of electrons, including electrons with a primary beam electron energy that varies as the acceleration voltage is varied along with low energy electrons produced by ionization of the Ar gas atoms in the process chamber by the injected beam electrons. When combining the HC Ar beam electron with a remote CF4/O2 electron cyclotron wave resonance plasma, the EEPF of both the low energy plasma electron and beam electron components decreases. Additionally, the authors studied surface etching of Si3N4 and polycrystalline Si (poly-Si) thin films as a function of process parameters, including the acceleration voltage (0–70 V), discharge current of the HC discharge (1–2 A), pressure (2–100 mTorr), source to substrate distance (2.5–5 cm), and feed gas composition (with or without CF4/O2). The direction of the incident beam electrons was perpendicular to the surface. Si3N4 and polycrystalline silicon etching are seen and indicate an electron-neutral synergy effect. Little to no remote plasma spontaneous etching was observed for the conditions used in this study, and the etching is confined to the substrate area irradiated by the injected beam electrons. The electron etched Si3N4 surface etching rate profile distribution is confined within a ∼30 mm diameter circle, which is slightly broader than the area for which poly-Si etching is seen, and coincides closely with the spatial profile of beam electrons as determined by the Langmuir probe measurements. The magnitude of the poly-Si etching rate is by a factor of two times smaller than the Si3N4 etching rate. The authors discuss possible explanations of the data and the role of surface charging.
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Publication: Phys. Rev. E 101, 032213 (2020)Doi: 10.1103/PhysRevE.101.032213
Recently, it was shown that certain systems with large time-varying delay exhibit different types of chaos, which are related to two types of time-varying delay: conservative and dissipative delays. The known high-dimensional turbulent chaos is characterized by strong fluctuations. In contrast, the recently discovered low-dimensional laminar chaos is characterized by nearly constant laminar phases with periodic durations and a chaotic variation of the intensity from phase to phase. In this paper we extend our results from our preceding publication [Hart, Roy, Müller-Bender, Otto, and Radons, Phys. Rev. Lett. 123, 154101 (2019)], where it is demonstrated that laminar chaos is a robust phenomenon, which can be observed in experimental systems. We provide a time series analysis toolbox for the detection of robust features of laminar chaos. We benchmark our toolbox by experimental time series and time series of a model system which is described by a nonlinear Langevin equation with time-varying delay. The benchmark is done for different noise strengths for both the experimental system and the model system, where laminar chaos can be detected, even if it is hard to distinguish from turbulent chaos by a visual analysis of the trajectory.
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Publication: Nano Lett. 20, 741 (2020)Doi: 10.1021/acs.nanolett.9b04679
Rare-earth ion ensembles doped in single crystals are a promising materials system with widespread applications in optical signal processing, lasing, and quantum information processing. Incorporating rare-earth ions into integrated photonic devices could enable compact lasers and modulators, as well as on-chip optical quantum memories for classical and quantum optical applications. To this end, a thin film single crystalline wafer structure that is compatible with planar fabrication of integrated photonic devices would be highly desirable. However, incorporating rare-earth ions into a thin film form-factor while preserving their optical properties has proven challenging. We demonstrate an integrated photonic platform for rare-earth ions doped in a single crystalline thin film lithium niobate on insulator. The thin film is composed of lithium niobate doped with Tm3+. The ions in the thin film exhibit optical lifetimes identical to those measured in bulk crystals. We show narrow spectral holes in a thin film waveguide that require up to 2 orders of magnitude lower power to generate than previously reported bulk waveguides. Our results pave the way for scalable on-chip lasers, optical signal processing devices, and integrated optical quantum memories.
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Publication: Opt. Exp. 28, 11463 (2020)Doi: 10.1364/OE.389393
High intensity laser filamentation in air has recently demonstrated that, through plasma generation and its associated shockwave, fog can be cleared around the beam, leaving an optically transparent path to transmit light. However, for practical applications like free-space optical communication (FSO), channels of multi-centimeter diameters over kilometer ranges are required, which is extremely challenging for a plasma based method. Here we report a radically different approach, based on quantum control. We demonstrate that fog clearing can also be achieved by producing molecular quantum wakes in air, and that neither plasma generation nor filamentation are required. The effect is clearly associated with the rephasing time of the rotational wave packet in N2.Pump excitation provided in the form of resonant trains of 8 pulses separated by the revival time are able to transmit optical data through fog with initial extinction as much as −6 dB.
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Publication: Chem. Mater. 32, 7226 (2020)Doi: 10.1021/acs.chemmater.0c01478
Vanadium oxides are widely seen as strong candidates for next-generation energy-saving electrochemical devices, ranging from their use as cathode materials in inherently safe high energy all-solid-state batteries to smart windows that employ their wide color range of electrochromic response. However, critical questions about these materials remain largely unanswered: interfacial reactions and the evolution of the electrode material as delithiation takes place. Distinguishing between topotactic (i.e., reversible) intercalation, conversion, and alloying reactions in ion tunable vanadium oxide devices, in operando, at a resolution that matches the size of structural building units, is a particularly challenging task. In this work, we investigated the effects of lithiation on the structural and optical characteristics of a model thin film system - LixV2O5 - as a function of depth, using several highly sensitive and nondestructive spectroscopic methods with different depth sensitivities. We exploit (1) LixV2O5 electrochromic properties to utilize in operando optical response, (2) depth-resolved cathodoluminescence spectroscopy (DRCLS), and (3) Raman spectroscopy to monitor the changes in LixV2O5 electronic structure from the surface to the bulk of the thin film with nanoscale resolution. We find that the degradation of electrochemical performance with deep discharge of LixV2O5 is associated with drastic band structure changes that accompany octahedral distortion, rather than with a chemical conversion reaction. Elongation along the c axis and charge redistribution induced by varying levels of V(3d)–O(2p) hybridization in the presence of the Li considerably affect the electronic band structure. The coexistence of multiple metastable phases, strong electron correlation, and deviation from an ideal cubic symmetry results in lower structural reversibility with a higher bandgap. Beyond these specific inferences, these results suggest that these optical techniques—Raman, optical absorption/reflection, and cathodoluminescence—can be a powerful combination to reveal electrochemical behavior of ion-tunable transition metal oxides materials and associated reaction mechanisms.
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Publication: Phys. Rev. X 10, 011044 (2020)Doi: 10.1103/PhysRevX.10.011044
We report on a new type of chimera state that attracts almost all initial conditions and exhibits power-law switching behavior in networks of coupled oscillators. Such switching chimeras consist of two symmetric configurations, which we refer to as subchimeras, in which one cluster is synchronized and the other is incoherent. Despite each subchimera being linearly stable, switching chimeras are extremely sensitive to noise: Arbitrarily small noise triggers and sustains persistent switching between the two symmetric subchimeras. The average switching frequency scales as a power law with the noise intensity, which is in contrast with the exponential scaling observed in typical stochastic transitions. Rigorous numerical analysis reveals that the power-law switching behavior originates from intermingled basins of attraction associated with the two subchimeras, which, in turn, are induced by chaos and symmetry in the system. The theoretical results are supported by experiments on coupled optoelectronic oscillators, which demonstrate the generality and robustness of switching chimeras.
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Publication: Phys. Rev. Appl. 13, 024067 (2020)Doi: 10.1103/PhysRevApplied.13.024067
Monolithic optical cavities employing total internal reflections are appealing compact platforms for a wide range of applications from fundamental physics to industry applications. We characterize the transverse patterns of a millimeter-size monolithic nonplanar ring vortex laser pumped by a Gaussian beam. Vortex beams carrying orbital angular momentum (OAM) up to 30 together with vortex crystals and petal patterns are observed. The rigid nature of the monolithic cavity brings along its low-noise lasing performance for these beams. A frequency noise floor as low as 0.1Hz/Hz1/2 corresponding to sub-Hz instantaneous linewidth is measured. Furthermore, we demonstrate low-noise dual-frequency lasing with phase noise as low as −118dBc/Hz at 10-kHz offset for the beatnote at 8.2 GHz. We expect that such versatile and compact OAM laser sources can facilitate multiple applications including optical communications, optical tweezers, and laser metrology such as dual-frequency Doppler lidar.
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Publication: J. Vac. Sci. Technol. B 38, 032208 (2020)Doi: 10.1116/1.5143538
An etching system based on the interaction of electrons extracted from a direct current hollow cathode (HC) Ar plasma and injected toward an Si3N4 covered silicon substrate located in the downstream reactive environment created by an additional remote CF4/O2 plasma source was developed and evaluated. By controlling the properties of the injected beam electrons, this approach allows to deliver energy to a surface functionalized by exposure to reactive species and initiate surface etching. The energy of the primary beam electrons is controlled by the acceleration voltage relative to the HC discharge. Ar atoms flow from the high-pressure HC discharge into the low pressure downstream reactive environment in the process chamber. For an acceleration voltage greater than the ionization potential of Ar and/or process gas species, the energetic primary beam electrons produce a secondary plasma in the process chamber and can also cause additional dissociation. The authors have characterized the properties of the secondary plasma and also surface etching of Si3N4 as a function of process parameters, including acceleration voltage (0–80 V), discharge current of the HC discharge (1–2 A), pressure (3.5–20 mTorr), source to substrate distance (1.5–5 cm), and feed gas composition (20% and 80% O2 in CF4/O2). The electron energy probability function measured with a Langmuir probe about 2.5 cm below the extraction ring suggests several major groups of electrons for this situation, including high energy primary beam electrons with an energy that varies as the acceleration voltage is changed and low-energy electrons produced by beam electron-induced ionization of the Ar gas in the process chamber. When a remote CF4/O2 plasma is additionally coupled to the process chamber, Si3N4 surfaces can be functionalized, and by varying the energy of the beam electrons, Si3N4 etching can be induced by electron-neutral synergy effect with plasma-surface interaction. For conditions without beam electron injection, the remote plasma etching rate of Si3N4 depends strongly on the O2 concentration in the CF4/O2 processing gas mixture and can be suppressed for O2-rich process conditions by the formation of an SiONF passivation layer on the Si3N4 surface. The combination of the HC electron beam (HCEB) source with the remote plasma source makes it possible to induce Si3N4 etching for O2-rich remote plasma conditions where remote plasma by itself produces negligible Si3N4 etching. The electron enhanced etching of Si3N4 depends strongly on the O2/CF4 mixing ratio reflecting changing arrival rates of O and F species at the surface. Optical emission spectroscopy was used to estimate the ratio of gas phase F and O densities and found to be controlled by the gas mixing ratio and independent of HCEB operating conditions. At this time, the detailed sequence of events operative in the etching mechanism is unclear. While the increase of the electron energy is ultimately responsible for initiating surface etching, presently, the authors cannot rule out a role of ions from the simultaneously produced secondary plasma in plasma-surface interaction mechanisms.
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Publication: Opt. Exp. 28, 30656 (2020)Doi: 10.1364/OE.404791
We propose a time-domain model to analyze the dynamical behavior of miniature optoelectronic oscillators (OEOs) based on whispering-gallery mode resonators. In these systems, the whispering-gallery mode resonator features a quadratic nonlinearity and operates as an electrooptical modulator, thereby eliminating the need for an integrated Mach-Zehnder modulator. The narrow optical resonances also eliminate the need for both an optical fiber delay line and an electric bandpass filter in the optoelectronic feedback loop. The architecture of miniature OEOs therefore appears as significantly simpler than the one of their traditional counterparts and permits us to achieve competitive metrics in terms of size, weight, and power. Our theoretical approach is based on the closed-loop coupling between the optical intracavity modes and the microwave signal generated via the photodetection of the output electrooptical comb. The resulting nonlinear oscillator model involves the slowly-varying envelopes of the microwave and optical fields, and its stability analysis permits the analytical determination the critical value of the feedback gain needed to trigger self-sustained oscillations. This stability analysis also allows us to understand how key parameters of the system such as cavity detuning or coupling efficiency influence the onset of the radiofrequency oscillation. Our study is complemented by time-domain simulations for the microwave and optical signals, which are in excellent agreement with the analytical predictions.
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Publication: Phys. Plasmas 27, 022502 (2020)Doi: 10.1063/1.5127948
An optimization study of Quasi-Axisymmetric (QA) stellarators with varied elongation has been carried out using the optimization code STELLOPT. The starting point of our optimization is a previously obtained QA stellarator with three field periods and an aspect ratio of 6. A series of QA stellarators are obtained at zero plasma beta with the varied elongation value ranging from 2.5 to 3.7. A good quasi-symmetry is kept when the elongation value is reduced from the original value of 3.7. The rotational transform profile and aspect ratio are kept fixed. The plasma volume is ether kept fixed or varied linearly with elongation. Furthermore, finite beta QA stellarators are considered. The corresponding bootstrap currents are calculated using the kinetic code SFINCS. A series of kink-stable QA stellarators are obtained via optimization with varied plasma beta up to 5% and self-consistent bootstrap current. This work demonstrates that good QA stellarators with finite beta and varied elongation exist that are stable to external kink modes.
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Publication: Geophys. Res. Lett. 47, e2020GL087776 (2020)Doi: 10.1029/2020GL087776
The paper investigates the applicability of machine learning (ML) to weather prediction by building a reservoir computing-based, low-resolution, global prediction model. The model is designed to take advantage of the massively parallel architecture of a modern supercomputer. The forecast performance of the model is assessed by comparing it to that of daily climatology, persistence, and a numerical (physics-based) model of identical prognostic state variables and resolution. Hourly resolution 20-day forecasts with the model predict realistic values of the atmospheric state variables at all forecast times for the entire globe. The ML model outperforms both climatology and persistence for the first three forecast days in the midlatitudes, but not in the tropics. Compared to the numerical model, the ML model performs best for the state variables most affected by parameterized processes in the numerical model.
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Publication: Optica 7, 291 (2020)Doi: 10.1364/OPTICA.384118
The goal of integrated quantum photonics is to combine components for the generation, manipulation, and detection of nonclassical light in a phase-stable and efficient platform. Solid-state quantum emitters have recently reached outstanding performance as single-photon sources. In parallel, photonic integrated circuits have been advanced to the point that thousands of components can be controlled on a chip with high efficiency and phase stability. Consequently, researchers are now beginning to combine these leading quantum emitters and photonic integrated circuit platforms to realize the best properties of each technology. In this paper, we review recent advances in integrated quantum photonics based on such hybrid systems. Although hybrid integration solves many limitations of individual platforms, it also introduces new challenges that arise from interfacing different materials. We review various issues in solid-state quantum emitters and photonic integrated circuits, the hybrid integration techniques that bridge these two systems, and methods for chip-based manipulation of photons and emitters. Finally, we discuss the remaining challenges and future prospects of on-chip quantum photonics with integrated quantum emitters.
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Publication: Optica 7, 506 (2020)Doi: 10.1364/OPTICA.386347
Synthetic dimensions provide a promising platform for photonic quantum simulations. Manipulating the flow of photons in these dimensions requires an electric field. However, photons do not have charge and do not directly interact with electric fields. Therefore, alternative approaches are needed to realize electric fields in photonics. One approach is to use engineered gauge fields that can mimic the effect of electric fields and produce the same dynamical behavior. Here, we demonstrate such an electric field for photons propagating in a two-dimensional synthetic space. Generation of electric fields in a two-dimensional synthetic lattice provides the possibility to guide photons and to trap them through the creation of quantum confined structures. We achieve this using a linearly time-varying gauge field generated by direction-dependent phase modulations. We show that the generated electric field leads to Bloch oscillations and the revival of the state after a certain number of steps dependent on the field strength. We measure the probability of the revival and demonstrate a good agreement between the observed values and the theoretically predicted results. Furthermore, by applying a nonuniform electric field, we show the possibility of waveguiding photons. Ultimately, our results open up new opportunities for manipulating the propagation of photons with potential applications in photonic quantum simulations.
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Publication: J. Vac. Sci. Technol. A 38, 032601 (2020)Doi: 10.1116/1.5143247
The early work of John Coburn and Harold Winters revealed a synergistic effect in ion-enhanced Si etching by the concurrent irradiation of Ar+ and XeF2. This finding provided an important foundation for the development of plasma dry etching processes. The experimental results of Coburn and Winters also found effects that are useful for the development of atomic layer etching (ALE) processes characterized by a self-limited etch rate. ALE approaches are widely established and can be utilized in either directional or isotropic etching by employing proper surface modification and product removal steps. Nevertheless, the development of material selective ALE processes is still limited. An approach that combines substrate-selective deposition with etching opens a new processing window for selective ALE. The authors studied the deposition behavior of mixtures of methane (CH4) with trifluoromethane (CHF3) and mixtures of methane with octafluorocyclobutane (C4F8) on HfO2 and Si surfaces. The experimental results show that a CH4/C4F8 mixture produces a comparable fluorocarbon (FC) deposition thickness on both HfO2 and Si during the deposition step. In contrast, a CH4/CHF3 mixture deposits an FC film on Si, whereas it fluorinates the HfO2 surface with negligible FC deposition. Utilizing these behaviors allows for an ALE process based on CH4/CHF3 for selective removal of HfO2 over Si. Surface characterization data that provide mechanistic insights into these processes are also provided and discussed.
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Publication: Phys. Plasmas 27, 073103 (2020)Doi: 10.1063/5.0010377
It is known that the gyrotron theory is developed in a general form that allows one to draw many important conclusions about gyrotron operation, which are valid for gyrotrons operating in arbitrary modes, at arbitrary frequencies, and driven by electron beams with different voltages and currents. One of important issues in this theory is the analysis of possible start-up scenarios, i.e., the methods allowing, first, to excite the desired mode prior to competitors in the region of soft self-excitation of this mode and, then, drive it into the zone of hard self-excitation where, as a rule, the operation with high efficiency is possible. So far, in all studies, these zones of soft and hard self-excitation were defined for specific voltages. In the present paper, it is shown how one can determine these zones in a more general manner that makes the results applicable to gyrotrons operating at arbitrary voltages. The study also includes consideration of the no-start-current zones and the role of electron velocity spread.
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Publication: Phys. Rev. Res. 2, 043422 (2020)Doi: 10.1103/PhysRevResearch.2.043422
Modern electronic systems operate in complex electromagnetic environments and must handle noise and unwanted coupling. The capability to isolate or reject unwanted signals for mitigating vulnerabilities is critical in any practical application. In this work, we describe the use of a binary programmable metasurface to (i) control the spatial degrees of freedom for waves propagating inside an electromagnetic cavity and demonstrate the ability to create nulls in the transmission coefficient between selected ports, and (ii) create the conditions for coherent perfect absorption. Both objectives are performed at arbitrary frequencies. In the first case, an effective optimization algorithm is presented that selectively generates cold spots over a single-frequency band or simultaneously over multiple-frequency bands. We show that this algorithm is successful with multiple input port configurations and varying optimization bandwidths. In the second case, we establish how this technique can be used to establish a multiport coherent perfect absorption state for the cavity.
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Publication: High Energy Density Phys. 37, 100881 (2020)Doi: 10.1016/j.hedp.2020.100881
Collisional effects on relativistic electron beam transport through high-density magnetized plasma are studied numerically and theoretically. An electron beam injected into cold high-density plasma induces the Weibel instability generating magnetic field components transverse to the direction of beam propagation. This field scatters the beam electrons. While an applied magnetic field suppresses the instability, collisions in the background plasma enhance the instability. This interesting result is verified by a dispersion relation derived based on a set of two fluid equations. We give a physical interpretation using a simple theoretical model. We also analyze the nonlinear evolution of the collisional magnetized beam-plasma interaction using a hybrid simulation code. As a result, it is found that a large scale structure is produced in the collisional plasma by an enhanced induced longitudinal electric field.
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Publication: J. Vac. Sci. Technol. A 38, 033012 (2020)Doi: 10.1116/6.0000123
Atmospheric pressure plasma jets have great potential for the surface modification of polymers. In this work, the authors report on polystyrene etching by a radio frequency driven atmospheric pressure plasma jet with a focus on the role of H⋅H⋅, O⋅O⋅, and ⋅OH⋅OH radicals in this process. The absolute flux of H⋅H⋅, O⋅O⋅, and ⋅OH⋅OH radicals reaching the surface of the polymer was determined by a comsol multiphysics reacting fluid dynamics model incorporating detailed transport phenomena in the boundary layer near the substrate. The simulated results of H⋅H⋅ and ⋅OH⋅OH densities in the jet effluent were experimentally verified by two-photon absorption laser induced fluorescence and laser induced fluorescence, respectively. The carbon atom removal flux from the polystyrene surface was taken from previously reported measurements using the same plasma source. The authors show that the boundary layer effects in the interfacial region above the substrate can have a significant impact on the calculated etching probabilities. The reaction probability (β) has a significant uncertainty although a variation of 2 orders of magnitude in β leads to uncertainties of approximately 1 order of magnitude variation in the determined etching probability. The etching probability of polystyrene by ⋅OH⋅OH radicals was confirmed to be at least an order of magnitude larger than the polystyrene etching probability by O⋅O⋅ radicals. The authors also confirmed the weak polystyrene etching probability by H⋅H⋅ radicals. The model suggests that the presence of a 30 ppm O2O2 impurity can lead to the production of ⋅OH⋅OH radicals in the far effluent of the Ar+1%H2Ar+1%H2 plasma jet close to the substrate at sufficient densities to enable effective etching.
2019
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Publication: Adv. Energy Mater. 10, 1903161 (2019)Doi: 10.1002/aenm.201903161
Metal halide perovskites exhibit optimal properties for optoelectronic devices, ranging from photovoltaics to light-emitting diodes, utilizing simple fabrication routes that produce impressive electrical and optical tunability. As perovskite technologies continue to mature, an understanding of their fundamental properties at length scales relevant to their morphology is critical. In this review, an overview is presented of the key insights into perovskite material properties provided by measurement methods based on the atomic force microscopy (AFM). Specifically, the manner in which AFM-based techniques supply valuable information regarding electrical and chemical heterogeneity, ferroelectricity and ferroelasticity, surface passivation and chemical modification, ionic migration, and material/device stability is discussed. Continued advances in perovskite materials will require multimodal approaches and machine learning, where the output of these scanning probe measurements is combined with high spatial resolution structural and chemical information to provide a complete nanoscale description of materials behavior and device performance.
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Publication: Opt. Exp. 17, 16882 (2019)Doi: 10.1364/OE.27.016882
Integration of single-photon sources and detectors to silicon-based photonics opens the possibility of complex circuits for quantum information processing. In this work, we demonstrate integration of quantum dots with a silicon photonic add-drop filter for on-chip filtering and routing of telecom photons. A silicon microdisk resonator acts as a narrow filter that transfers the quantum dot emission and filters the background over a wide wavelength range. Moreover, by tuning the quantum dot emission wavelength over the resonance of the microdisk, we can control the transmission of the quantum dot emission to the drop and through channels of the add-drop filter. This result is a step toward the on-chip control of single photons using silicon photonics for applications in quantum information processing, such as linear optical quantum computation and boson sampling.
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Publication: ACS Nano 13, 8481 (2019)Doi: 10.1021/acsnano.9b04390
The rapidly growing demand for portable electronics, electric vehicles, and grid storage drives the pursuit of high-performance electrical energy storage (EES). A key strategy for improving EES performance is exploiting nanostructured electrodes that present nanoconfined environments of adjacent electrolytes, with the goal to decrease ion diffusion paths and increase active surface areas. However, fundamental gaps persist in understanding the interface-governed electrochemistry in such nanoconfined geometries, in part because of the imprecise and variable dimension control. Here, we report quantification of lithium insertion under nanoconfinement of the electrolyte in a precise lithography-patterned nanofluidic cell. We show a mechanism that enhances ion insertion under nanoconfinement, namely, selective ion accumulation when the confinement length is comparable to the electrical double layer thickness. The nanofabrication approach with uniform and accurate dimensional control provides a versatile model system to explore fundamental mechanisms of nanoscale electrochemistry, which could have an impact on practical energy storage systems.
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Publication: Rev. Mod. Phys. 91, 035006 (2019)Doi: 10.1103/RevModPhys.91.035006
Time-delayed optoelectronic oscillators are at the center of a large body of scientific literature. The complex behavior of these nonlinear oscillators has been thoroughly explored both theoretically and experimentally, leading to a better understanding of their dynamical properties. Beyond fundamental research, these systems have also inspired a wide and diverse set of applications, such as optical chaos communications, pseudorandom number generation, optoelectronic machine learning based on reservoir computing, ultrapure microwave generation, optical pulse-train synthesis, and sensing. The aim of this review is to provide a comprehensive survey of this field, to outline the latest achievements, and discuss the main challenges ahead.
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Publication: ACS Appl. Mater. Interfaces 11, 24919 (2019)Doi: 10.1021/acsami.9b01389
Pd-containing alloys are promising materials for catalysis. Yet, the relationship of the structure–property performance strongly depends on their chemical composition, which is currently not fully resolved. Herein, we present a physical vapor deposition methodology for developing PdxAu1–x alloys with fine control over the chemical composition. We establish direct correlations between the composition and these materials’ structural and electronic properties with its catalytic activity in an ethanol (EtOH) oxidation reaction. By combining X-ray diffraction (XRD) and X-ray photelectron spectroscopy (XPS) measurements, we validate that the Pd content within both bulk and surface compositions can be finely controlled in an ultrathin-film regime. Catalytic oxidation of EtOH on the PdxAu1–x electrodes presents the largest forward-sweeping current density for x = 0.73 at ∼135 mA cm–2, with the lowest onset potential and largest peak activity of 639 A gPd–1 observed for x = 0.58. Density functional theory (DFT) calculations and XPS measurements demonstrate that the valence band of the alloys is completely dominated by Pd particularly near the Fermi level, regardless of its chemical composition. Moreover, DFT provides key insights into the PdxAu1–x ligand effect, with relevant chemisorption activity descriptors probed for a large number of surface arrangements. These results demonstrate that alloys can outperform pure metals in catalytic processes, with fine control of the chemical composition being a powerful tuning knob for the electronic properties and, therefore, the catalytic activity of ultrathin PdxAu1–x catalysts. Our high-throughput experimental methodology, in connection with DFT calculations, provides a unique foundation for further materials’ discovery, including machine-learning predictions for novel alloys, the development of Pd-alloyed membranes for the purification of reformate gases, binder-free ultrathin electrocatalysts for fuel cells, and room temperature lithography-based development of nanostructures for optically driven processes.
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Publication: Vac. Sci. Technol. B 37, 011202 (2019)Doi: 10.1116/1.5042540
Long-distance quantum communication relies on the ability to efficiently generate and prepare single photons at telecom wavelengths. Low-density InAs quantum dots on InP surfaces are grown in a molecular beam epitaxy system using a modified Stranski–Krastanov growth paradigm. This material is a source of bright and indistinguishable single photons in the 1.3 μm telecom band. Here, the exploration of the growth parameters is presented as a phase diagram, while low-temperature photoluminescence and atomic resolution images are presented to correlate structure and spectral performance. This work identifies specific stacking faults and V-shaped defects that are likely causes of the observed low brightness emission at 1.55 μm telecom wavelengths. The different locations of the imaged defects suggest possible guidance for future development of InAs/InP single photon sources for c-band, 1.55 μm wavelength telecommunication systems.
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Publication: IEEE Trans. Electron Devices 66, 2409 (2019)Doi: 10.1109/TED.2019.2907061
We present a computationally efficient method for the calculation of impedance (Z-) matrices for a large class of standing- and traveling-wave structures used in klystrons, traveling-wave tubes, and other RF vacuum electronic devices. We apply joining formulas by which the Z-matrix of a structure can be constructed from the Z-matrices of its component parts without a full-scale finite-element (FE) electromagnetic (EM) simulation of the whole structure. For the inverse operation, we apply subtraction formulas that define the Z-matrix of a structure from which a selected section has been removed. New Z-matrix modification equations are derived which determine the Z-matrix of a modified structure, without recomputing the entire structure Z-matrix. The approach and examples are extended to multibeam RF structures. The obtained Z-matrices are suitable for the large-signal 1-D and 2-D beam-wave interaction codes CHRISTINE-CC and TESLA-Z. Application of the techniques described here greatly facilitates the accurate calculation of Z-matrices and optimization of large, complex circuits that are difficult to model whole in a single FE simulation.
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Activation of Microwave Signals in Nanoscale Magnetic Tunnel Junctions by Neuronal Action Potentials
Publication: IEEE Magnetics Lett. 10, 3101405 (2019)Doi: 10.1109/LMAG.2019.2896307Action potentials are the basic unit of information in the nervous system, and their reliable detection and decoding holds the key to understanding how the brain generates complex thought and behavior. Transduction of these signals into microwave signal oscillations can enable wireless sensors that report on brain activity through magnetic induction. In this letter, we demonstrate that action potentials from the lateral giant neurons of crayfish can induce microwave oscillations in nanoscale magnetic tunnel junctions (NMTJs). We show that action potentials activate microwave oscillations in NMTJs with an amplitude that follows the action potential signal, demonstrating that the device has both the sensitivity and temporal resolution to respond to action potentials from a single neuron. The activation of magnetic oscillations by action potentials, together with the small surface area and the high-frequency tunability, makes these devices potential candidates for high-resolution sensing of bioelectric signals from neural tissues. These device attributes may be useful for the design of high-throughput bidirectional brain–machine interfaces.
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Publication: Phys. Plasmas 26, 082903 (2019)Doi: 10.1063/1.5098888
We describe a systematic development of kinetic entropy as a diagnostic in fully kinetic particle-in-cell (PIC) simulations and use it to interpret plasma physics processes in heliospheric, planetary, and astrophysical systems. First, we calculate kinetic entropy in two forms—the “combinatorial” form related to the logarithm of the number of microstates per macrostate and the “continuous” form related to flnf, where f is the particle distribution function. We discuss the advantages and disadvantages of each and discuss subtleties about implementing them in PIC codes. Using collisionless PIC simulations that are two-dimensional in position space and three-dimensional in velocity space, we verify the implementation of the kinetic entropy diagnostics and discuss how to optimize numerical parameters to ensure accurate results. We show the total kinetic entropy is conserved to three percent in an optimized simulation of antiparallel magnetic reconnection. Kinetic entropy can be decomposed into a sum of a position space entropy and a velocity space entropy, and we use this to investigate the nature of kinetic entropy transport during collisionless reconnection. We find the velocity space entropy of both electrons and ions increases in time due to plasma heating during magnetic reconnection, while the position space entropy decreases due to plasma compression. This project uses collisionless simulations, so it cannot address physical dissipation mechanisms; nonetheless, the infrastructure developed here should be useful for studies of collisional or weakly collisional heliospheric, planetary, and astrophysical systems. Beyond reconnection, the diagnostic is expected to be applicable to plasma turbulence and collisionless shocks.
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Publication: Opt. Lett. 44, 3078 (2019)Doi: 10.1364/OL.44.003078
We demonstrate that extended dissipative structures in Kerr-nonlinear whispering-gallery mode resonators undergo a spatiotemporal instability, as the pumping parameters are varied. We show that the dynamics of the patterns beyond this bifurcation yield specific Kerr comb and sub-comb spectra that can be subjected to a phase of frequency-locking when optimal conditions are met. Our numerical results are found to be in agreement with experimental measurements.
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Publication: Phys. Today 72, 70 (2019)Doi: 10.1063/PT.3.4234
The everyday concept of optics is of transparent elements, such as glass lenses, that bend beams of light in useful ways. The small lenses in our smartphones are now almost as ubiquitous as the lenses in our eyes. In both cases, the lenses redirect the rays of light scattered from, say, a tree and project them to form an image of the tree on the camera’s photosensitive chip or on our retinas.
Suppose you directed a laser beam into your smartphone lens. (Don’t even think about doing the same with your eye.) The lens redirects the beam to a near-point-like focal spot on the chip. The milliwatts of power delivered by common laser pointers is more than enough to damage your smartphone. But what if you dialed up the beam’s power enough that the beam damaged the lens before arriving at the focus? For a high-average-power continuous-wave beam, the small fractional optical absorption that always takes place inside transparent dielectric materials would eventually heat and thermally stress the lens until it fractures and melts. The lens would be ruined.
Another type of beam, though, has a radically different effect on the lens: That beam is an ultrahigh-peak-power laser pulse formed by packing a modest amount of energy into an extremely short-duration pulse. Half of the 2018 Nobel Prize in Physics was awarded for precisely that feat of compression (see Physics Today, December 2018, page 18). If such a now-routine pulse—typically of a peak intensity up to 1022 W/cm2 and a duration shorter than 100 fs—is incident on the lens, the laser electric field would cause electrons to nearly instantaneously tunnel out of the bound states of surface atoms. The laser-induced tunneling would form a solid-density plasma with optical properties akin to a highly polished metal mirror, and the pulse would specularly reflect from the surface.
To generate the plasma, one would need to focus the beam on the surface, and the interaction would need to take place in vacuum to prevent the laser ionization of air that would defocus the pulse well before it arrived at the surface. Long after the pulse is gone, damage follows on a nanosecond acoustic time scale as the dense hot plasma (with temperature on the order of 106 K and a pressure of 107 atmospheres) launches an impulsive pressure spike into the bulk of the glass and causes significant local damage.
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Publication: Sci. Adv. 5, eaav6804 (2019)Doi: 10.1126/sciadv.aav6804
Remote detection of a distant, shielded sample of radioactive material is an important goal, but it is made difficult by the finite spatial range of the decay products. Here, we present a proof-of-principle demonstration of a remote detection scheme using mid-infrared (mid-IR) (λ = 3.9 μm) laser–induced avalanche breakdown of air. In the scheme’s most basic version, we observe on-off breakdown sensitivity to the presence of an external radioactive source. In another realization of the technique, we correlate the shift of the temporal onset of avalanche to the degree of seed ionization from the source. We present scaling of the interaction with laser intensity, verify observed trends with numerical simulations, and discuss the use of mid-IR laser–driven electron avalanche breakdown to detect radioactive material at range.
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Publication: ACS Photonics 6, 302 (2019)Doi: 10.1021/acsphotonics.8b01499
Plasmons in subwavelength-structured graphene surfaces exhibit strong light–matter interaction and prominent resonance effects in the terahertz/mid-IR frequency range. Due to its exceptionally small electronic specific heat, graphene shows strong photoinduced hot electron effects that significantly alter the plasmon response. This can enable fast control of plasmon resonance through transient heating of carriers. We employ nonlinear pump–probe measurements on subwavelength graphene ribbons to explore the effect of photoinduced hot carriers on graphene plasmons. Measurements taken above and below the plasmon resonance frequency clearly demonstrate an optically induced red-shift of the plasmon resonance, which is a signature of hot carriers in the graphene. The observed photoinduced change in plasmon resonance exhibits very strong (of order 10%) and fast response times (few picoseconds), which are governed by the cooling rate of hot electrons. The results presented here contribute to the understanding of plasmonic hot carriers in graphene and can find applications in fast terahertz modulation and switching.
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Publication: Phys. Plasmas 26, 013109 (2019)Doi: 10.1063/1.5079629
We develop a sensitivity function for the design of electron optics using an adjoint approach based on a form of reciprocity implicit in Hamilton's equations of motion. The sensitivity function, which is computed with a small number of time-reversed runs of a beam optics code, allows for the determination of the effects on specific beam quality figures of merit of small, but arbitrary changes in electrode potentials, positions, and shapes and in magnet strengths and locations. The sensitivity function can thus be used in an optimization cycle of a focusing system's design and/or to predict the sensitivity of a particular design to manufacturing, assembly, and alignment errors.
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Publication: Opt. Lett. 44, 203 (2019)Doi: 10.1364/OL.44.000203
We report the emission of localized orbital angular momentum (OAM) crystals in a millimeter-size monolithic Nd:YAG nonplanar ring laser. Narrow-linewidth single-frequency lasing in the kilohertz level featuring crystal-like vortices is obtained via phase locking of Laguerre-Gaussian modes in the cavity. It is found that the spatially degenerate OAM of high-order LG modes can be easily broken by superimposing a low-order mode, leading to crystal-like vortices. Our theoretical analysis is found to be in agreement with the experimental results for both intensity and interference patterns.
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Publication: Plasma Processes Polymers 16, 1900019 (2019)Doi: 10.1002/ppap.201900019
Thin layers of polypropylene (PP) have been treated by argon low-temperature plasmas in an inductively coupled plasma setup. The etched thickness of PP was monitored in situ by means of single-wavelength ellipsometry. The ellipsometric model of the polymer surface exposed to plasma consists of a UV-modified layer, a dense amorphous carbon layer because of ion bombardment, and an effective medium approximation layer, which accounts for moderate surface roughness. The etching behavior has been compared to a model based on argon ion beam irradiation experiments. In this approach, surface processes are described in terms of etching yields and crosslinking probabilities as a function of incident fluxes and energies of Ar ions and UV photons. The ion beam model fits well with the plasma etching results.
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Publication: Joule 3, 2057 (2019)Doi: 10.1016/j.joule.2019.07.010
Despite growing public concerns and international agreements, few concrete actions have been taken to fix our changing climate. In fact, the Earth is now warming faster than expected, and greenhouse gas emissions are still on the rise. The path forward has been clear: a reduction in CO2 emission is needed through an increase in energy efficiency and cleaner power production. However, failure to act is making these solutions harder to realize, because the CO2 that we put in the atmosphere today can persist for decades. The Earth has already warmed by 1°C above pre-industrial levels and is expected to reach 1.5°C in the next 10 to 20 years. With time running out, we may need to turn to additional mitigation strategies.
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Publication: Chaos 29, 123130 (2019)Doi: 10.1063/1.5123753
We describe the continuous-time dynamics of networks implemented on Field Programable Gate Arrays (FPGAs). The networks can perform Boolean operations when the FPGA is in the clocked (digital) mode; however, we run the programed FPGA in the unclocked (analog) mode. Our motivation is to use these FPGA networks as ultrafast machine-learning processors, using the technique of reservoir computing. We study both the undriven dynamics and the input response of these networks as we vary network design parameters, and we relate the dynamics to accuracy on two machine-learning tasks.
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Efficient Terahertz and Brunel Harmonic Generation from Air Plasma via Mid-Infrared Coherent Control
Publication: Optica 6, 1338 (2019)Doi: 10.1364/OPTICA.6.001338Nonlinear light conversion involves one or more bound–bound, bound–free, free–free, and free–bound transitions. It is often challenging to interpret the exact conversion mechanisms. Here we use a femtosecond mid-infrared laser to enhance free–free transitions in terahertz and Brunel harmonic generation from air plasma. Microscopically, both THz and harmonics originate from a common source–ionization-induced plasma currents–and are greatly enhanced when driven by intense long-wavelength pulses. We observe 1% laser-to-terahertz conversion efficiency. Using two-color laser fields, we generate coherent radiation from terahertz to petahertz and investigate the interplay among tunneling ionization, terahertz, and harmonic generation with coherent control.
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Publication: Chaos 29, 121104 (2019)Doi: 10.1063/1.5134845
We introduce and test a general machine-learning-based technique for the inference of short term causal dependence between state variables of an unknown dynamical system from time-series measurements of its state variables. Our technique leverages the results of a machine learning process for short time prediction to achieve our goal. The basic idea is to use the machine learning to estimate the elements of the Jacobian matrix of the dynamical flow along an orbit. The type of machine learning that we employ is reservoir computing. We present numerical tests on link inference of a network of interacting dynamical nodes. It is seen that dynamical noise can greatly enhance the effectiveness of our technique, while observational noise degrades the effectiveness. We believe that the competition between these two opposing types of noise will be the key factor determining the success of causal inference in many of the most important application situations.
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Publication: Opt. Lett. 44, 4710 (2019)Doi: 10.1364/OL.44.004710
We present an electro-optic downconverting mixer with image rejection capabilities. By using a dual-drive Mach-Zehnder modulator (DD-MZM) to modulate an optical carrier with both a signal and a local oscillator, and an asymmetric Mach-Zehnder interferometer (AMZI) to filter the optical spectrum into two separate ports, we generate photocurrents with a phase relationship controlled via direct current (DC) bias voltage applied to the DD-MZM. By choosing these photocurrents to be in quadrature and combining them in a 90-degree electrical hybrid we achieve over 40 dB of image rejection, with a 3 dB bandwidth of approximately 20 GHz limited mainly by the AMZI free spectral range. We demonstrate downconversion of a 1 Gbaud quadrature phase-shift keyed (QPSK) signal even in the presence of a strong interfering image tone.
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Spectral Broadening of a KrF Laser via Propagation through Xe in the Negative Nonlinear Index Regime
Publication: Phys. Rev. A 100, 023831 (2019)Doi: 10.1103/PhysRevA.100.023831In inertial confinement (ICF) experiments at the NIKE laser facility, the high-power krypton fluoride (KrF) laser output beams propagate through long (∼75m) air paths to achieve angular multiplexing, which is required because the KrF medium does not store energy for a sufficiently long time. Recent experiments and simulations have shown that, via stimulated rotational Raman scattering, this propagation can spectrally broaden the laser beam well beyond the ∼1 THz laser linewidth normally achieved by the induced spatial incoherence (ISI) technique used in NIKE. These enhanced bandwidths may be enough to suppress the laser-plasma instabilities which limit the maximum intensity that can be incident on the ICF target. In this paper we investigate an alternative technique that achieves spectral broadening by self-phase modulation in Xe gas, which has a large, negative nonlinear refractive index ∼248 nm, and thus completely avoids transverse filamentation issues. The collective, nonlinear atomic response to the chaotic, nonsteady state ISI light is modeled using a two-photon vector model, and the effect of near-resonant behavior on the spectral broadening is studied.
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Publication: Phys. Today 72, 74 (2019)Doi: 10.1063/PT.3.4327
One usually imagines a vacuum as empty space devoid of any matter. That picture isn’t quite accurate when quantum mechanics is taken into account. Emptiness turns out to be an illusion: The real vacuum is full of activity in the form of quantum fluctuations—sometimes thought of as virtual particles that appear and disappear so quickly that they don’t violate Heisenberg’s uncertainty principle. In this Quick Study, I discuss how electromagnetic fluctuations can give rise to forces and even torques between macroscopic objects without the need for any other interactions. Indeed, the quantum mechanics of a vacuum may prove to be an exciting tool for engineering nanoscale devices.
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Publication: APL Photonics 4, 034502 (2019)Doi: 10.1063/1.5046732
Black phosphorus possesses several attractive properties for optoelectronics, notably a direct and layer dependent bandgap that varies from the visible to mid-infrared and the ability to transfer the material to nearly arbitrary substrates. A less utilized property of black phosphorus for optoelectronics is the nonlinear photoresponse. The photocarrier lifetime in black phosphorus exhibits a strong nonlinear dependence on the excitation density that is utilized in the present work for optoelectronic mixing. In this scheme, two telecommunications-band lasers are intensity-modulated by a radio frequency (RF) and local oscillator (LO) frequency and focused onto a black phosphorus photoconductive detector. Above the saturation carrier density, the photocurrent is proportional to the square root of the optical power which produces photocurrents at the sum and difference frequencies of the input beams. The bandwidth of the mixing process increases from 10 to 100 MHz for incident powers of 0.01 to 1 mW, respectively. An excess carrier model accurately describes the power dependence of the cutoff frequency and mixing conversion, which are both limited by photocarrier recombination. Optimizing our device geometry to support larger bias fields and decreased carrier transit times could increase the maximum RF/LO frequency beyond a GHz by reducing the excess carrier lifetime. Frequency mixing based on the photocarrier nonlinearity in multilayer black phosphorus demonstrated here can be readily extended to mid-infrared wavelengths as long as 4 µm.
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Publication: Phys. Plasmas 26, 093101 (2019)Doi: 10.1063/1.5090886
We consider the development of a highly efficient, gridless tetrode as a megawatt-level RF source in the 3 to 10 MHz range for application in mobile ionospheric heaters. Such a heater has potential advantages over the stationary facilities, such as High-Frequency Active Auroral Research Program, found at high latitudes. The considered device operates in class D mode with an annular electron beam allowing realization of high efficiency. The present study, based on numerical simulations using the Particle in Cell code Michelle [Petillo et al., IEEE Trans. Electron Devices Sci. 52, 742 (2005)], examines the optimization of device geometry. In particular, the dependence of efficiency on spacing between electrodes is studied. In addition, the role of secondary electrons emitted at the collector is examined. Both static and time dependent operations are simulated. In the time dependent case, it is found that during the portion of the RF cycle when the beam current is on, secondaries emitted from the collector are driven back into the collector by the incoming primary beam. When the beam is switched off, secondaries can stream back into the tetrode and have a small negative impact on efficiency. We present a design in which the secondary electrons are eventually absorbed at the collector rather than at the cathode or anode.
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Publication: Metal Nanostructures Photonics 87 (2019)Doi: 10.1016/B978-0-08-102378-5.00005-2
In this chapter, we review the near- and far-field optical responses of metallic nanostructures, ranging from pure metals, heterometallic systems, and metallic alloys. We utilize numerical simulations to discuss the near- and far-field optical properties of self-assembled Au and Ag-Au heterometallic nanostructures with geometries that mimic molecules such as CH4 and WCl6. We discuss the unique, polarization-independent response for structures containing at least threefold rotational symmetry. We quantify the absorption characteristics of Au and Au/Ag metasurfaces formed by ordered arrays of linear trimers and show that by modifying the arrangement and relative spacing between the nanoparticles, the absorption is reversibly switched between high and low within the visible range of the spectrum. Further, we present alloyed metallic nanoparticles of Au and Ag and map their optical response at the nanoscale using near-field scanning optical microscopy. We spatially resolve high transmittance centered directly beneath the nanoparticles at resonance, which is in excellent agreement with our numerical simulations. Overall, this study demonstrates alternative materials with on-demand optical properties for applications in photonics such as waveguides, sensors, polarization converters, and color displays.
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Publication: Langmuir 35, 4270 (2019)Doi: 10.1021/acs.langmuir.9b00316
We report on the chemical analysis of ultrathin (10 nm) polymer films using the attenuated total reflectance–Fourier transform infrared (ATR-FTIR) technique based on p-polarized infrared light and two types of enhancing substrates, that is, metallic (Au) and dielectric (Si). We selected low-temperature plasma-treated ∼10 nm thick polystyrene films as a test case for demonstrating the capability of the p-polarized ATR-FTIR, whose performance was further compared with the conventional X-ray photoelectron spectroscopy (XPS) techniques. Although ATR-FTIR cannot be used for quantitatively determining elemental compositions in polymers at which XPS excels, it is able to be operated under nonvacuum conditions and allows the study of hydrogen-containing moieties. By correcting the contact condition between the polymer surface and the ATR prism, the relative concentration of the chemical bonds from different samples can be compared. Because ATR-FTIR and XPS provide complementary information on chemical bonds, their combination provides a powerful approach for studying the chemical composition of polymers.
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Publication: ACS Photonics 6, 272 (2019)Doi: 10.1021/acsphotonics.8b01299
Optical reconfigurability has enabled the realization of photonic devices that change in functionality, including modulators, sensors, and signal processors. Yet, most approaches to date require the application of power, which severely limits their usage in portable devices. We demonstrate the concept of transient photonics based on Mg, a burgeoning material for (nano)photonics. We realize dynamic Mg/MgO/Mg color pixels covering the entire sRGB gamut color spectrum, where all hues vanish completely in less than 10 min upon exposure to water at room temperature and neutral pH, ideal for encryption. This scalable thin-film architecture has a robust angular response, maintaining vivid colors up to 80 degrees of incidence. Our transient photonics approach using materials that are earth-abundant and CMOS-compatible opens the door for the implementation of reconfigurable devices with controlled responses in the UV–IR that can disappear without leaving any trace after stable operation, relevant for healthcare, defense, and energy applications.
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Publication: ACS Appl. Mater. Interfaces 11, 45057 (2019)Doi: 10.1021/acsami.9b14244
PdxAu1–x alloys have recently shown great promise for next-generation optical hydrogen sensors due to their increased chemical durability while their optical sensitivity to small amounts of hydrogen gas is maintained. However, the correlation between chemical composition and the dynamic optical behavior upon hydrogenation/dehydrogenation is currently not well understood. A complete understanding of this relation is necessary to optimize future sensors and nanophotonic devices. Here, we quantify the dynamic optical, chemical, and mechanical properties of thin film PdxAu1–x alloys as they are exposed to H2 by combining in situ ellipsometry with gravimetric and stress measurements. We demonstrate the dynamic optical property dependence of the film upon hydrogenation and directly correlate it with the hydrogen content up to a maximum of 7 bar of H2. With this measurement, we find that the thin films exhibit their strongest optical sensitivity to H2 in the near-infrared. We also discover higher hydrogen-loading amounts as compared to previous measurements for alloys with low atomic percent Pd. Specifically, a measurable optical and gravimetric hydrogen response in alloys as low as 34% Pd is found, when previous works have suggested a disappearance of this response near 55% Pd. This result suggests that differences in film stress and microstructuring play a crucial role in the sorption behavior. We directly measure the thin film stress and morphology upon hydrogenation and show that the alloys have a substantially higher relative stress change than pure Pd, with the pure Pd data point falling 0.9 GPa below the expected trend line. Finally, we use the measured optical properties to illustrate the applicability of these alloys as grating structures and as a planar physical encryption scheme, where we show significant and variable changes in reflectivity upon hydrogenation. These results lay the foundation for the composition and design of next-generation hydrogen sensors and tunable photonic devices.
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Publication: ACS Nano 13, 1438 (2019)Doi: 10.1021/acsnano.8b07295
Perovskite solar cells that incorporate small concentrations of Cs in their A-site have shown increased lifetime and improved device performance. Yet, the development of fully stable devices operating near the theoretical limit requires understanding how Cs influences perovskites’ electrical properties at the nanoscale. Here, we determine how the chemical composition of three perovskites (MAPbBr3, MAPbI3, and Cs-mixed) affects their short- and long-term voltage stabilities, with <50 nm spatial resolution. We map an anomalous irreversible electrical signature on MAPbBr3 at the mesoscale, resulting in local Voc variations of ∼400 mV, and in entire grains with negative contribution to the Voc. These measurements prove the necessity of high spatial resolution mapping to elucidate the fundamental limitations of this emerging material. Conversely, we capture the fully reversible voltage response of Cs-mixed perovskites, composed by Cs0.06(MA0.17FA0.83)0.94Pb(I0.83Br0.17)3, demonstrating that the desired electrical output persists even at the nanoscale. The Cs-mixed material presents no spatial variation in Voc, as ion motion is restricted. Our results show that the nanoscale electrical behavior of the perovskites is intimately connected to their chemical composition and macroscopic response.
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Publication: ACS Appl. Mater. Interfaces 11, 47516 (2019)Doi: 10.1021/acsami.9b14953
Due to recent breakthroughs in silicon photonics, sub-band-gap photodetection in silicon (Si) has become vital to the development of next-generation integrated photonic devices for telecommunication systems. In particular, photodetection in Si using complementary metal-oxide semiconductor (CMOS) compatible materials is in high demand for cost-effective integration. Here, we achieve broad-band near-infrared photodetection in Si/metal-oxide Schottky junctions where the photocurrent is generated from interface defects induced by aluminum-doped zinc oxide (AZO) films deposited on a Si substrate. The combination of photoexcited carrier generation from both interface defect states and intrinsic Si bulk defect states contributes to a photoresponse of 1 mA/W at 1325 nm and 0.22 mA/W at 1550 nm with zero-biasing. From a fit to the Fowler equation for photoemission, we quantitatively determine the individual contributions from these effects. Finally, using this analysis, we demonstrate a gold-nanoparticle-coated photodiode that has three distinct photocurrent responses resulting from hot carriers in the gold, interface defects from the AZO, and bulk defects within the Si. The hot carrier response is found to dominate near the band gap of Si, while the interface defects dominate for longer wavelengths.
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Publication: Nature 570, 45 (2019)Doi: 10.1038/s41586-019-1256-6
The 1989 claim of ‘cold fusion’ was publicly heralded as the future of clean energy generation. However, subsequent failures to reproduce the effect heightened scepticism of this claim in the academic community, and effectively led to the disqualification of the subject from further study. Motivated by the possibility that such judgement might have been premature, we embarked on a multi-institution programme to re-evaluate cold fusion to a high standard of scientific rigour. Here we describe our efforts, which have yet to yield any evidence of such an effect. Nonetheless, a by-product of our investigations has been to provide new insights into highly hydrided metals and low-energy nuclear reactions, and we contend that there remains much interesting science to be done in this underexplored parameter space.
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Publication: Plasma Proc. Polymers 16, e1800193 (2019)Doi: 10.1002/ppap.201800193
The use of surface microdischarge (SMD) for the bacterial decontamination of raw produce was evaluated. With 1 min of SMD treatment, >2 logarithmic reduction in Escherichia coli O157:H7 was consistently observed. The scanning electron microscopy of E. coli O157:H7 show that SMD damages the cell membrane, leads to cell expansion, and eventually lysis. The attenuated total reflectance-Fourier-transform infrared spectroscopy characterization of E. coli O157:H7 and lipopolysaccharides (LPS) shows that SMD causes (a) the oxidation of cellular components by forming COOH and COO − groups inside and on the cell wall, and (b) the modification of polysaccharides and phosphorus-containing groups found in phospholipids and DNA. Further characterization with X-ray photoelectron spectroscopy suggests SMD mainly modifies the O-chain and core-polysaccharide part of LPS.
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Publication: Phys. Rev. A 99, 033805 (2019)Doi: 10.1103/PhysRevA.99.033805
The archetypal quantum interferometry experiment yields an interference pattern that results from the indistinguishability of two spatiotemporal paths available to a photon or to a pair of entangled photons. A fundamental challenge in quantum interferometry is to perform such experiments with a higher number of paths and over large distances. We demonstrate that using indistinguishable frequency paths instead of spatiotemporal ones allows for robust, high-dimensional quantum interferometry in optical fibers. In our system, twin photons from an Einstein-Podolsky-Rosen pair are offered up to nine frequency paths after propagation in long-haul optical fibers and we show that the multipath quantum interference patterns can be faithfully restored after the photons travel a total distance of up to 60km.
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Publication: J. Plasma Phys. 85, 7958050501 (2019)Doi: 10.1017/S0022377819000527
Stellarators are a promising route to steady-state fusion power. However, to achieve the required confinement, the magnetic geometry must be highly optimized. This optimization requires navigating high-dimensional spaces, often necessitating the use of gradient-based methods. The gradient of the neoclassical fluxes is expensive to compute with classical methods, requiring flux computations, where is the number of parameters. To reduce the cost of the gradient computation, we present an adjoint method for computing the derivatives of moments of the neoclassical distribution function for stellarator optimization. The linear adjoint method allows derivatives of quantities which depend on solutions of a linear system, such as moments of the distribution function, to be computed with respect to many parameters from the solution of only two linear systems. This reduces the cost of computing the gradient to the point that the finite-collisionality neoclassical fluxes can be used within an optimization loop. With the neoclassical adjoint method, we compute solutions of the drift kinetic equation and an adjoint drift kinetic equation to obtain derivatives of neoclassical quantities with respect to geometric parameters. When the number of parameters in the derivative is large ( ), this adjoint method provides up to a factor of 200 reduction in cost. We demonstrate adjoint-based optimization of the field strength to obtain minimal bootstrap current on a surface. With adjoint-based derivatives, we also compute the local sensitivity to magnetic perturbations on a flux surface and identify regions where tight tolerances on error fields are required for control of the bootstrap current or radial transport. Furthermore, the solve for the ambipolar electric field is accelerated using a Newton method with derivatives obtained from the adjoint method.
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Publication: Chem. Mater. 31, 8969 (2019)Doi: 10.1021/acs.chemmater.9b03089
Tuning the halide composition in semiconductor perovskite materials is relevant for light-emitting and absorbing applications, as it significantly affects the dynamics of both the optical and electrical properties. Yet, a precise understanding of how the halide species influence the electrical behavior of the perovskite remains vague and speculative. In this work, we elucidate the transient voltage of two pure-halide perovskite film compositions (CH3NH3PbBr3 and CH3NH3PbI3) to directly compare the role of the halide in ionic species migration. We capture the photovoltage rise and residual voltage relaxation upon switching the illumination ON and subsequently OFF using Kelvin-probe force microscopy. We discover a unique and unforeseen wavelength-dependent voltage decay for CH3NH3PbBr3. Here, high-energy photons induce a more than 1 order of magnitude slower voltage decline toward equilibrium (i.e., dark conditions) than low-energy photons. Conversely, we find that the CH3NH3PbI3 perovskite composition has a wavelength-independent decay rate. The difference in electrical response occurs primarily because of the halide composition, as ion migration rates are reduced with higher Br content. The results detailed here yield new experimental insights about ion/defect activation energies in different perovskite films and devices, underlining a new parameter, photon energy (wavelength), which must be considered when assessing the fundamental photophysics within these materials.
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Publication: J. Vac. Sci. Technol. B 37, 031802 (2019)Doi: 10.1116/1.5092969
The authors investigate the effect of substrate temperature on the migration of fluorocarbon film precursor species into a model high aspect ratio feature with precise temperature control and shielding from direct plasma line of sight interactions. Increased substrate temperature shows fluorocarbon deposition further into the high aspect ratio feature and scales with aspect ratio for two different width gap sizes. Modeling of the deposition behavior suggests that multiple neutral species contribute to the deposition behavior, which have different survival rates as they travel into the high aspect ratio feature and experience encounters with surfaces. The work shows how slight changes in substrate temperature can be used to control migration behavior of neutral species in high aspect ratio features.
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Publication: PLOS One 14, e0218312 (2019)Doi: 10.1371/journal.pone.0218312
Jury deliberations provide a quintessential example of collective decision-making, but few studies have probed the available data to explore how juries reach verdicts. We examine how features of jury dynamics can be better understood from the joint distribution of final votes and deliberation time. To do this, we fit several different decision-making models to jury datasets from different places and times. In our best-fit model, jurors influence each other and have an increasing tendency to stick to their opinion of the defendant’s guilt or innocence. We also show that this model can explain spikes in mean deliberation times when juries are hung, sub-linear scaling between mean deliberation times and trial duration, and unexpected final vote and deliberation time distributions. Our findings suggest that both stubbornness and herding play an important role in collective decision-making, providing a nuanced insight into how juries reach verdicts, and more generally, how group decisions emerge.
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Publication: J. Vac. Sci. Technol. A 37, 031305 (2019)Doi: 10.1116/1.5092272
The authors evaluate the effect of water vapor on the plasma processing of materials using a model system consisting of a well-characterized radio-frequency plasma jet, controlled gaseous environment, and polystyrene as target material. The authors find that the effluent of Ar/H2O plasma jet is capable of (1) etching polymers with relatively high etch rate and (2) weakly oxidizing the etched polymer surface by forming O containing moieties. When increasing the treatment distance between the polymer and the Ar/H2O plasma, the authors find that the polymer etch rate drops exponentially, whereas the O elemental composition of the etched surface shows a maximum at intermediate treatment distance. The OH density in the Ar/H2O jet was measured near the substrate surface by laser induced fluorescence, and the density change of the OH radicals with treatment distance is found to be consistent with the exponential decrease of polymer etch rate, which indicates that OH may play a dominant role in the polymer etching process. A control experiment of Ar/H2 plasma shows that the observed fast polymer etching by Ar/H2O plasma cannot be attributed to H atoms. By correlating the OH flux with the polymer etch rate, the authors estimated the etching reaction coefficient of OH radicals (number of C atoms removed per OH radical from the gas phase) as ∼10−2. The polymer etch rate of Ar/H2O plasma is enhanced as the substrate temperature is lowered, which can be explained by the enhanced surface adsorption of gas phase species. For the same molecular admixture concentration and plasma power, the authors find that Ar/H2O/O2 plasma has much reduced etching efficiency compared to either Ar/H2O or Ar/O2 plasma.
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Publication: Phys. Rev. A 100, 022508 (2019)Doi: 10.1103/PhysRevA.100.022508
Quantum electrodynamic fluctuations cause an attractive force between metallic surfaces. At separations where the finite speed of light affects the interaction, it is called the Casimir force. Thermal motion determines the fundamental sensitivity limits of its measurement at room temperature, but several other systematic errors contribute uncertainty as well and become more significant in air relative to vacuum. Here we discuss the viability of the force modulation measurement technique in air (compared to frequency modulation, which is typically used in vacuum, and quasi-static deflection, which is usually used in fluid), characterize its sensitivity and accuracy by identifying several dominant sources of uncertainty, and compare the results to the fundamental sensitivity limits dictated by thermal motion and to the uncertainty inherent to calculations of the Casimir force. Finally, we explore prospects for mitigating the sources of uncertainty to enhance the range and accuracy of Casimir force measurements.
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Publication: Electrochimica Acta 323, 134778 (2019)Doi: 10.1016/j.electacta.2019.134778
A comprehensive photogenerated carrier assisted etching investigation is carried out on the Si polar surface of high-quality p- and n-type (0001) 4H–SiC epilayers. The epilayers have intentional or unintentional doping densities in the 1014–1018 cm−3 range. Cyclic voltammetry and chronoamperometry studies under a non-focused above bandgap (280–400 nm) light illumination (≤0.66 Wcm−2) in a highly basic etching medium (1 wt% KOH solution, pH ∼12) complemented by Mott-Schottky characterization reveal a dependence of the etch voltage (within ±1 V) and etch rate (∼20–60 nm/min) on doping concentration and type. Our results demonstrate separate mutually exclusive etch voltage windows (∼± 0.3–0.7 V) in the diffusion-limited regime for a pair of doping concentrations with opposite conductivities to establish smooth preferential etching. These results provide insight in the design and fabrication of higher performance three dimensional SiC homo/hetero-junctions for various applications including photonic crystal cavities.
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Publication: J. Mater. Sci. 54, 3671 (2019)Doi: 10.1007/s10853-018-3093-7
Tuning the halide composition in semiconductor perovskite materials is relevant for light-emitting and absorbing applications, as it significantly affects the dynamics of both the optical and electrical properties. Yet, a precise understanding of how the halide species influence the electrical behavior of the perovskite remains vague and speculative. In this work, we elucidate the transient voltage of two pure-halide perovskite film compositions (CH3NH3PbBr3 and CH3NH3PbI3) to directly compare the role of the halide in ionic species migration. We capture the photovoltage rise and residual voltage relaxation upon switching the illumination ON and subsequently OFF using Kelvin-probe force microscopy. We discover a unique and unforeseen wavelength-dependent voltage decay for CH3NH3PbBr3. Here, high-energy photons induce a more than 1 order of magnitude slower voltage decline toward equilibrium (i.e., dark conditions) than low-energy photons. Conversely, we find that the CH3NH3PbI3 perovskite composition has a wavelength-independent decay rate. The difference in electrical response occurs primarily because of the halide composition, as ion migration rates are reduced with higher Br content. The results detailed here yield new experimental insights about ion/defect activation energies in different perovskite films and devices, underlining a new parameter, photon energy (wavelength), which must be considered when assessing the fundamental photophysics within these materials.
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Publication: Phys. Today 72, 36 (2019)Doi: 10.1063/PT.3.4202
In the summer of 2013, a cube of uranium two inches on a side and weighing about five pounds found its way to us at the University of Maryland. If the sudden appearance of the unusual metal cube wasn’t intriguing enough, it came with a note that read, “Taken from the reactor that Hitler tried to build. Gift of Ninninger.”
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Publication: Phys. Rev. Lett. 123, 150503 (2019)Doi: 10.1103/PhysRevLett.123.150503
Temporal multiplexing provides an efficient and scalable approach to realize a quantum random walk with photons that can exhibit topological properties. But two-dimensional time-multiplexed topological quantum walks studied so far have relied on generalizations of the Su-Shreiffer-Heeger model with no synthetic gauge field. In this work, we demonstrate a two-dimensional topological quantum random walk where the nontrivial topology is due to the presence of a synthetic gauge field. We show that the synthetic gauge field leads to the appearance of multiple band gaps and, consequently, a spatial confinement of the quantum walk distribution. Moreover, we demonstrate topological edge states at an interface between domains with opposite synthetic fields. Our results expand the range of Hamiltonians that can be simulated using photonic quantum walks.
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Publication: J. Vac. Sci. Technol. A 37, 051303 (2019)Doi: 10.1116/1.5109651
Atmospheric pressure plasma (APP) sources are able to generate a variety of reactive species that have different effects on materials, such as functionalization, etching, and deposition. In this article, the authors study the effect of long-lived reactive neutral species on polymers using a model plasma-surface interaction system that consists of ultrathin (∼10 nm) polystyrene (PS) films and a surface microdischarge (SMD) reactor operated with various N2/O2 working gas mixtures. The authors characterized and quantified the reactive species generated by SMD using IR and UV absorption, and they found that O3, N2O5, N2O, and HNO3 are the dominant long-lived reactants near the target surface. When exposing PS films to these reactive species, the authors observed material responses including film thickness expansion, surface and bulk oxidation, and surface organic nitrate formation. The quantity of these changes varied with the N2/O2 working gas composition. By correlating material response with gas phase species, the authors find that the chemical modification of PS strongly depends on the density of O3 in the gas phase, which is indicative of an essential role of O3 in the remote APP treatment of polymers. Authors’ results show that O3 causes polymer surface oxidation, participates in the diffusion-reaction process in the polymer bulk, and results in aromatic ring cleavage and the formation of carbonyl groups. In contrast, they did not find a correlation between surface organic nitrate and individual long-lived reactive species mentioned above. This indicates that the organic nitrate formation on polymer surfaces might result from the interaction ofmultiple species, including O3 and nitrogen containing reactive species. A model for the interphase mass transfer of reactive species from gas to solid was also described.
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Publication: Phys. Rev. Lett. 122, 058301 (2019)Doi: 10.1103/PhysRevLett.122.058301
Symmetries are ubiquitous in network systems and have profound impacts on the observable dynamics. At the most fundamental level, many synchronization patterns are induced by underlying network symmetry, and a high degree of symmetry is believed to enhance the stability of identical synchronization. Yet, here we show that the synchronizability of almost any symmetry cluster in a network of identical nodes can be enhanced precisely by breaking its structural symmetry. This counterintuitive effect holds for generic node dynamics and arbitrary network structure and is, moreover, robust against noise and imperfections typical of real systems, which we demonstrate by implementing a state-of-the-art optoelectronic experiment. These results lead to new possibilities for the topological control of synchronization patterns, which we substantiate by presenting an algorithm that optimizes the structure of individual clusters under various constraints.
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Publication: J. Vac. Sci. Technol. A 37, 051503 (2019)Doi: 10.1116/1.5110524
Hexagonal boron nitride (hBN) thin films were grown by plasma-enhanced chemical beam epitaxy (PE-CBE) on epitaxial graphene (EG) on macrostepped 4°-offcut 4H-SiC(0001) substrates. The choice of growth conditions in this system allowed for two prominent in-plane hBN/EG rotational alignments: a direct alignment of the hBN and EG lattices or a 30° in-plane rotational twist such that the ⟨1120⟩hBN and ⟨1010⟩EG directions are parallel. The use of nitrogen plasma in conjunction with borazine at growth temperatures of 1450 °C increased the crystallinity of the few-monolayer-thick films relative to films grown by CBE without plasma exposure. In vacuo x-ray photoelectron spectroscopy showed that films grown with nitrogen plasma exposure were stoichiometric to nitrogen-rich, depending on growth conditions, and exhibited no bonding indicative of additional phase formation. This PE-CBE process was shown to produce films with atomically abrupt interfaces between the hBN and EG lattices, as determined by cross-sectional transmission electron microscopy (TEM). Annular dark field and bright field scanning TEM paired with energy dispersive x-ray spectroscopy confirmed that the EG persisted throughout this deposition and no intercalative growth of hBN under the EG was detected. Higher PE-CBE growth rates produced hBN domains that nucleated uniformly across the substrate with little preferred orientation of their edges. In comparison, lower growth rates appeared to cause preferential nucleation on the macrostep edges with a 30° in-plane rotation relative to the EG, as confirmed by cross-sectional TEM. By correlating the hBN nuclei shape in AFM to the atomic registry of the hBN to the substrate, it was found that the triangular, macrostep-edge nuclei were arm-chair edge terminated. The ability to select different rotational alignments by changing epitaxial growth conditions may be used in future wafer-scale growth of hBN/graphene heterostructures to achieve varying degrees of graphene band structure modulation.
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Publication: Optica 6, 811 (2019)Doi: 10.1364/OPTICA.6.000811
We demonstrate that avalanche ionization breakdown of air with picosecond mid-infrared (mid-IR) laser pulses is an exceptionally sensitive and quantitative probe of extremely low concentrations of charged species. By exponentially increasing the electron density in the vicinity of a single seed atom or molecule to detectable levels, mid-IR electron avalanche is an analogue of single photon detection in photomultiplier tubes and can be useful in a range of applications. We apply the technique to meter-scale standoff detection of a radioactive source, sensitive to extremely low concentrations of radiation-induced negative ions down to ∼103 cm−3, limited only by background. By imaging the location of spatially isolated avalanche breakdown sites, we directly measure these low densities and benchmark the performance of standoff detection diagnostics. We discuss implementation of this radiation detection scheme at ranges of 10–100 m and adapting the avalanche probe to detection of other low-density plasmas.
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Publication: ACS Photonics 6, 2238 (2019)Doi: 10.1021/acsphotonics.9b00449
Light absorption in metal films can excite hot carriers, which are useful for photodetection, solar energy conversion, and many other applications. However, metals are highly reflective, and therefore, careful optical design is required to achieve high absorption in these films. Here we utilize a subwavelength Fabry-Pérot-like resonance in conjunction with an index-near-zero (INZ) substrate to achieve near-unity absorption and hot carrier photocurrent in nanoscale metal films. By employing aluminum-doped zinc oxide (AZO) as the INZ medium in the near-infrared range, we enhance the metal film absorption by nearly a factor of 2. To exploit this absorption enhancement in an optoelectronic device, we fabricate a Schottky photodiode and find that the photocurrent generated in Pt on Si is enhanced by >80% with the INZ substrate. The enhancement arises from a combination of improved carrier generation and carrier transport resulting from the addition of the AZO film.
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Publication: Chaos 29, 033124 (2019)Doi: 10.1063/1.5084965
This paper considers a recently introduced
D -dimensional generalized Kuramoto model for many (N ≫ 1) interacting agents, in which the agent states areD -dimensional unit vectors. It was previously shown that, for even (but not odd) D, similar to the original Kuramoto model (D = 2), there exists a continuous dynamical phase transition from incoherence to coherence of the time asymptotic attracting state (time t→∞) as the coupling parameter K increases through a critical value which we denote K(+)c > 0. We consider this transition from the point of view of the stability of an incoherent state, where an incoherent state is defined as one for which the N→∞ distribution function is time-independent and the macroscopic order parameter is zero. In contrast with D = 2, for even D > 2, there is an infinity of possible incoherent equilibria, each of which becomes unstable with increasing K at a different point K=Kc. Although there are incoherent equilibria for which Kc = K(+)c , there are also incoherent equilibria with a range of possible Kc values below K(+)c, (K(+)c/2) ≤ Kc < K(+)c. How can the possible instability of incoherent states arising at K = Kc < K(+)c be reconciled with the previous finding that, at large time (t→∞), the state is always incoherent unless K > K(+)c ? We find, for a given incoherent equilibrium, that, if K is rapidly increased from K < Kc to Kc < K < K(+)c , due to the instability, a short, macroscopic burst of coherence is observed, in which the coherence initially grows exponentially, but then reaches a maximum, past which it decays back into incoherence. Furthermore, after this decay, we observe that the equilibrium has been reset to a new equilibrium whose Kc value exceeds that of the increased K. Thus, this process, which we call “Instability-Mediated Resetting,” leads to an increase in the effective Kc with continuously increasing K, until the equilibrium has been effectively set to one for which Kc ≈ K(+)c . Thus, instability-mediated resetting leads to a unique critical point of the t→∞ time asymptotic state (K = K(+)c ) in spite of the existence of an infinity of possible pretransition incoherent states. -
Publication: Nano Lett. 19, 7072 (2019)Doi: 10.1021/acs.nanolett.9b02443
Charged quantum dots containing an electron or hole spin are bright solid-state qubits suitable for quantum networks and distributed quantum computing. Incorporating such quantum dot spin into a photonic crystal cavity creates a strong spin–photon interface in which the spin can control a photon by modulating the cavity reflection coefficient. However, previous demonstrations of such spin–photon interfaces have relied on quantum dots that are charged randomly by nearby impurities, leading to instability in the charge state, which causes poor contrast in the cavity reflectivity. Here we demonstrate a strong spin–photon interface using a quantum dot that is charged deterministically with a diode structure. By incorporating this actively charged quantum dot in a photonic crystal cavity, we achieve strong coupling between the cavity mode and the negatively charged state of the dot. Furthermore, by initializing the spin through optical pumping, we show strong spin-dependent modulation of the cavity reflectivity, corresponding to a cooperativity of 12. This spin-dependent reflectivity is important for mediating entanglement between spins using photons, as well as generating strong photon–photon interactions for applications in quantum networking and distributed quantum computing.
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Publication: Phys. Rev. Lett. 123, 154101 (2019)Doi: 10.1103/PhysRevLett.123.154101
A new type of dynamics called laminar chaos was recently discovered through a theoretical analysis of a scalar delay differential equation with time-varying delay. Laminar chaos is a low-dimensional dynamics characterized by laminar phases of nearly constant intensity with periodic durations and a chaotic variation of the intensity from one laminar phase to the next laminar phase. This is in stark contrast to the typically observed higher-dimensional turbulent chaos, which is characterized by strong fluctuations. In this Letter we provide the first experimental observation of laminar chaos by studying an optoelectronic feedback loop with time-varying delay. The noise inherent in the experiment requires the development of a nonlinear Langevin equation with variable delay. The results show that laminar chaos can be observed in higher-order systems, and that the phenomenon is robust to noise and a digital implementation of the variable time delay.
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Publication: Phys. Rev. E 100, 042905 (2019)Doi: 10.1103/PhysRevE.100.042905
We analyze reversibility of displacements and rotations of spherical grains in three-dimensional compression experiments. Using transparent acrylic beads with cylindrical holes and index matching techniques, we are not only capable of tracking displacements but also analyzing reversibility of rotations. We observe that for moderate compression amplitudes, up to one bead diameter, the translational displacements of the beads after each cycle become mostly reversible after an initial transient. By contrast, granular rotations are largely irreversible. We find a weak correlation between translational and rotational displacements, indicating that rotational reversibility depends on more subtle changes in contact distributions and contact forces between grains compared with displacement reversibility. Three-dimensional rotations in dense granular systems are particularly important, since frictional losses associated with rotations are the dominant mechanism for energy dissipation. As such our work provides a first step toward a thorough study of rotations and tangential forces to understand the granular dynamics in dense systems.
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Publication: Opt. Exp. 27, 21367 (2019)Doi: 10.1364/OE.27.021367
Chiral light-matter interactions can enable polarization to control the direction of light emission in a photonic device. Most realizations of chiral light-matter interactions require external magnetic fields to break time-reversal symmetry of the emitter. One way to eliminate this requirement is to utilize strong spin-orbit coupling present in transition metal dichalcogenides that exhibit a valley-dependent polarized emission. Such interactions were previously reported using plasmonic waveguides, but these structures exhibit short propagation lengths due to loss. Chiral dielectric structures exhibit much lower loss levels and could therefore solve this problem. We demonstrate chiral light-matter interactions using spin-valley states of transition metal dichalcogenide monolayers coupled to a dielectric waveguide. We use a photonic crystal glide-plane waveguide that exhibits chiral modes with high field intensity, coupled to monolayer WSe2. We show that the circularly polarized emission of the monolayer preferentially couples to one direction of the waveguide, with a directionality as high as 0.35, limited by the polarization purity of the bare monolayer emission. This system enables on-chip directional control of light and could provide new ways to control spin and valley degrees of freedom in a scalable photonic platform.
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Publication: J. Plasma Phys. 85, 815850601 (2019)Doi: 10.1017/S0022377819000783
A method is given to rapidly compute quasisymmetric stellarator magnetic fields for plasma confinement, without the need to call a three-dimensional magnetohydrodynamic equilibrium code inside an optimization iteration. The method is based on direct solution of the equations of magnetohydrodynamic equilibrium and quasisymmetry using Garren & Boozer's expansion about the magnetic axis (Phys Fluids B, vol. 3, 1991, p. 2805), and it is several orders of magnitude faster than the conventional optimization approach. The work here extends the method of Landreman et al. (J. Plasma Phys., vol. 85, 2019, 905850103), which was limited to flux surfaces with elliptical cross-section, to higher order in the aspect-ratio expansion. As a result, configurations can be generated with strong shaping that achieve quasisymmetry to high accuracy. Using this construction, we give the first numerical demonstrations of Garren and Boozer's ideal scaling of quasisymmetry breaking with the cube of the inverse aspect ratio. We also demonstrate a strongly non-axisymmetric configuration (vacuum rotational transform ℓ > 0.4) in which symmetry-breaking mode amplitudes throughout a finite volume are < 2 × 10-7, the smallest ever reported. To generate boundary shapes of finite-minor-radius configurations, a careful analysis is given of the effect of substituting a finite minor radius into the near-axis expansion. The approach here can provide analytic insight into the space of possible quasisymmetric stellarator configurations, and it can be used to generate good initial conditions for conventional stellarator optimization.
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Publication: Phys. Rev. X 9, 011002 (2019)Doi: 10.1103/PhysRevX.9.011002
The Kuramoto model, originally proposed to model the dynamics of many interacting oscillators, has been used and generalized for a wide range of applications involving the collective behavior of large heterogeneous groups of dynamical units whose states are characterized by a scalar angle variable. One such application in which we are interested is the alignment of orientation vectors among members of a swarm. Despite being commonly used for this purpose, the Kuramoto model can only describe swarms in two dimensions, and hence the results obtained do not apply to the often relevant situation of swarms in three dimensions. Partly based on this motivation, as well as on relevance to the classical, mean-field, zero-temperature Heisenberg model with quenched site disorder, in this paper we study the Kuramoto model generalized to D dimensions. We show that in the important case of three dimensions, as well as for any odd number of dimensions, the D-dimensional generalized Kuramoto model for heterogeneous units has dynamics that are remarkably different from the dynamics in two dimensions. In particular, for odd D the transition to coherence occurs discontinuously as the interunit coupling constant K is increased through zero, as opposed to the D=2 case (and, as we show, also the case of even D) for which the transition to coherence occurs continuously as K increases through a positive critical value Kc. We also demonstrate the qualitative applicability of our results to related models constructed specifically to capture swarming and flocking dynamics in three dimensions.
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Publication: Chaos 29, 033104 (2019)Doi: 10.1063/1.5064679
We propose a framework for the analysis of the integro-differential delay Ikeda equations ruling the dynamics of bandpass optoelectronic oscillators (OEOs). Our framework is based on the normal form reduction of OEOs and helps in the determination of the amplitude and the frequency of the primary Hopf limit-cycles as a function of the time delay and other parameters. The study is carried for both the negative and the positive slopes of the sinusoidal transfer function, and our analytical results are confirmed by the numerical and experimental data.
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Publication: Joule 3, 325 (2019)Doi: 10.1016/j.joule.2018.11.010
High-performing and low-cost photovoltaics (PV) are critical to the continued adoption of renewable energy sources. While promising, perovskite solar materials show a dynamic optoelectronic response when exposed to H2O, O2, bias, temperature, or light that severely impacts their performance, preventing commercialization. We posit a reap-rest-recovery cycle to avoid permanent material degradation and achieve long-term power conversion efficiency through machine learning (ML). First, the influence of each above-mentioned parameter must be investigated individually and in combination, from the nano- to the macroscale. With sufficient data for ML, provided by a shared-knowledge repository, monitoring frameworks for perovskite solar cells will be developed to maximize long-term operation by using predictive methods to determine the ideal pathways to recovery through rest. With these milestones achieved, we expect perovskite PV to reach the 25 years T80 lifetime requirement.
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Publication: Plasma Proc. Polymers 16, 1900026 (2019)Doi: 10.1002/ppap.201900026
Ion bombardment of photoresist materials during plasma etching results in the formation of a surface dense amorphous carbon (DAC) layer that contributes to both etch resistance and the development of surface roughness. Real-time ellipsometric measurements/analysis reveals that a C4F8-containing plasma interacts with an Ar-plasma-formed DAC layer to produce a modified DAC/fluorocarbon (FC) layer by FC deposition/diffusion of fluorine into the surface. The depletion of the DAC layer via modification and ion bombardment causes the etch rate of the bulk layer to increase. As the modified surface layer is formed, a noticeable decrease in surface roughness decrease is observed. These findings provide an understanding of the mechanisms of atomic layer etching processes in photoresist materials.
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Publication: Chaos 29, 033133 (2019)Doi: 10.1063/1.5085653
The Random Coupling Model (RCM) is a statistical approach for studying the scattering properties of linear wave chaotic systems in the semi-classical regime. Its success has been experimentally verified in various over-moded wave settings, including both microwave and acoustic systems. It is of great interest to extend its use in nonlinear systems. This paper studies the impact of a nonlinear port on the measured statistical electromagnetic properties of a ray-chaotic complex enclosure in the short wavelength limit. A Vector Network Analyzer is upgraded with a high power option, which enables calibrated scattering (S) parameter measurements up to +43 dBm. By attaching a diode to the excitation antenna, amplitude-dependent S-parameters and Wigner reaction matrix (impedance) statistics are observed. We have systematically studied how the key components in the RCM are affected by this nonlinear port, including the radiation impedance, short ray orbit corrections, and statistical properties. By applying the newly developed radiation efficiency extension to the RCM, we find that the diode admittance increases with the excitation amplitude. This reduces the amount of power entering the cavity through the port so that the diode effectively acts as a protection element. As a result, we have developed a quantitative understanding of the statistical scattering properties of a semi-classical wave chaotic system with a nonlinear coupling channel.
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Publication: Appl. Phys. Lett. 114, 071105 (2019)Doi: 10.1063/1.5082560
InAs/InP quantum dots are excellent sources of telecom single-photon emission and are among the most promising candidates for scalable quantum photonic circuits. However, geometric differences in each quantum dot lead to slightly different emission wavelengths and hinder the possibility of generating multiple identical quantum emitters on the same chip. Stark tuning is an efficient technique to overcome this issue as it can control the emission energy of individual quantum dots through the quantum-confined Stark effect. Realizing this technique in InAs/InP quantum dots has previously been limited to shifts of less than 0.8 meV due to jumps in the emission energy because of additional charges at high electric field intensities. We demonstrate up to 5.1 meV of Stark tuning in the emission wavelength of InAs/InP quantum dots. To eliminate undesirable jumps to the charged state, we use a thin oxide insulator to prevent carrier injection from the contacts, thereby significantly improving the tuning range of the Stark effect. Moreover, the single-photon nature and narrow linewidth of the quantum dot emission are preserved under a wide range of applied electric fields. Using photoluminescence intensity measurements and time-resolved lifetime spectroscopy, we confirmed that this Stark tuning range is limited by carrier tunneling at high electric fields. This result is an important step toward integrating multiple identical quantum emitters at telecom wavelengths on a chip, which is crucial for realizing complex quantum photonic circuits for quantum information processing.
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Publication: Appl. Phys. Lett. 114, 171101 (2019)Doi: 10.1063/1.5089907
Fiber-coupled single photon sources are considered important components of photonics-based quantum information processors. Most fiber-coupled single photon sources require careful alignment between fibers and quantum emitters. In this work, we present an alignment-free fiber-integrated single photon source based on an InAs/InP quantum dot emitting at telecom wavelengths. We designed a nanobeam containing the quantum dots attached to a fiber taper. The adiabatic tapered coupler of the nanobeam enables efficient light coupling to the fiber taper. Using a tungsten probe in a focused ion beam system, we transferred the nanobeam to the fiber taper. The observed fiber-coupled single photon emission occurs with a brightness of 1.4% and a purity of 83%. This device provides a building block for fiber-optic quantum circuits that have various applications, such as quantum communication and distributed quantum computing.
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Publication: Chaos 29, 053107 (2019)Doi: 10.1063/1.5093038
Previous results have shown that a large class of complex systems consisting of many interacting heterogeneous phase oscillators exhibit an attracting invariant manifold. This result has enabled reduced analytic system descriptions from which all the long term dynamics of these systems can be calculated. Although very useful, these previous results are limited by the restriction that the individual interacting system components have one-dimensional dynamics, with states described by a single, scalar, angle-like variable (e.g., the Kuramoto model). In this paper, we consider a generalization to an appropriate class of coupled agents with higher-dimensional dynamics. For this generalized class of model systems, we demonstrate that the dynamics again contain an invariant manifold, hence enabling previously inaccessible analysis and improved numerical study, allowing a similar simplified description of these systems. We also discuss examples illustrating the potential utility of our results for a wide range of interesting situations.
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Publication: IEEE J. Quantum Electron. 55, 5000106 (2019)Doi: 10.1109/JQE.2019.2920694
In this work, we propose a theoretical and experimental study of an optoelectronic oscillators with a nonlinear filter. The system displays higher frequency oscillation. We expect our system to harness new dynamical behaviors that could find applications in photonic information processing. This work results from a three-year-long collaboration between three laboratories located at the University of Dschang (Cameroon), the University of Yaoundé I (Cameroon) and the University of Maryland (USA).
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Publication: Plasma Proc. Polymers 16, e1900103 (2019)Doi: 10.1002/ppap.201900103
Cold atmospheric plasma (CAP) produces reactive oxygen species and reactive nitrogen species, which may disproportionally damage tumor cells, resulting in potentially selective cancer therapy. Here, we compare the effects of two CAP sources, that is, the atmospheric pressure plasma jet and the surface micro discharge, on the selectivity of CAP-treated cell-culture media. CAP-treated media were applied to metastatic breast tumor cells and their normal breast epithelial cell counterparts to assess treatment selectivity, while systematically varying common cell-culture media and cell-matrix binding moieties. We show that media compositions are crucial in a CAP-treated media selectivity, while binding moieties (specifically, collagen I, fibronectin, and poly-d-lysine) play a lesser role. These data have further implications in the translation of CAP to in vivo use.
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Publication: J. Opt. Soc. Am. B-Opt. Phys. 36, 346 (2019)Doi: 10.1364/JOSAB.36.000346
The effect of laser noise on the atmospheric propagation of high-power CW lasers and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. High-power CW laser systems have relatively large fractional levels of intensity noise and frequency noise. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. A paraxial wave equation, containing dispersion and nonlinear effects, is expanded in terms of fluctuations in the intensity and phase. Longitudinal and transverse intensity noise and frequency noise are considered. The laser propagation model includes group velocity dispersion, Kerr, delayed Raman response, and optical self-steepening effects. A set of coupled linearized equations are derived for the evolution of the laser intensity and frequency fluctuations. In certain limits, these equations can be solved analytically. For example, we find that, in a dispersive medium, frequency noise can couple to and induce intensity noise (fluctuations) and vice versa. At high intensities, the Kerr effect can reduce this intensity noise. In addition, significant spectral modification can occur if the initial intensity noise level is sufficiently high. Finally, our model is used to study the transverse and longitudinal modulational instabilities. We present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm, 1 μm, and 10.6 μm.
2018
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Publication: J. Vac. Sci. Technol. A 36, 040601 (2018)Doi: 10.1116/1.5035291
The authors demonstrate that complex hydrofluorocarbon (HFC) precursors offer significant advantages relative to gas mixtures of comparable elemental ratios for plasma-based selective atomic layer etching (ALE). This work compares mixtures of a fluorocarbon precursor and H2 with an HFC precursor, i.e., mixtures of octafluorocyclobutane (C4F8) with H2 and 3,3,3-trifluoropropene (C3H3F3), for SiO2 ALE and etching of SiO2 selective to Si3N4 or Si. For continuous plasma etching, process gas mixtures, e.g., C4F8/H2, have been employed and enable highly selective material removal based on reduction of the fluorine content of deposited steady-state HFC films; however this approach is not successful for ALE since hydrogen-induced etching reduces the thickness of the ultrathin HFC passivation layer which is required for both etching of SiO2 and passivation of the Si3N4 and Si underlayers, leading to lower materials etching selectivity. Conversely, the experimental results show that C3H3F3-based ALE enables ultrahigh ALE selectivity of SiO2 over Si3N4 and Si. The hydrogen in the precursor structure allows to reduce the fluorine content of the deposited HFC film without suppressing the formation of the passivation layer on the surface. Gas pulsing of complex reactive precursors in ALE provides the prospect of utilizing the precursor chemical structure for achieving high materials selectivity in ALE.
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Publication: ACS Photon. 5, 3466 (2018)Doi: 10.1021/acsphotonics.8b00580
Plasmonic nanostructures provide an efficient way to control and enhance the radiative properties of quantum emitters. Coupling these structures to single defects in two-dimensional materials provides a particularly promising material platform to study emitter–plasmon interactions because these emitters are not embedded in a surrounding dielectric. They can therefore approach a near-field plasmonic mode to nanoscale distances, potentially enabling strong light–matter interactions. However, this coupling requires precise alignment of the emitters to the plasmonic mode of the structures, which is particularly difficult to achieve in a site-controlled structure. We present a technique to generate quantum emitters in two-dimensional tungsten diselenide coupled to site-controlled plasmonic nanopillars. The plasmonic nanopillar induces strains in the two-dimensional material which generate quantum emitters near the high-field region of the plasmonic mode. The electric field of the nanopillar mode is nearly parallel to the two-dimensional material and is therefore in the correct orientation to couple to the emitters. We demonstrate both an enhanced spontaneous emission rate and increased brightness of emitters coupled to the nanopillars. This approach may enable bright site-controlled nonclassical light sources for applications in quantum communication and optical quantum computing.
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Publication: J. Vac. Sci. Technol. A 36, 021304 (2018)Doi: 10.1116/1.5009640
The development of new photoresists for semiconductor manufacturing applications requires an understanding of the material properties that control the material's plasma etching behavior. Ion bombardment at ion energies of the order 100 s of eV is typical of plasma-based pattern-transfer processes and results in the formation of a dense amorphous carbon (DAC) layer on the surface of a photoresist, such as the PR193-type of photoresist that currently dominates the semiconductor industry. Prior studies have examined the physical properties of the DAC layer, but the correlation between these properties and the photoresist etching behavior had not been established. In this work, the authors studied the real-time evolution of a steady-state DAC layer as it is selectively depleted using an admixture of oxygen into an argon plasma. Observations of the depletion behavior for various DAC layer thicknesses motivate a new model of DAC layer depletion. This model also correlates the impact of the DAC layer thickness with the etch rate of the bulk photoresist. The authors find that up to a 40% depletion of the DAC layer thickness does not have a significant impact on the bulk photoresist etch rate. However, further depletion results in an exponential increase in the etch rate, which can be up to ten times greater at full depletion than for the fully formed DAC layer. Thus, with these trends the authors show that the photoresist etch rate is controlled by the thickness of the DAC layer. Furthermore, thickness loss of the DAC layer in an O2-containing plasma coincides with a chemical modification of the layer into an oxygen-rich surface overlayer with properties that are intermediate between those of the DAC layer and the bulk photoresist. Support for this interpretation was provided via x-ray photoelectron spectroscopy characterization. Atomic force microscopy was used to gauge the impact on surface roughness as the DAC layer is formed and depleted. The trends established in this work will provide a benchmark in our development of new photoresists, which will be suitable for pattern transfer processes that will ultimately be a part of enabling smaller semiconductor device feature sizes and pitches.
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Publication: Phys. Plasmas 25, 102120 (2018)Doi: 10.1063/1.5050530
The outflow velocity of jets produced by collisionless magnetic reconnection is shown to be reduced by the ion exhaust temperature in fully kinetic particle in cell simulations and in situ satellite observations. We derive a scaling relationship for the outflow velocity based on the upstream Alfvén speed and the parallel ion exhaust temperature, which is verified in kinetic simulations and observations. The outflow speed reduction is shown to be due to the firehose instability criterion, and so, for large enough guide fields, this effect is suppressed and the outflow speed reaches the upstream Alfvén speed based on the reconnecting component of the magnetic field.
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Publication: Phys. Plasmas 25, 012514 (2018)Doi: 10.1063/1.5003359
The use of supersonic rotation of a plasma in mirror geometry has distinct advantages for thermonuclear fusion. The device is steady state, there are no disruptions, the loss cone is almost closed, sheared rotation stabilizes magnetohydrodynamic instabilities as well as plasma turbulence, there are no runaway electrons, and the coil configuration is simple. In this work, we examine the effect of rotation on mirror confinement using a full cyclotron orbit code. The full cyclotron simulations give a much more complete description of the particle energy distribution and losses than the use of guiding center equations. Both collisionless loss as a function of rotation and the effect of collisions are investigated. Although the cross field diffusion is classical, we find that the local rotating Maxwellian is increased to higher energy, increasing the fusion rate and also enhancing the radial diffusion. We find a loss channel not envisioned with a guiding center treatment, but a design can be chosen that can satisfy the Lawson criterion for ions. Of course, the rotation has a minimal effect on the alpha particle birth distribution, so there is initially loss through the usual loss cone, just as in a mirror with no rotation. However after this loss, the alphas slow down on the electrons with little pitch angle scattering until reaching low energy, so over half of the initial alpha energy is transferred to the electrons. The important problem of energy confinement, with losses primarily through the electron channel, is not addressed in this work. We also discuss the use of rotating mirror geometry to produce an ion thruster.
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Publication: Appl. Phys. Lett. 113, 221102 (2018)Doi: 10.1063/1.5054865
The integration of quantum emitters with integrated photonics enables complex quantum photonic circuits that are necessary for photonic implementation of quantum simulators, computers, and networks. Thin-film lithium niobate is an ideal material substrate for quantum photonics because it can tightly confine light in small waveguides and has a strong electro-optic effect that can switch and modulate single photons at low power and high speed. However, lithium niobate lacks efficient single-photon emitters, which are essential for scalable quantum photonic circuits. We demonstrate deterministic coupling of single-photon emitters with a lithium niobate photonic chip. The emitters are composed of InAs quantum dots embedded in an InP nanobeam, which we transfer to a lithium niobate waveguide with nanoscale accuracy using a pick-and-place approach. An adiabatic taper transfers single photons emitted into the nanobeam to the lithium niobate waveguide with high efficiency. We verify the single photon nature of the emission using photon correlation measurements performed with an on-chip beamsplitter. Our results demonstrate an important step toward fast, reconfigurable quantum photonic circuits for quantum information processing.
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Publication: Chaos 28, 061104 (2018)Doi: 10.1063/1.5039508
A machine-learning approach called “reservoir computing” has been used successfully for short-term prediction and attractor reconstruction of chaotic dynamical systems from time series data. We present a theoretical framework that describes conditions under which reservoir computing can create an empirical model capable of skillful short-term forecasts and accurate long-term ergodic behavior. We illustrate this theory through numerical experiments. We also argue that the theory applies to certain other machine learning methods for time series prediction.
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Publication: Phys. Rev. E 98, 062306 (2018)Doi: 10.1103/PhysRevE.98.062306
With the increasing abundance of “digital footprints” left by human interactions in online environments, e.g., social media and app use, the ability to model complex human behavior has become increasingly possible. Many approaches have been proposed, however, most previous model frameworks are fairly restrictive. We introduce a new social modeling approach that enables the creation of models directly from data with minimal a priori restrictions on the model class. In particular, we infer the minimally complex, maximally predictive representation of an individual's behavior when viewed in isolation and as driven by a social input. We then apply this framework to a heterogeneous catalog of human behavior collected from 15 000 users on the microblogging platform Twitter. The models allow us to describe how a user processes their past behavior and their social inputs. Despite the diversity of observed user behavior, most models inferred fall into a small subclass of all possible finite-state processes. Thus, our work demonstrates that user behavior, while quite complex, belies simple underlying computational structures.
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Publication: Astrophys. J. 867, 154 (2018)Doi: 10.3847/1538-4357/aae393
Heat flux suppression in collisionless plasmas for a large range of plasma β is explored using two-dimensional particle-in-cell simulations with a strong, sustained thermal gradient. We find that a transition takes place between whistler-dominated (high-β) and double-layer-dominated (low-β) heat flux suppression. Whistlers saturate at small amplitude in the low beta limit and are unable to effectively suppress the heat flux. Electrostatic double layers (DLs) suppress the heat flux to a mostly constant factor of the free-streaming value once this transition happens. The DL physics is an example of ion–electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of β explored. The range of plasma-βs studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including the intracluster medium of galaxy clusters, hot accretion flows, stellar and accretion disk coronae, and the solar wind.
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Publication: J. Phys. Chem. Lett. 9, 3463 (2018)Doi: 10.1021/acs.jpclett.8b01357
Hybrid organic–inorganic perovskites containing Cs are a promising new material for light-absorbing and light-emitting optoelectronics. However, the impact of environmental conditions on their optical properties is not fully understood. Here, we elucidate and quantify the influence of distinct humidity levels on the charge carrier recombination in CsxFA1–xPb(IyBr1–y)3 perovskites. Using in situ environmental photoluminescence (PL), we temporally and spectrally resolve light emission within a loop of critical relative humidity (rH) levels. Our measurements show that exposure up to 35% rH increases the PL emission for all Cs (10–17%) and Br (17–38%) concentrations investigated here. Spectrally, samples with larger Br concentrations exhibit PL redshift at higher humidity levels, revealing water-driven halide segregation. The compositions considered present hysteresis in their PL intensity upon returning to a low-moisture environment due to partially reversible hydration of the perovskites. Our findings demonstrate that the Cs/Br ratio strongly influences both the spectral stability and extent of light emission hysteresis. We expect our method to become standard when testing the stability of emerging perovskites, including lead-free options, and to be combined with other parameters known for affecting material degradation, e.g., oxygen and temperature.
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Publication: Opt. Lett. 43, 1131 (2018)Doi: 10.1364/OL.43.001131
►We report on, to the best of our knowledge, the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared (IR) laser pulses (lambda = 3.9 μm, 100 fs, 0.25 TW), which enable near- and above-critical density interactions with moderate-density gas jets. Relativistic electron acceleration up to ∼12 MeV ∼ 12 MeV occurs when the jet width exceeds the threshold scale length for relativistic self-focusing. We present scaling trends in the accelerated beam profiles, charge, and spectra, which are supported by particle-in-cell simulations and time-resolved images of the interaction. For similarly scaled conditions, we observe significant increases in the accelerated charge, compared to previous experiments with near-infrared (lambda = 800 nm) pulses.
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Publication: J. Geophys. Res.-Space Phys. 123, 6327 (2018)Doi: 10.1029/2018JA025604
The structure of dipolarization jets with finite width in the dawn-dusk direction relevant to magnetic reconnection in the Earth's magnetotail is explored with particle-in-cell simulations. We carry out Riemann simulations of the evolution of the jet in the dawn-dusk, north-south plane to investigate the dependence of the jet structure on the jet width in the dawn-dusk direction. We find that the magnetic field and Earth-directed ion flow structure depend on the dawn-dusk width. A reversal in the usual Hall magnetic field near the center of the current sheet on the duskside of larger jets is observed. For small widths, the maximum velocity of the earthward flow is significantly reduced below the theoretical limit of the upstream Alfvén speed. However, the ion flow speed approaches this limit once the width exceeds the ion Larmor radius based on the normal magnetic field, Bz.
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Publication: Adv. Energy Mater. 8, 1702615 (2018)Doi: 10.1002/aenm.201702615
Lithium-ion batteries (LIBs) are integral parts of modern technology, but can raise safety concerns because of their flammable organic electrolytes with low flash points. Aqueous electrolytes can be used in LIBs to overcome the safety issues that come with organic electrolytes while avoiding poor kinetics associated with solid state electrolytes. Despite advances in aqueous electrolytes, current collectors for aqueous battery systems have been neglected. Current collectors used in today's aqueous battery systems are usually metal-based materials, which are heavy, expensive, bulky, and prone to corrosion after prolonged use. Here, a carbon nanotube (CNT)–cellulose nanofiber (CNF) all-fiber composite is developed that takes advantage of the high conductivity of CNT while achieving high mechanical strength through the interaction between CNT and CNF. By optimizing the CNT/CNF weight ratio, this all-fiber current collector can be made very thin while maintaining high conductivity (≈700 S cm−1) and strength (>60 MPa), making it an ideal replacement for heavy metal current collectors in aqueous battery systems.
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Publication: Sci. Rep. 8, 16487 (2018)Doi: 10.1038/s41598-018-34595-1
Metal films deposited on graphene are known to influence its electronic properties, but little is known about graphene’s interactions with very low work function rare earth metals. Here we report on the work functions of a wide range of metals deposited on n-type epitaxial graphene (EG) as measured by Kelvin Probe Force Microscopy (KPFM). We compare the behaviors of rare earth metals (Pr, Eu, Er, Yb, and Y) with commonly used noble metals (Cr, Cu, Rh, Ni, Au, and Pt). The rare earth films oxidize rapidly, and exhibit unique behaviors when on graphene. We find that the measured work function of the low work function group is consistently higher than predicted, unlike the noble metals, which is likely due to rapid oxidation during measurement. Some of the low work function metals interact with graphene; for example, Eu exhibits bonding anomalies along the metal-graphene perimeter. We observe no correlation between metal work function and photovoltage, implying the metal-graphene interface properties are a more determinant factor. Yb emerges as the best choice for future applications requiring a low-work function electrical contact on graphene. Yb films have the strongest photovoltage response and maintains a relatively low surface roughness, ~5 nm, despite sensitivity to oxidation.
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Publication: Phys. Chem. Chem. Phys. 20, 2517 (2018)Doi: 10.1039/c7cp06312a
Batteries based on magnesium chemistry are being widely investigated as an alternative energy storage system to replace lithium-ion batteries. Mg batteries have multiple challenges, especially on the cathode side. The divalent Mg ion has slow insertion kinetics in many metal oxide cathodes conventionally used in Li-ion batteries. One solution that has been explored is adding water molecules into an organic electrolyte, which has been shown to aid in Mg insertion and improve performance of manganese oxide (MnO2) cathodes. While there have been studies on Mg insertion mechanisms into MnO2 in solely aqueous or organic electrolytes for some crystalline MnO2 polymorphs, our work is focused on water-containing organic electrolyte, where an H2O to Mg ratio of 6 : 1 is present. In this study, we report results based on ex situ XPS experiments, including both angle resolved and depth profiling studies to assess the surface reactions and determine the mechanism of Mg insertion into an amorphous, electrodeposited MnO2 cathode. We propose that in this mixed electrolyte system, there is a combined insertion/conversion reaction mechanism whereby Mg and H2O molecules co-insert into the MnO2 structure and a reaction between H2O and Mg creates an observable Mg(OH)2 layer at the surface of the MnO2. A more full understanding of the role of the water molecules is important to aid in the future design of cathode materials, especially when determining potential ways to integrate metal oxides in Mg batteries.
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Publication: Nano Lett. 18, 4734 (2018)Doi: 10.1021/acs.nanolett.8b01133
Future scalable photonic quantum information processing relies on the ability of integrating multiple interacting quantum emitters into a single chip. Quantum dots provide ideal on-chip quantum light sources. However, achieving quantum interaction between multiple quantum dots on-a-chip is a challenging task due to the randomness in their frequency and position, requiring local tuning technique and long-range quantum interaction. Here, we demonstrate quantum interactions between separated two quantum dots on a nanophotonic waveguide. We achieve a photon-mediated long-range interaction by integrating the quantum dots to the same optical mode of a nanophotonic waveguide and overcome spectral mismatch by incorporating on-chip thermal tuners. We observe their quantum interactions of the form of super-radiant emission, where the two dots collectively emit faster than each dot individually. Creating super-radiant emission from integrated quantum emitters could enable compact chip-integrated photonic structures that exhibit long-range quantum interactions. Therefore, these results represent a major step toward establishing photonic quantum information processors composed of multiple interacting quantum emitters on a semiconductor chip.
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Publication: Phys. Rev. E 97, 062220 (2018)Doi: 10.1103/PhysRevE.97.062220
The Random Coupling Model (RCM) predicts the statistical properties of waves inside a ray-chaotic enclosure in the semiclassical regime by using Random Matrix Theory, combined with system-specific information. Experiments on single cavities are in general agreement with the predictions of the RCM. It is now desired to test the RCM on more complex structures, such as a cascade or network of coupled cavities, that represent realistic situations but that are difficult to test due to the large size of the structures of interest. This paper presents an experimental setup that replaces a cubic-meter-scale microwave cavity with a miniaturized cavity, scaled down by a factor of 20 in each dimension, operated at a frequency scaled up by a factor of 20 and having wall conductivity appropriately scaled up by a factor of 20. We demonstrate experimentally that the miniaturized cavity maintains the statistical wave properties of the larger cavity. This scaled setup opens the opportunity to study wave properties in large structures such as the floor of an office building, a ship, or an aircraft, in a controlled laboratory setting.
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Publication: Nucl. Fusion 58, 026003 (2018)Doi: 10.1088/1741-4326/aa9a50
Breaking the up–down symmetry of the tokamak poloidal cross-section can significantly increase the spontaneous rotation due to turbulent momentum transport. In this work, we optimize the shape of flux surfaces with both tilted elongation and tilted triangularity in order to maximize this drive of intrinsic rotation. Nonlinear gyrokinetic simulations demonstrate that adding optimally-tilted triangularity can double the momentum transport of a tilted elliptical shape. This work indicates that tilting the elongation and triangularity in an ITER-like device can reduce the energy transport and drive intrinsic rotation with an Alfvén Mach number of roughly 1%. This rotation is four times larger than the rotation expected in ITER and is approximately what is needed to stabilize MHD instabilities. It is shown that this optimal shape can be created using the shaping coils of several present-day experiments.
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Publication: Sci. Rep. 8, 15930 (2018)Doi: 10.1038/s41598-018-34381-z
Opto-mechanical forces result from the momentum transfer that occurs during light-matter interactions. One of the most common examples of this phenomenon is the radiation pressure that is exerted on a reflective surface upon photon reflection. For an ideal mirror, the radiation pressure is independent of the wavelength of light and depends only on the incident power. Here we consider a different regime where, for a constant input optical power, wavelength-dependent radiation pressure is observed due to coherent thin film Fabry-Perot interference effects. We perform measurements using a Si microcantilever and utilize an in-situ optical transmission technique to determine the local thickness of the cantilever and the light beam’s angle of incidence. Although Si is absorptive in the visible part of the spectrum, by exploiting the Fabry-Perot modes of the cantilever, we can determine whether momentum is transferred via reflection or absorption by tuning the incident wavelength by only ~20 nm. Finally, we demonstrate that the tunable wavelength excitation measurement can be used to separate photothermal effects and radiation pressure.
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Publication: Adv. Opt. Mater. 6, 1700830 (2018)Doi: 10.1002/adom.201700830
Superabsorbers based on metasurfaces have recently enabled the control of light at the nanoscale in unprecedented ways. Nevertheless, the sub-wavelength features needed to modify the absorption band usually require complex fabrication methods, such as electron-beam lithography. To overcome the scalability limitations associated with the fabrication of metallic nanostructures, engineering the optical response of superabsorbers by metal alloying is proposed, instead of tuning the geometry/size of the nanoscale building blocks. The superior performance of thin film AlCu alloys as the metallic component of planar bilayer superabsorbers is numerically demonstrated. This alloy outperforms its pure constituents as well as other metals, such as Ag, Au, and Cr. As a model system, a Si/AlCu structure is analyzed that presents >99% absorption at selected wavelength ranging from the visible to the near-infrared regions of the spectrum, depending on the subwavelength thickness of the semiconductor. The multi-wavelength near-unity absorption behavior of Si/AlCu persists even for oblique angle of incidence, up to 70°. Additionally, the findings are validated by fabricating and testing a-Si/AlCu superabsorbers, where good agreement is found between the numerically and experimentally determined optical response. The system investigated here is relevant for integration in complementary metal-oxide-semiconductor (CMOS) technologies.
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Publication: Phys. Rev. E 97, 053203 (2018)Doi: 10.1103/PhysRevE.97.053203
We present experiments and numerical simulations which demonstrate that fully ionized, low-density plasma channels could be formed by hydrodynamic expansion of plasma columns produced by optical field ionization. Simulations of the hydrodynamic expansion of plasma columns formed in hydrogen by an axicon lens show the generation of 200 mm long plasma channels with axial densities of order ne(0) = 1 × 1017cm−3 and lowest-order modes of spot size WM ≈ 40 μm. These simulations show that the laser energy required to generate the channels is modest: of order 1 mJ per centimeter of channel. The simulations are confirmed by experiments with a spherical lens which show the formation of short plasma channels with 1.5 × 1017cm−3 ≲ ne(0) ≲ 1 × 1018cm−3 and 61 μm ≳ WM ≳ 33 μm. Low-density plasma channels of this type would appear to be well suited as multi-GeV laser-plasma accelerator stages capable of long-term operation at high pulse repetition rates.
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Publication: Phys. Chem. Chem. Phys. 20, 29708 (2018)Doi: 10.1039/c8cp04706e
Considering the tortuous, random porous nanostructures existing in many battery electrodes, it is essential to understand electronic and ionic behaviors in such a confined nanoscale porous geometry in which electron and ion transports can change dynamically. Here, we have carefully designed three dimensional (3D) interconnected porous electrode structures and performed experiments to probe how the ion and electron transport is impacted within these controlled geometries. By using anodized aluminum oxide as a template, we were able to fabricate both 1D array electrodes and 3D electrodes with varying numbers of interconnections, utilizing vanadium oxide (V2O5) as the active material. We demonstrate that the inherent properties of the electrode material in combination with the structural properties of the electrodes can both positively and negatively impact electrochemical characteristics. Most notably, electrodes with seven interconnecting layers in their structure had 19.7% less capacity at 25C than electrodes with zero interconnecting layers, demonstrating the negative effect of interconnections combined with poor electronic conductivity of V2O5 upon lithiation beyond one Li insertion. These results indicate that a careful consideration of the material and structural properties is needed for the design of high performance battery systems.
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Publication: Adv. Opt. Mater. 6, 1701323 (2018)Doi: 10.1002/adom.201701323
Optoelectronic materials are the backbone of today's high-tech industry. To customize their response, one can directly modify the atomic arrangement, chemical composition, lattice strain, or doping of the semiconductor. However, these processes frequently cause undesirable effects resulting from induced defects. Here, a novel concept is demonstrated to actively tune the optoelectronic response of a material through tailored photon recycling. Without altering the material's intrinsic structure, doping, or temperature, the reabsorption of emitted photons within GaAs is modulated to control its carrier density. This approach is used to create a diode that can change its emission wavelength, a solar cell with improved open-circuit voltage, and an actively controlled, gate-free current modulator. These results represent a new platform to enable materials with tailored optoelectronic response based on photonic manipulation rather than semiconductor engineering.
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Publication: Sci. 359, 666 (2018)Doi: 10.1126/science.aaq0327
Exploiting topological properties of a system allows certain properties to be protected against the disorder and scattering caused by defects. Barik et al. demonstrate a strong light-matter interaction in a topological photonic structure (see the Perspective by Amo). They created topological edge states at the interface between two photonic, topologically distinct regions and coupled them to a single quantum emitter. The chiral nature of single-photon emission was used to inject single photons of opposite polarization into counterpropagating topological edge states. Such a topological quantum optics interface may provide a powerful platform for developing robust integrated quantum optical circuits.
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Publication: Phys. Rev. E 98, 943206 (2018)Doi: 10.1103/PhysRevE.98.043206
The injection of electrons into a laser wakefield accelerator (LWFA) is observed to generate an intense coherent ultra-broadband and ultrashort pulse radiation flash, consistent with the acceleration of electrons from rest to nearly the speed of light in a distance <∼1μm. Under certain conditions, the radiation occurs at harmonics of the local plasma frequency. The flash is sufficiently bright to induce large nonlinear refractive index shifts in optical materials; we estimate a source brightness temperature of ∼1018K. We present measurements of the flash spectra, coherence, pulse duration, polarization, and angular distribution, providing a detailed picture of electron injection dynamics in LWFA. These are characteristic of laser-assisted injection of off-axis electrons, which preserves wake coherence.
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Publication: Appl. Phys. Lett. 113,191105 (2018)Doi: 10.1063/1.5045727
Coupling single photon emitters to surface plasmons provides a versatile ground for on chip quantum photonics. However, achieving good coupling efficiency requires precise alignment of both the position and dipole orientation of the emitter relative to the plasmonic mode. We demonstrate coupling of single emitters in the 2-D semiconductor, WSe2 self-aligned with propagating surface plasmon polaritons in silver-air-silver, metal-insulator-metal waveguides. The waveguide produces strain induced defects in the monolayer which are close to the surface plasmon mode with favorable dipole orientations for optimal coupling. We measure an average enhancement in the rate of spontaneous emission by a factor of 1.89 for coupling the single defects to the plasmonic waveguide. This architecture provides an efficient way of coupling single photon emitters to propagating plasmons which is an important step towards realizing active plasmonic circuits on chip.
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Publication: Geophys. Res. Lett. 45, 4639 (2018)Doi: 10.1029/2018GL077874
Kinetic aspects of energy conversion and dissipation near a dipolarization front (DF) in the magnetotail are considered using fully kinetic 3-D particle-in-cell simulations. The energy conversion is described in terms of the pressure dilatation, as well as the double contraction of deviatoric pressure tensor and traceless strain rate tensor, also known as the Pi-D parameter in turbulence studies. It is shown that in contrast to the fluid dissipation measure, the Joule heating rate, which cannot distinguish between ion and electron dissipation and reveals deep negative dips at the DF, the Pi-D parameters, as kinetic analogs of the Joule heating rate, are largely positive and drastically different for ions and electrons. Further analysis of these parameters suggests that ions are heated at and ahead of the DF due to their reflection from the front, while electrons are heated at and behind the DF due to the long-wavelength lower-hybrid drift instability.
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Publication: Plasma Processes Polymers 15, e1700217 (2018)Doi: 10.1002/ppap.201700217
Treatments of polymer films using either a MHz atmospheric pressure plasma jet (APPJ) or an atmospheric pressure surface micro-discharge (SMD) plasma are investigated. While the typical approach to determine relevant reactive species is to correlate surface effects with gas phase species measurement, this does not capture potential synergistic or other complex effects that may be occurring. Activation energy and directionality of the etching process can characterize what is occurring at the surface for these processes in more detail. The APPJ source shows an apparent activation energy of ∼0.18 eV at 8 mm distance and up to ∼0.34 eV at 16 mm distance for a temperature range of 20–80 °C tested with thin polymer films. The APPJ source shows directional etching at 8 mm distance with less anisotropy the more distance is increased. The SMD source has an apparent activation energy of ∼0.8–0.9 eV at a distance of 3 mm. The SMD also only shows isotropic etching behavior. However the SMD surface chemistry changes significantly to less oxidation with increased temperature while the APPJ source induced modifications remain very similar with temperature change. The lower apparent activation energy of the APPJ-induced etching reactions as compared with low pressure work (0.5 eV) and observation of line-of-sight contribution to etching suggests the involvement of a directional species at closer distances facilitating the etching which falls off with increasing distance. The high activation energy of the SMD suggests that species with less capability for etching is responsible compared to the APPJ and low pressure plasma. The high surface oxidation from low temperature SMD treatments shows that the surface is being oxidized but not sufficiently to reach the desorption step of the etching process.
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Publication: Opt. Soc. Am. B - Opt. Phys. 35, 2822 (2018)Doi: 10.1364/JOSAB.35.002822
We have developed a single-shot imaging technique that can capture ultrafast events occurring on femtosecond to picosecond time scales. The technique is based on an optical pump-probe method, in which multiple time-delayed femtosecond pulses simultaneously probe a pump-excited sample. Here we use two sets of 2-by-2 mirror arrays for spatial/temporal division and routing of multiple probe pulses. This single-shot scheme is successfully applied to capture femtosecond ionization fronts propagating at the speed of light in air, as well as laser-induced ablation of solid targets.
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Publication: 2D Mater. 5, 011033 (2018)Doi: 10.1088/2053-1583/aa90b1
Graphene layers grown epitaxially on SiC substrates are attractive for a variety of sensing and optoelectronic applications because the graphene acts as a transparent, conductive, and chemically responsive layer that is mated to a wide-bandgap semiconductor with large breakdown voltage. Recent advances in control of epitaxial growth and doping of SiC epilayers have increased the range of electronic device architectures that are accessible with this system. In particular, a recently-introduced Schottky-emitter bipolar phototransistor (SEPT) based on an epitaxial graphene (EG) emitter grown on a p-SiC base epilayer has been found to exhibit a maximum common emitter current gain of 113 and a UV responsivity of 7.1 A W−1. The behavior of this device, formed on an n+-SiC substrate that serves as the collector, was attributed to a very large minority carrier injection efficiency at the EG/p-SiC Schottky contact. This large minority carrier injection efficiency is in turn related to the large built-in potential found at a EG/p-SiC Schottky junction. The high performance of this device makes it critically important to analyze the sub bandgap visible response of the device, which provides information on impurity states and polytype inclusions in the crystal. Here, we employ scanning photocurrent microscopy (SPCM) with sub-bandgap light as well as a variety of other techniques to clearly demonstrate a localized response based on the graphene transparent electrode and an approximately 1000-fold difference in responsivity between 365 nm and 444 nm excitation. A stacking fault propagating from the substrate/epilayer interface, assigned as a single layer of the 8H-SiC polytype within the 4H-SiC matrix, is found to locally increase the photocurrent substantially. The discovery of this polytype heterojunction opens the potential for further development of heteropolytype devices based on the SEPT architecture.
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Publication: Rev. Sci. Instrum. 89, 085106 (2018)Doi: 10.1063/1.5040503
We present an apparatus that allows for the simultaneous measurement of mass change, heat evolution, and stress of thin film samples deposited on quartz crystal microbalances (QCMs). We show device operation at 24.85 ± 0.05 °C under 9.31 ± 0.02 bars of H2 as a reactive gas. Using a 335 nm palladium film, we demonstrate that our apparatus quantifies curvature changes of 0.001 m−1. Using the QCM curvature to account for stress induced frequency changes, we demonstrate the measurement of mass changes of 13 ng/cm2 in material systems exhibiting large stress fluctuations. We use a one-state nonlinear lumped element model to describe our system with thermal potentials measured at discrete positions by three resistance temperature devices lithographically printed on the QCM. By inputting known heat amounts through lithographically defined Cr/Al wires, we demonstrate a 150 μW calorimetric accuracy and 20 μW minimum detectable power. The capabilities of this instrument will allow for a more complete characterization of reactions occurring in nanoscale systems, such as the effects of hydrogenation in various metal films and nanostructures, as well as allow for direct stress compensation in QCM measurements.
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Publication: Nanophotonics 7, 735 (2018)Doi: 10.1515/nanoph-2017-0100
Graphene is an ideal material for hot-electron bolometers due to its low heat capacity and weak electron-phonon coupling. Nanostructuring graphene with quantum-dot constrictions yields detectors of electromagnetic radiation with extraordinarily high intrinsic responsivity, higher than 1×109 V W−1 at 3 K. The sensing mechanism is bolometric in nature: the quantum confinement gap causes a strong dependence of the electrical resistance on the electron temperature. Here, we show that this quantum confinement gap does not impose a limitation on the photon energy for light detection and these quantum-dot bolometers work in a very broad spectral range, from terahertz through telecom to ultraviolet radiation, with responsivity independent of wavelength. We also measure the power dependence of the response. Although the responsivity decreases with increasing power, it stays higher than 1×108 V W−1 in a wide range of absorbed power, from 1 pW to 0.4 nW.
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Publication: Nature 564, 386 (2018)Doi: 10.1038/s41586-018-0777-8
Intermolecular forces are pervasive in nature and give rise to various phenomena including surface wetting1, adhesive forces in biology2,3, and the Casimir effect4, which causes two charge-neutral, metal objects in vacuum to attract each other. These interactions are the result of quantum fluctuations of electromagnetic waves and the boundary conditions imposed by the interacting materials. When the materials are optically anisotropic, different polarizations of light experience different refractive indices and a torque is expected to occur that causes the materials to rotate to a position of minimum energy5,6. Although predicted more than four decades ago, the small magnitude of the Casimir torque has so far prevented direct measurements of it. Here we experimentally measure the Casimir torque between two optically anisotropic materials—a solid birefringent crystal (calcite, lithium niobite, rutile or yttrium vanadate) and a liquid crystal (5CB). We control the sign and strength of the torque, and its dependence on the rotation angle and the separation distance between the materials, through the choice of materials. The values that we measure agree with calculations, verifying the long-standing prediction that a mechanical torque induced by quantum fluctuations can exist between two separated objects. These results open the door to using the Casimir torque as a micro- or nanoscale actuation mechanism, which would be relevant for a range of technologies, including microelectromechanical systems and liquid crystals.
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Publication: ACS Photonics 5, 306 (2018)Doi: 10.1021/acsphotonics.7b01021
Silicon is the most widely used material for visible photodetection, with extensive applications in both consumer and industrial products. Further, its excellent optoelectronic properties and natural abundance have made it nearly ideal for microelectronic devices and solar cells. However, its lack of absorption in the infrared precludes its use in infrared detectors and imaging sensors, severely constraining its implementation in telecommunications. Here we show that this limitation can be overcome by exploiting resonant absorption in ultrathin metal films (<20 nm). Through appropriate optical design, a zeroth-order Fabry–Perot resonance is achieved, enabling ∼80% light absorption below the bandgap of the semiconductor. Absorption within the metal film results in excitation and injection of hot carriers through a Schottky junction into the Si. We experimentally demonstrate this phenomenon with four ultrathin planar metal films (Pt, Fe, Cr, and Ti), chosen to satisfy the resonant condition over a wide range of wavelengths (1200–1600 nm), and realize a near-infrared imaging detector. Our approach paves the way to implement a scalable, lithography free, and low-cost route to obtain silicon-based optoelectronics beyond the material bandgap.
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Publication: Nano Lett. 18, 1644 (2018)Doi: 10.1021/acs.nanolett.7b04518
Li metal is the preferred anode material for all-solid-state Li batteries. However, a stable plating and stripping of Li metal at the anode–solid electrolyte interface remains a significant challenge particularly at practically feasible current densities. This problem usually relates to high and/or inhomogeneous Li-electrode–electrolyte interfacial impedance and formation and growth of high-aspect-ratio dendritic Li deposits at the electrode–electrolyte interface, which eventually shunt the battery. To better understand details of Li metal plating, we use operando electron microscopy and Auger spectroscopy to probe nucleation, growth, and stripping of Li metal during cycling of a model solid-state Li battery as a function of current density and oxygen pressure. We find a linear correlation between the nucleation density of Li clusters and the charging rate in an ultrahigh vacuum, which agrees with a classical nucleation and growth model. Moreover, the trace amount of oxidizing gas (≈10–6 Pa of O2) promotes the Li growth in a form of nanowires due to a fine balance between the ion current density and a growth rate of a thin lithium-oxide shell on the surface of the metallic Li. Interestingly, increasing the partial pressure of O2 to 10–5 Pa resumes Li plating in a form of 3D particles. Our results demonstrate the importance of trace amounts of preexisting or ambient oxidizing species on lithiation processes in solid-state batteries.
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Publication: Phys. Plasmas 25, 103166 (2018)Doi: 10.1063/1.5052183
The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be affected by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, high frequency source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. For developing a mobile heater with a much smaller antenna array, a new highly efficient megawatt-class Radio Frequency (RF) source is required to reduce the overall power demands on a fully deployable system. The concept of such a source has been described previously [Beaudoin et al., J. Electromagn. Waves Appl. 31(17), 1786–1801 (2017)]. Here, experimental results using an electron beam produced by a gridded thermionic electron gun to drive an external lumped element circuit for a high efficiency RF generation are reported. The gun produces an electron beam bunched at the driving frequency with a narrow phase angle spread that is then collected by an external circuit for resonant impedance matching to the load. The results showed that effects, such as the internal resistance of the inductor and deflection of the beam electrons by the induced RF voltages on the beam collector, are important considerations to be included in the design of a practical device using this configuration for high efficiency RF generation.
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Publication: Phys. Plasmas 25, 093108 (2018)Doi: 10.1063/1.5045317
It is known that gyrotrons (as well as other electron beam driven microwave and millimeter-wave oscillators) can operate in the regime of either soft or hard self-excitation. In the regime of soft self-excitation, the beam current exceeds its starting value; thus, the oscillations can start to grow from the noise produced by electrons. In the regime of hard self-excitation, the beam current is less than its starting value. Therefore, for exciting the oscillations, a certain start-up scenario is required, which may include the variation of the mod-anode and/or beam voltage or the guiding magnetic field. It was found recently [O. Dumbrajs and G. S. Nusinovich, Phys. Plasmas 25, 013121 (2018)] that some gyrotrons can also operate in the region of magnetic fields where there is no start current at all. In the present paper, it is shown that this sort of operation can be attributed to the presence of the axial dependence of the phase of the resonator field.
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Publication: J. Opt. Soc. Am. B - Opt. Phys. 35, 2718 (2018)Doi: 10.1364/JOSAB.35.002718
High-average-power ultra-broadband mid-IR radiation can be generated by illuminating a nonlinear medium with a multi-line laser radiation. Propagation of a multi-line pulsed CO2CO2 laser beam in a nonlinear medium, e.g., gallium arsenide or chalcogenide, can generate directed broadband IR radiation in the atmospheric window (2–13 μm). A 3D laser code for propagation in a nonlinear medium has been developed to incorporate extreme spectral broadening resulting from the beating of several wavelengths. The code has the capability to treat coupled forward and backward-propagating waves, as well as transverse and full linear dispersion effects. Methods for enhancing the spectral broadening are proposed and analyzed. Grading the refractive index radially or using a cladding will tend to guide the CO2CO2 radiation and extend the interaction distance, allowing for enhanced spectral broadening. Nonlinear coupling of the CO2CO2 laser beam to a backward-propagating reflected beam can increase the rate of spectral broadening in the anomalous dispersion regime of a medium. Laser phase noise associated with the finite CO2CO2 linewidths can significantly enhance the spectral broadening, as well. In a dispersive medium, laser phase noise results in laser intensity fluctuations. These intensity fluctuations result in spectral broadening due to the self-phase modulation mechanism. Finally, we present propagation through a chalcogenide fiber as an alternative for extreme spectral broadening of a frequency-doubled CO2CO2 multi-line laser beam.
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Publication: Chem. Mater. 30, 2526 (2018)Doi: 10.1021/acs.chemmater.7b04666
Major advances in thin-film solid-state batteries (TFSSBs) may capitalize on 3D structuring using high-aspect-ratio substrates such as nanoscale pits, pores, trenches, flexible polymers, and textiles. This will require conformal processes such as atomic layer deposition (ALD) for every active functional component of the battery. Here we explore the deposition and electrochemical properties of SnO2, SnNy, and SnOxNy thin films as TFSSB anode materials, grown by ALD using tetrakisdimethylamido(tin), H2O, and N2 plasma as precursors. By controlling the dose ratio between H2O and N2, the N–O fraction can be tuned between 0% N and 95% N. The electrochemical properties of these materials were tested across a composition range varying from pure SnO2, to SnON intermediates, and pure SnNy. In TFSSBs, the SnNy anodes are found to be more stable during cycling than the SnO2 or SnOxNy films, with an initial reversible capacity beyond that of Li–Sn alloying, retaining 75% of their capacity over 200 cycles compared to only 50% for SnO2. Furthermore, the performance of the SnOxNy anodes indicates that SnNy anodes should not be negatively impacted by small levels of O contamination.
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Publication: Complexity 2018, 6953836 (2018)Doi: 10.1155/2018/6953836
We investigate the ways in which a machine learning architecture known as Reservoir Computing learns concepts such as “similar” and “different” and other relationships between image pairs and generalizes these concepts to previously unseen classes of data. We present two Reservoir Computing architectures, which loosely resemble neural dynamics, and show that a Reservoir Computer (RC) trained to identify relationships between image pairs drawn from a subset of training classes generalizes the learned relationships to substantially different classes unseen during training. We demonstrate our results on the simple MNIST handwritten digit database as well as a database of depth maps of visual scenes in videos taken from a moving camera. We consider image pair relationships such as images from the same class; images from the same class with one image superposed with noise, rotated 90°, blurred, or scaled; images from different classes. We observe that the reservoir acts as a nonlinear filter projecting the input into a higher dimensional space in which the relationships are separable; i.e., the reservoir system state trajectories display different dynamical patterns that reflect the corresponding input pair relationships. Thus, as opposed to training in the entire high-dimensional reservoir space, the RC only needs to learns characteristic features of these dynamical patterns, allowing it to perform well with very few training examples compared with conventional machine learning feed-forward techniques such as deep learning. In generalization tasks, we observe that RCs perform significantly better than state-of-the-art, feed-forward, pair-based architectures such as convolutional and deep Siamese Neural Networks (SNNs). We also show that RCs can not only generalize relationships, but also generalize combinations of relationships, providing robust and effective image pair classification. Our work helps bridge the gap between explainable machine learning with small datasets and biologically inspired analogy-based learning, pointing to new directions in the investigation of learning processes.
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Publication: Phys. Rev. Lett. 120, 183901 (2018)Doi: 10.1103/PhysRevLett.120.183901
We present absolute space- and time-resolved measurements of the ultrafast laser-driven nonlinear polarizability in argon, krypton, xenon, nitrogen, and oxygen up to ionization fractions of a few percent. These measurements enable determination of the strongly nonperturbative bound-electron nonlinear polarizability well beyond the ionization threshold, where it is found to remain approximately quadratic in the laser field, a result normally expected at much lower intensities where perturbation theory applies.
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Publication: Phys. Rev. A 97, 022323 (2018)Doi: 10.1103/PhysRevA.97.022323
Most research regarding quantum adiabatic optimization has focused on stoquastic Hamiltonians, whose ground states can be expressed with only real non-negative amplitudes and thus for whom destructive interference is not manifest. This raises the question of whether classical Monte Carlo algorithms can efficiently simulate quantum adiabatic optimization with stoquastic Hamiltonians. Recent results have given counterexamples in which path-integral and diffusion Monte Carlo fail to do so. However, most adiabatic optimization algorithms, such as for solving MAX-k-SAT problems, use k-local Hamiltonians, whereas our previous counterexample for diffusion Monte Carlo involved n-body interactions. Here we present a 6-local counterexample which demonstrates that even for these local Hamiltonians there are cases where diffusion Monte Carlo cannot efficiently simulate quantum adiabatic optimization. Furthermore, we perform empirical testing of diffusion Monte Carlo on a standard well-studied class of permutation-symmetric tunneling problems and similarly find large advantages for quantum optimization over diffusion Monte Carlo.
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Publication: ACS Photonics 5, 4677 (2018)Doi: 10.1021/acsphotonics.8b01243
Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices, such as optical hydrogen sensors and switchable mirrors. While some of these metals, such as palladium, have been well studied, many other promising materials have only been characterized over a limited optical range and lack direct in situ measurements of hydrogen loading, limiting their potential applications. Further, there have been no systematic studies that allow for a clear comparison between these metals. In this work, we present such a systematic study of the dynamically tunable optical properties of Pd, Mg, Zr, Ti, and V throughout hydrogenation with a wavelength range of 250–1690 nm. These measurements were performed in an environmental chamber, which combines mass measurements via a quartz crystal microbalance with ellipsometric measurements in up to 7 bar of hydrogen gas, allowing us to determine the optical properties during hydrogen loading. In addition, we demonstrate a further tunability of the optical properties of titanium and its hydride by altering annealing conditions, and we investigate the optical and gravimetric hysteresis that occurs during hydrogenation cycling of palladium. Finally, we demonstrate several nanoscale optical and plasmonic structures based on these dynamic properties. We show structures that, upon hydrogenation, demonstrate 5 orders of magnitude change in reflectivity, resonance shifts of >200 nm, and relative transmission switching of >3000%, suggesting a wide range of applications.
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Publication: IEEE Trans. Plasma Sci. 46, 2848 (2018)Doi: 10.1109/TPS.2018.2849379
Spectroscopic applications have stimulated strong interest in frequency tunable gyrotrons. One of the possibilities to tune gyrotron frequency is known as frequency pulling, which is based on the effect of the electron beam on the oscillation frequency. Recently, the linear theory of this effect was developed, and the results were obtained for a sequence of modes with different axial indices. This paper is intended to analyze saturation effects in such gyrotrons. The nonlinear theory describing the frequency pulling and the efficiency in gyrotrons is developed in the cold-cavity approximation. The results are obtained for the modes with different axial indices (from 1 to 4). The frequency shift and the efficiency are calculated in a wide range of the dimensionless parameters characterizing the beam current and the external magnetic field. These results related to the frequency pulling are compared with those of the linear theory. Also, comparison is made with the results of the self-consistent theory and experiments.
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Publication: Phys. Rev. Appl. 9, 054013 (2018)Doi: 10.1103/PhysRevApplied.9.054013
We demonstrate optical readout of a single spin using cavity quantum electrodynamics. The spin is based on a single trapped electron in a quantum dot that has a poor branching ratio of 0.43. Selectively coupling one of the optical transitions of the quantum dot to the cavity mode results in a spin-dependent cavity reflectivity that enables spin readout by monitoring the reflected intensity of an incident optical field. Using this approach, we demonstrate spin-readout fidelity of 0.61. Achieving this fidelity using resonance fluorescence from a bare dot would require 43 times improvement in photon collection efficiency.
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Publication: Nucl. Fusion 58, 076023 (2018)Doi: 10.1088/1741-4326/aac197
Tight tolerances have been a leading driver of cost in recent stellarator experiments, so improved definition and control of tolerances can have significant impact on progress in the field. Here we relate tolerances to the shape gradient representation that has been useful for shape optimization in industry, used for example to determine which regions of a car or aerofoil most affect drag, and we demonstrate how the shape gradient can be computed for physics properties of toroidal plasmas. The shape gradient gives the local differential contribution to some scalar figure of merit (shape functional) caused by normal displacement of the shape. In contrast to derivatives with respect to quantities parameterizing a shape (e.g. Fourier amplitudes), which have been used previously for optimizing plasma and coil shapes, the shape gradient gives spatially local information and so is more easily related to engineering constraints. We present a method to determine the shape gradient for any figure of merit using the parameter derivatives that are already routinely computed for stellarator optimization, by solving a small linear system relating shape parameter changes to normal displacement. Examples of shape gradients for plasma and electromagnetic coil shapes are given. We also derive and present examples of an analogous representation of the local sensitivity to magnetic field errors; this magnetic sensitivity can be rapidly computed from the shape gradient. The shape gradient and magnetic sensitivity can both be converted into local tolerances, which inform how accurately the coils should be built and positioned, where trim coils and structural supports for coils should be placed, and where magnetic material and current leads can best be located. Both sensitivity measures provide insight into shape optimization, enable systematic calculation of tolerances, and connect physics optimization to engineering criteria that are more easily specified in real space than in Fourier space.
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Publication: Geophys. Res. Lett. 45, 1237 (2018)Doi: 10.1002/2017GL076809
Data from the NASA Magnetospheric Multiscale mission are used to investigate asymmetric magnetic reconnection at the dayside boundary between the Earth's magnetosphere and the solar wind. High-resolution measurements of plasmas and fields are used to identify highly localized (~15 electron Debye lengths) standing wave structures with large electric field amplitudes (up to 100 mV/m). These wave structures are associated with spatially oscillatory energy conversion, which appears as alternatingly positive and negative values of J · E. For small guide magnetic fields the wave structures occur in the electron stagnation region at the magnetosphere edge of the electron diffusion region. For larger guide fields the structures also occur near the reconnection X-line. This difference is explained in terms of channels for the out-of-plane current (agyrotropic electrons at the stagnation point and guide field-aligned electrons at the X-line).
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Publication: Phys. Rev. Lett. 120, 024102 (2018)Doi: 10.1103/PhysRevLett.120.024102
We demonstrate the effectiveness of using machine learning for model-free prediction of spatiotemporally chaotic systems of arbitrarily large spatial extent and attractor dimension purely from observations of the system’s past evolution. We present a parallel scheme with an example implementation based on the reservoir computing paradigm and demonstrate the scalability of our scheme using the Kuramoto-Sivashinsky equation as an example of a spatiotemporally chaotic system.
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Publication: Phys. Rev. Lett. 120, 040401 (2018)Doi: 10.1103/PhysRevLett.120.040401
Complex interaction geometries offer a unique opportunity to modify the strength and sign of the Casimir force. However, measurements have traditionally been limited to sphere-plate or plate-plate configurations. Prior attempts to extend measurements to different geometries relied on either nanofabrication techniques that are limited to only a few materials or slight modifications of the sphere-plate geometry due to alignment difficulties of more intricate configurations. Here, we overcome this obstacle to present measurements of the Casimir force between two gold spheres using an atomic force microscope. Force measurements are alternated with topographical scans in the x−y plane to maintain alignment of the two spheres to within approximately 400 nm (∼1% of the sphere radii). Our experimental results are consistent with Lifshitz’s theory using the proximity force approximation (PFA), and corrections to the PFA are bounded using nine sphere-sphere and three sphere-plate measurements with spheres of varying radii.
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Publication: Geophys. Res. Lett. 45, 5260 (2018)Doi: 10.1029/2017GL076862
We analyze a high-resolution simulation of magnetopause reconnection observed by the Magnetospheric Multiscale mission and explain the occurrence of strongly localized dissipation with an amplitude more than an order of magnitude larger than expected. Unlike symmetric reconnection, wherein reconnection of the ambient reversed magnetic field drives the dissipation, we find that the annihilation of the self-generated, out-of-plane (Hall) magnetic field plays the dominant role. Electrons flow along the magnetosheath separatrices, converge in the diffusion region, and jet past the X-point into the magnetosphere. The resulting accumulation of negative charge generates intense parallel electric fields that eject electrons along the magnetospheric separatrices and produce field-aligned beams. Many of these features match Magnetospheric Multiscale observations.
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Publication: J. Plasma Phys. 84, 905840616 (2018)Doi: 10.1017/S0022377818001289
The confinement of the guiding-centre trajectories in a stellarator is determined by the variation of the magnetic field strength B in Boozer coordinates (r,θ,φ), but B(r,θ,φ) depends on the flux surface shape in a complicated way. Here we derive equations relating B(r,θ,φ) in Boozer coordinates and the rotational transform to the shape of flux surfaces in cylindrical coordinates, using an expansion in distance from the magnetic axis. A related expansion was done by Garren and Boozer (Phys. Fluids B, vol. 3, 1991a, 2805) based on the Frenet-Serret frame, which can be discontinuous anywhere the magnetic axis is straight, a situation that occurs in the interesting case of omnigenity with poloidally closed B contours. Our calculation in contrast does not use the Frenet-Serret frame. The transformation between the Garren-Boozer approach and cylindrical coordinates is derived, and the two approaches are shown to be equivalent if the axis curvature does not vanish. The expressions derived here help enable optimized plasma shapes to be constructed that can be provided as input to VMEC and other stellarator codes, or to generate initial configurations for conventional stellarator optimization.
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Publication: Phys. Rev. A 98, 063832 (2018)Doi: 10.1103/PhysRevA.98.063832
In this paper, we have developed a method for describing the dynamics of an arbitrary quantum system under a bidirectional time-delayed feedback loop. For this purpose, we have described the evolution in terms of the time propagation of the quantum system of interest without feedback together with several identical systems, which represent the history of the quantum system under study. This technique provides a numerically efficient solution for describing a system's dynamics in the case of significant time delays in which direct investigation of the state of the reservoirs becomes numerically intractable. Using this method, we have studied two scenarios of multiple scatterings of photons incident on a cavity with a two-level atom positioned inside it, coupled to two waveguides that are connected at their ends. In the first scenario, two photons impinge on the cavity through separate waveguides with a delay between them. We have demonstrated that the maximum difference between the two output photon numbers occurs when the delay between the incident photons becomes close to the inverse of their linewidth. In the second scenario, multiple photons impinge on the cavity through the same waveguide and go through multiple interactions. We have shown that, for a fixed atom-cavity coupling rate, the transmission rate enhances as the number of photons increases and have quantified this enhancement. The developed method enables us to study a broad range of nonlinear dynamics in complex quantum networks.
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Publication: Chaos 28, 041101 (2018)Doi: 10.1063/1.5028373
A model-based approach to forecasting chaotic dynamical systems utilizes knowledge of the mechanistic processes governing the dynamics to build an approximate mathematical model of the system. In contrast, machine learning techniques have demonstrated promising results for forecasting chaotic systems purely from past time series measurements of system state variables (training data), without prior knowledge of the system dynamics. The motivation for this paper is the potential of machine learning for filling in the gaps in our underlying mechanistic knowledge that cause widely-used knowledge-based models to be inaccurate. Thus, we here propose a general method that leverages the advantages of these two approaches by combining a knowledge-based model and a machine learning technique to build a hybrid forecasting scheme. Potential applications for such an approach are numerous (e.g., improving weather forecasting). We demonstrate and test the utility of this approach using a particular illustrative version of a machine learning known as reservoir computing, and we apply the resulting hybrid forecaster to a low-dimensional chaotic system, as well as to a high-dimensional spatiotemporal chaotic system. These tests yield extremely promising results in that our hybrid technique is able to accurately predict for a much longer period of time than either its machine-learning component or its model-based component alone.
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Publication: ACS Appl. Mater. Interfaces 10, 38850 (2018)Doi: 10.1021/acsami.8b08097
The spatial resolution of atomic force microscopy (AFM) needed to resolve material interfaces is limited by the tip–sample separation (d) dependence of the force used to record an image. Here, we present a new multiscale functional imaging technique that allows for in situ tunable spatial resolution, which can be applied to a wide range of inhomogeneous materials, devices, and interfaces. Our approach uses a multifrequency method to generate a signal whose d-dependence is controlled by mixing harmonics of the cantilever’s oscillation with a modulated force. The spatial resolution of the resulting image is determined by the signal’s d-dependence. Our measurements using harmonic mixing (HM) show that we can change the d-dependence of a force signal to improve spatial resolution by up to a factor of two compared to conventional methods. We demonstrate the technique with both Kelvin probe force microscopy (KPFM) and bimodal AFM to show its generality. Bimodal AFM with harmonic mixing actuation separates conservative from dissipative forces and is used to identify the regions of adhesive residue on exfoliated graphene. Our electrostatic measurements with open-loop KPFM demonstrate that multiple force modulations may be applied at once. Further, this method can be applied to any tip–sample force that can be modulated, for example, electrostatic, magnetic, and photoinduced forces, showing its universality. Because HM enables in situ switching between high sensitivity and high spatial resolution with any periodic driving force, we foresee this technique as a critical advancement for multiscale functional imaging.
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Publication: Science 361, 57 (2018)Doi: 10.1126/science.aat3581
Single-photon switches and transistors generate strong photon-photon interactions that are essential for quantum circuits and networks. However, the deterministic control of an optical signal with a single photon requires strong interactions with a quantum memory, which has been challenging to achieve in a solid-state platform. We demonstrate a single-photon switch and transistor enabled by a solid-state quantum memory. Our device consists of a semiconductor spin qubit strongly coupled to a nanophotonic cavity. The spin qubit enables a single 63-picosecond gate photon to switch a signal field containing up to an average of 27.7 photons before the internal state of the device resets. Our results show that semiconductor nanophotonic devices can produce strong and controlled photon-photon interactions that could enable high-bandwidth photonic quantum information processing.
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Publication: Chemsuschem 11, 1956 (2018)Doi: 10.1002/cssc.201800027
Detailed understanding of solid–solid interface structure–function relationships is critical for the improvement and wide deployment of all-solid-state batteries. The interfaces between lithium phosphorous oxynitride (LiPON) solid electrolyte material and lithium metal anode, and between LiPON and LixCoO2 cathode, have been reported to generate solid–electrolyte interphase (SEI)-like products and/or disordered regions. Using electronic structure calculations and crystalline LiPON models, we predict that LiPON models with purely P−N−P backbones are kinetically inert towards lithium at room temperature. In contrast, transfer of oxygen atoms from low-energy LixCoO2(104) surfaces to LiPON is much faster under ambient conditions. The mechanisms of the primary reaction steps, LiPON structural motifs that readily reacts with lithium metal, experimental results on amorphous LiPON to partially corroborate these predictions, and possible mitigation strategies to reduce degradations are discussed. LiPON interfaces are found to be useful case studies for highlighting the importance of kinetics-controlled processes during battery assembly at moderate processing temperatures.
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Publication: IEEE J. Quantum Electron. 54, 5000207 (2018)Doi: 10.1109/JQE.2017.2782319
We present a theoretical and experimental study of a low-frequency optoelectronic oscillator featuring both laser-diode and Van der Pol-like nonlinearities. In this architecture, the device performing the electrical-to-optical conversion is the laser-diode itself instead of an external electro-optical modulator, while the electric branch of the oscillator is characterized by a Van der Pol nonlinear transfer function. We show that the system displays a complex autonomous dynamics, induced by the competition between these two nonlinearities. In the case of small delay, the system displays harmonic and relaxation oscillations. When the delay is large, the interplay between the two nonlinearities leads to a period-doubling route of bifurcations as the feedback gain is increased, and ultimately to fully developed chaos. Our experimental measurements are in good agreement with the theoretical analysis.
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Publication: Phys. Plasmas 22, 20705 (2015)Doi: 10.1063/1.4908545
A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks.
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Publication: ACS Nano 12, 4286 (2018)Doi: 10.1021/acsnano.7b08751
Three-dimensional thin-film solid-state batteries (3D TSSB) were proposed by Long et al. in 2004 as a structure-based approach to simultaneously increase energy and power densities. Here, we report experimental realization of fully conformal 3D TSSBs, demonstrating the simultaneous power-and-energy benefits of 3D structuring. All active battery components—electrodes, solid electrolyte, and current collectors—were deposited by atomic layer deposition (ALD) onto standard CMOS processable silicon wafers microfabricated to form arrays of deep pores with aspect ratios up to approximately 10. The cells utilize an electrochemically prelithiated LiV2O5 cathode, a very thin (40–100 nm) Li2PO2N solid electrolyte, and a SnNx anode. The fabrication process occurs entirely at or below 250 °C, promising compatibility with a variety of substrates as well as integrated circuits. The multilayer battery structure enabled all-ALD solid-state cells to deliver 37 μAh/cm2·μm (normalized to cathode thickness) with only 0.02% per-cycle capacity loss. Conformal fabrication of full cells over 3D substrates increased the areal discharge capacity by an order of magnitude while simulteneously improving power performance, a trend consistent with a finite element model. This work shows that the exceptional conformality of ALD, combined with conventional semiconductor fabrication methods, provides an avenue for the successful realization of long-sought 3D TSSBs which provide power performance scaling in regimes inaccessible to planar form factor cells.
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Publication: Adv. Opt. Mater. 6, 1800218 (2018)Doi: 10.1002/adom.201800218
Surface plasmon polaritons (SPPs) enable the deep subwavelength confinement of an electromagnetic field, which can be used in optical devices ranging from sensors to nanoscale lasers. However, the limited number of metals that satisfy the required boundary conditions for SPP propagation in a metal/dielectric interface severely limits its occurrence in the visible range of the electromagnetic spectrum. We introduce the strategy of engineering the band structure of metallic materials by alloying. We experimentally and theoretically establish the control of the dispersion relation in Ag–Au alloys by varying the film chemical composition. Through X-ray photoelectron spectroscopy (XPS) measurements and partial density-of-states calculations we deconvolute the d band contribution of the density-of-states from the valence band spectrum, showing that the shift in energy of the d band follows the surface plasmon resonance change of the alloy. Our density functional theory calculations of the alloys band structure predict the same variation of the threshold of the interband transition, which is in very good agreement with our optical and XPS experiments. By elucidating the correlation between the optical behavior and band structure of alloys, we anticipate the fine control of the optical properties of metallic materials beyond pure metals.
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Publication: Opt. Exp. 26, 20572 (2018)Doi: 10.1364/OE.26.020572
Single-shot supercontinuum spectral interferometry (SSSI) is an optical technique that can measure ultrafast transients in the complex index of refraction. This method uses chirped supercontinuum reference/probe pulses that need to be pre-characterized prior to use. Conventionally, the spectral phase (or chirp) of those pulses can be determined from a series of phase or spectral measurements taken at various time delays with respect to a pump-induced modulation. Here we propose a novel method to simplify this process and characterize reference/probe pulses up to the third order dispersion from a minimum of 2 snapshots taken at different pump-probe delays. Alternatively, without any pre-characterization, our method can retrieve both unperturbed and perturbed reference/probe phases, including the pump-induced modulation, from 2 time-delayed snapshots. From numerical simulations, we show that our retrieval algorithm is robust and can achieve high accuracy even with 2 snapshots. Without any apparatus modification, our method can be easily applied to any experiment that uses SSSI.
2016
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Publication: Acc. Chem. Res. 49, 2336 (2016)Doi: 10.1021/acs.accounts.6b00315
This Account summarizes recent findings related to thin electrode materials synthesized by atomic layer deposition (ALD) and electrochemical deposition (ECD), including nanowires, nanotubes, and thin films. Throughout the Account, we will show how these techniques enabled us to synthesize electrodes of interest with precise control over the structure and composition of the material. We will illustrate and discuss how the electrochemical response of thin electrodes made by these techniques can facilitate new mechanisms for ion storage, mediate the interfacial electrochemical response of the electrode, and address issues related to electrode degradation over time. The effects of nanosizing materials and their electrochemical response will be mechanistically reviewed through two categories of ion storage: (1) pseudocapacitance and (2) ion insertion. Additionally, we will show how electrochemical processes that are more complicated because of accompanying volumetric changes and electrode degradation pathways can be mediated and controlled by application of thin functional materials on the electrochemically active interface; examples include conversion electrodes, reactive lithium metal anodes, and complex reactions in a Li/O2 cathode system. The goal of this Account is to illustrate how careful design of thin materials either as active electrodes or as mediating layers can facilitate desirable interfacial electrochemical activity and resolve or shed light on mechanistic limitations of electrochemical processes related to micrometer size particles currently used in energy storage electrodes.
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Publication: RSC Adv. 6, 83954 (2016)Doi: 10.1039/C6RA17360H
The application of suspended graphene as electron transparent supporting media in electron microscopy, vacuum electronics, and micromechanical devices requires the least destructive and maximally clean transfer from their original growth substrate to the target of interest. Here, we use thermally evaporated anthracene films as the sacrificial layer for graphene transfer onto an arbitrary substrate. We show that clean suspended graphene can be achieved via desorbing the anthracene layer at temperatures in the 100 °C to 150 °C range, followed by two sequential annealing steps for the final cleaning, using a Pt catalyst and activated carbon. The cleanliness of the suspended graphene membranes was analyzed employing the high surface sensitivity of low energy scanning electron microscopy and X-ray photoelectron spectroscopy. A quantitative comparison with two other commonly used transfer methods revealed the superiority of the anthracene approach to obtain a larger area of clean, suspended CVD graphene. Our graphene transfer method based on anthracene paves the way for integrating cleaner graphene in various types of complex devices, including the ones that are heat and humidity sensitive.
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Publication: Angewandte Chemie Int. Edit. 55, 9898 (2016)Doi: 10.1002/anie.201603531
Aluminum metal is a promising anode material for next generation rechargeable batteries owing to its abundance, potentially dendrite-free deposition, and high capacity. The rechargeable aluminum/sulfur (Al/S) battery is of great interest owing to its high energy density (1340 Wh kg−1) and low cost. However, Al/S chemistry suffers poor reversibility owing to the difficulty of oxidizing AlSx. Herein, we demonstrate the first reversible Al/S battery in ionic-liquid electrolyte with an activated carbon cloth/sulfur composite cathode. Electrochemical, spectroscopic, and microscopic results suggest that sulfur undergoes a solid-state conversion reaction in the electrolyte. Kinetics analysis identifies that the slow solid-state sulfur conversion reaction causes large voltage hysteresis and limits the energy efficiency of the system.
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Publication: Geophys. Res. Lett. 43, 6020 (2016)Doi: 10.1002/2016GL069578
Two- and three-dimensional particle-in-cell simulations of a recent encounter of the Magnetospheric Multiscale Mission (MMS) with an electron diffusion region at the magnetopause are presented. While the two-dimensional simulation is laminar, turbulence develops at both the X line and along the magnetic separatrices in the three-dimensional simulation. The turbulence is strong enough to make the magnetic field around the reconnection island chaotic and produces both anomalous resistivity and anomalous viscosity. Each contribute significantly to breaking the frozen-in condition in the electron diffusion region. A surprise is that the crescent-shaped features in velocity space seen both in MMS observations and in two-dimensional simulations survive, even in the turbulent environment of the three-dimensional system. This suggests that MMS's measurements of crescent distributions do not exclude the possibility that turbulence plays an important role in magnetopause reconnection.
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Publication: Phys. Rev. X 6, 031037 (2016)Doi: 10.1103/PhysRevX.6.031037
We present the first experimental evidence, supported by theory and simulation, of spatiotemporal optical vortices (STOVs). A STOV is an optical vortex with phase and energy circulation in a spatiotemporal plane. Depending on the sign of the material dispersion, the local electromagnetic energy flow is saddle or spiral about the STOV. STOVs are a fundamental element of the nonlinear collapse and subsequent propagation of short optical pulses in material media, and conserve topological charge, constraining their birth, evolution, and annihilation. We measure a self-generated STOV consisting of a ring-shaped null in the electromagnetic field about which the phase is spiral, forming a dynamic torus that is concentric with and tracks the propagating pulse. Our results, here obtained for optical pulse collapse and filamentation in air, are generalizable to a broad class of nonlinearly propagating waves.
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Publication: Nature Nanotechnol. 11, 539 (2016)Doi: 10.1038/nnano.2015.334
Interactions between single spins and photons are essential for quantum networks and distributed quantum computation. Achieving spin–photon interactions in a solid-state device could enable compact chip-integrated quantum circuits operating at gigahertz bandwidths. Many theoretical works have suggested using spins embedded in nanophotonic structures to attain this high-speed interface. These proposals implement a quantum switch where the spin flips the state of the photon and a photon flips the spin state. However, such a switch has not yet been realized using a solid-state spin system. Here, we report an experimental realization of a spin–photon quantum switch using a single solid-state spin embedded in a nanophotonic cavity. We show that the spin state strongly modulates the polarization of a reflected photon, and a single reflected photon coherently rotates the spin state. These strong spin–photon interactions open up a promising direction for solid-state implementations of high-speed quantum networks and on-chip quantum information processors using nanophotonic devices.
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Publication: Phys. Res. X 6, 041054 (2016)Doi: 10.1103/PhysRevX.6.041054
Experimental and theoretical studies are reported, for the first time, on the observation and characterization of echo phenomena in oscillatory chemical reactions. Populations of uncoupled and coupled oscillators are globally perturbed. The macroscopic response to this perturbation dies out with time: At some time τ after the perturbation (where τ is long enough that the response has died out), the system is again perturbed, and the initial response to this second perturbation again dies out. Echoes can potentially appear as responses that arise at 2τ,3τ,... after the first perturbation. The phase-resetting character of the chemical oscillators allows a detailed analysis, offering insights into the origin of the echo in terms of an intricate structure of phase relationships. Groups of oscillators experiencing different perturbations are analyzed with a geometric approach and in an analytical theory. The characterization of echo phenomena in populations of chemical oscillators reinforces recent theoretical studies of the behavior in populations of phase oscillators [E. Ott et al., Chaos 18, 037115 (2008)]. This indicates the generality of the behavior, including its likely occurrence in biological systems.
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Publication: Chaos 26, 094811 (2016)Doi: 10.1063/1.4954275
Cells in the brain's Suprachiasmatic Nucleus (SCN) are known to regulate circadian rhythms in mammals. We model synchronization of SCN cells using the forced Kuramoto model, which consists of a large population of coupled phase oscillators (modeling individual SCN cells) with heterogeneous intrinsic frequencies and external periodic forcing. Here, the periodic forcing models diurnally varying external inputs such as sunrise, sunset, and alarm clocks. We reduce the dimensionality of the system using the ansatz of Ott and Antonsen and then study the effect of a sudden change of clock phase to simulate cross-time-zone travel. We estimate model parameters from previous biological experiments. By examining the phase space dynamics of the model, we study the mechanism leading to the difference typically experienced in the severity of jet-lag resulting from eastward and westward travel.
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Publication: J. Phys. Chem. Lett. 7, 211 (2016)Doi: 10.1021/acs.jpclett.5b02613
The lithium oxygen system, if fully harnessed in the form of a rechargeable battery, offers a tantalizing ideal energy density of 3445 W h L–1, more than doubling state-of-the-art Li ion technology. An impressive amount of recent research has attacked this system, and despite its complexity, there now exists a menagerie of different rechargeable Li–O2 battery chemistries and electrode architectures exhibiting some level of promise. However, the system still presents a multitude of unsolved fundamental issues, mostly related to the electrochemical stability of the cathode, electrolyte, and Li metal anode in the exceptionally harsh cell environment, which are often specific to the exact combination of materials utilized. A natural consequence is that drawing comparisons between different Li–O2 battery configurations is intrinsically very difficult.
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Publication: Phys. Rev. B 94, 174507 (2016)Doi: 10.1103/PhysRevB.94.174507
The response of nonlinear metamaterials and superconducting electronics to two-tone excitation is critical for understanding their use as low-noise amplifiers and tunable filters. A new setting for such studies is that of metamaterials made of radio frequency superconducting quantum interference devices (rf-SQUIDs). The two-tone response of self-resonant rf-SQUID meta-atoms and metamaterials is studied here via intermodulation (IM) measurement over a broad range of tone frequencies and tone powers. A sharp onset followed by a surprising strongly suppressed IM region near the resonance is observed. Using a two time scale analysis technique, we present an analytical theory that successfully explains our experimental observations. The theory predicts that the IM can be manipulated with tone power, center frequency, frequency difference between the two tones, and temperature. This quantitative understanding potentially allows for the design of rf-SQUID metamaterials with either very low or very high IM response.
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Publication: Phys. Chem. Chem. Phys. 18, 30605 (2016)Doi: 10.1039/c6cp05640g
Here we introduce a strategy for creating nanotube array electrodes which feature periodic regions of porous interconnections providing open pathways between adjacent nanotubes within the array, utilizing a combination of anodized aluminum oxide growth modification (AAO) and atomic layer deposition. These porous interconnected structures can then be used as testbed electrodes to explore the influence of mesoscale structure on the electrochemical properties of the interconnected mesoporous electrodes. Critically, these unique structures allow the solid state lithium diffusion pathways to be held essentially constant, while the larger structure is modified. While it was anticipated that this strategy would simply provide increased mass loading, the kinetics of the Li+ ion insertion reaction in the porous interconnected electrodes are dramatically improved, demonstrating significantly better capacity retention at high rates than their aligned counterparts. We utilize a charge deconvolution method to explore the kinetics of the charge storage reactions. We are able to trace the origin of the structural influence on rate performance to electronic effects within the electrodes.
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Publication: Phys. Rev. A 94, 063407 (2016)Doi: 10.1103/PhysRevA.94.063407
Nonlocal potential models have been used in place of the Coulomb potential in the Schrödinger equation as an efficient means of exploring high-field laser-atom interaction in previous works. Although these models have found use in modeling phenomena including photoionization and ejected electron momentum spectra, they are known to break electromagnetic gauge invariance. This paper examines if there is a preferred gauge for the linear field response and photoionization characteristics of nonlocal atomic binding potentials in the length and velocity gauges. It is found that the length gauge is preferable for a wide range of parameters.
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Publication: Plasma Phys. Control. Fusion 58, 074009 (2016)Doi: 10.1088/0741-3335/58/7/074009
We review in a tutorial fashion some of the causes of impurity density variations along field lines and radial impurity transport in the moment approach framework. An explicit and compact form of the parallel inertia force valid for arbitrary toroidal geometry and magnetic coordinates is derived and shown to be non-negligible for typical TJ-II plasma conditions. In the second part of the article, we apply the fluid model including main ion-impurity friction and inertia to observations of asymmetric emissivity patterns in neutral beam heated plasmas of the TJ-II stellarator. The model is able to explain qualitatively several features of the radiation asymmetry, both in stationary and transient conditions, based on the calculated in-surface variations of the impurity density.
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Publication: J. Vac. Sci. Technol. A 34, 040603 (2016)Doi: 10.1116/1.4949260
The authors studied the effect of the temperature and chemical state of the chamber wall on process performance for atomic layer etching of SiO2 using a steady-state Ar plasma, periodic injection of a defined number of C4F8 molecules, and synchronized plasma-based Ar+ ion bombardment. To evaluate these effects, the authors measured the quartz coupling window temperature. The plasma gas phase chemistry was characterized using optical emission spectroscopy. It was found that although the thickness of the polymer film deposited in each cycle is constant, the etching behavior changed, which is likely related to a change in the plasma gas phase chemistry. The authors found that the main gas phase changes occur after C4F8 injection. The C4F8 and the quartz window react and generate SiF and CO. The emission intensity changes with wall surface state and temperature. Therefore, changes in the plasma gas species generation can lead to a shift in etching performance during processing. During initial cycles, minimal etching is observed, while etching gradually increases with cycle number.
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Publication: Geophys. Res. Lett.43, 43 (2016)Doi: 10.1002/2015GL066980
A new scalar measure of the gyrotropy of a pressure tensor is defined. Previously suggested measures are shown to be incomplete by means of examples for which they give unphysical results. To demonstrate its usefulness as an indicator of magnetic topology, the new measure is calculated for electron data taken from numerical simulations of magnetic reconnection, shown to peak at separatrices and X points, and compared to the other measures. The new diagnostic has potential uses in analyzing spacecraft observations, and so a method for calculating it from measurements performed in an arbitrary coordinate system is derived.
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Publication: IEEE Trans. Electron Devices 63, 2531 (2016)Doi: 10.1109/TED.2016.2555801
A new computationally efficient 2-D large-signal code TESLA-FW has been developed to model traveling wave tubes (TWTs) using folded (or serpentine) waveguide (FW) slow-wave structures. The code incorporates a recently published shunt-loaded transmission line model of the structure and takes advantage of a new time advance algorithm recently implemented in the code TESLA-CC. The new code has been applied to modeling G -band (220 GHz) serpentine and FW TWTs. Results compare very favorably with those obtained from the 3-D particle-in-cell (PIC) code MAGIC3D and with experimental data. Typical running times of TESLA-FW are 1-2 orders of magnitude less than those of 3-D PIC codes.
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Inhibitory Neurons Promote Robust Critical Firing Dynamics in Networks of Integrate-and-Fire Neurons
Publication: Phys. Rev. E 94, 062309 (2016)Doi: 10.1103/PhysRevE.94.062309We study the firing dynamics of a discrete-state and discrete-time version of an integrate-and-fire neuronal network model with both excitatory and inhibitory neurons. When the integer-valued state of a neuron exceeds a threshold value, the neuron fires, sends out state-changing signals to its connected neurons, and returns to the resting state. In this model, a continuous phase transition from non-ceaseless firing to ceaseless firing is observed. At criticality, power-law distributions of avalanche size and duration with the previously derived exponents, −3/2 and −2, respectively, are observed. Using a mean-field approach, we show analytically how the critical point depends on model parameters. Our main result is that the combined presence of both inhibitory neurons and integrate-and-fire dynamics greatly enhances the robustness of critical power-law behavior (i.e., there is an increased range of parameters, including both sub- and supercritical values, for which several decades of power-law behavior occurs).
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Publication: J. Infrared Millim. Terahz. Waves 37, 515 (2016)Doi: 10.1007/s10762-016-0243-3
This review paper summarizes the results of studies of a novel concept of the remote detection of concealed radioactive materials by using focused high-power terahertz (THz) radiation. The concept is based on the known fact that the ambient electron density in air is low (one to three free electrons per cubic centimeter). These electrons can serve as seed electrons from which an avalanche breakdown in strong electromagnetic fields starts. When a powerful THz radiation is focused in a small spot, the breakdown-prone volume can be much smaller than a cubic centimeter. So, the probability of having some free electrons in this volume and, hence, the probability of breakdown are low in the absence of additional sources of air ionization. However, in the vicinity of radioactive materials (10–20 m), the electron density can be higher, and, hence, there are always some seed free electrons from which the avalanche ionization will start. Thus, the breakdown rate in this case can be close to 100 %. Realization of this concept requires studies of various physical and technical issues. First, it is necessary to develop a high-power source of (sub-) THz radiation whose power, frequency, and pulse duration are sufficient for realizing this goal. Second, it is necessary to analyze numerous issues important for realizing this concept. Among these issues are (a) enhancement of the ionization level of air molecules in the presence of hidden radioactive materials, (b) estimating the minimum detectable mass of radioactive material, (c) formation of breakdown-prone volumes in focused THz wave beams, and (d) effect of atmospheric conditions on the propagation and focusing of THz wave beams and on the optimal location of the breakdown-prone volume between a container with hidden radioactive material and a THz antenna. The results of these studies are described below.
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Publication: Plasma Phys. Control. Fusion 58, 085001 (2016)Doi: 10.1088/0741-3335/58/8/085001
We present a numerical study of collisional transport in a tokamak pedestal in the presence of non-trace impurities, using the radially global "delta f" neoclassical solver Perfect (Landreman et al 2014 Plasma Phys. Control. Fusion 56 045005). It is known that in a tokamak core with non-trace impurities present the radial impurity flux opposes the bulk ion flux to provide an ambipolar particle transport, with the electron transport being negligibly small. However, in a sharp density pedestal with sub-sonic ion flows the electron transport can be comparable to the ion and impurity flows. Furthermore, the neoclassical particle transport is not intrinsically ambipolar, and the non-ambipolarity of the fluxes extends outside the pedestal region by the radial coupling of the perturbations. The neoclassical momentum transport, which is finite in the presence of ion orbit-width scale profile variations, is significantly enhanced when impurities are present in non-trace quantities, even if the total parallel mass flow is dominated by the bulk ions.
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Publication: New J. Phys. 18, 093008 (2016)Doi: 10.1088/1367-2630/18/9/093008
Advances in single-photon creation, transmission, and detection suggest that sending quantum information over optical fibers may have losses low enough to be correctable using a quantum error correcting code (QECC). Such error-corrected communication is equivalent to a novel quantum repeater scheme, but crucial questions regarding implementation and system requirements remain open. Here we show that long-range entangled bit generation with rates approaching 108 entangled bits per second may be possible using a completely serialized protocol, in which photons are generated, entangled, and error corrected via sequential, one-way interactions with as few matter qubits as possible. Provided loss and error rates of the required elements are below the threshold for quantum error correction, this scheme demonstrates improved performance over transmission of single photons. We find improvement in entangled bit rates at large distances using this serial protocol and various QECCs. In particular, at a total distance of 500 km with fiber loss rates of 0.3 dB km−1, logical gate failure probabilities of 10−5, photon creation and measurement error rates of 10−5, and a gate speed of 80 ps, we find the maximum single repeater chain entangled bit rates of 51 Hz at a 20 m node spacing and 190,000 Hz at a 43 m node spacing for the and QECCs respectively as compared to a bare success rate of 1 × 10−140 Hz for single photon transmission.
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Publication: Astrophys. J. Lett. 830, 1 (2016)Doi: 10.3847/2041-8205/830/1/L9
Understanding the thermodynamic state of the hot intracluster medium (ICM) in a galaxy cluster requires knowledge of the plasma transport processes, especially thermal conduction. The basic physics of thermal conduction in plasmas with ICM-like conditions has yet to be elucidated, however. We use particle-in-cell simulations and analytic models to explore the dynamics of an ICM-like plasma (with small gyroradius, large mean free path, and strongly sub-dominant magnetic pressure) driven by the diffusive heat flux associated with thermal conduction. Linear theory reveals that whistler waves are driven unstable by electron heat flux, even when the heat flux is weak. The resonant interaction of electrons with these waves then plays a critical role in scattering electrons and suppressing the heat flux. In a 1D model where only whistler modes that are parallel to the magnetic field are captured, the only resonant electrons are moving in the opposite direction to the heat flux, and the electron heat flux suppression is small. In 2D or more, oblique whistler modes also resonate with electrons moving in the direction of the heat flux. The overlap of resonances leads to effective symmetrization of the electron distribution function and a strong suppression of heat flux. The results suggest that thermal conduction in the ICM might be strongly suppressed, possibly to negligible levels.
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Publication: Space Sci. Rev. 204, 49 (2016)Doi: 10.1007/s11214-016-0244-5
NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.
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Publication: Nano Lett. 16, 7061 (2016)Doi: 10.1021/acs.nanolett.6b03295
Interactions between solid-state quantum emitters and cavities are important for a broad range of applications in quantum communication, linear optical quantum computing, nonlinear photonics, and photonic quantum simulation. These applications often require combining many devices on a single chip with identical emission wavelengths in order to generate two-photon interference, the primary mechanism for achieving effective photon–photon interactions. Such integration remains extremely challenging due to inhomogeneous broadening and fabrication errors that randomize the resonant frequencies of both the emitters and cavities. In this Letter, we demonstrate two-photon interference from independent cavity-coupled emitters on the same chip, providing a potential solution to this long-standing problem. We overcome spectral mismatch between different cavities due to fabrication errors by depositing and locally evaporating a thin layer of condensed nitrogen. We integrate optical heaters to tune individual dots within each cavity to the same resonance with better than 3 μeV of precision. Combining these tuning methods, we demonstrate two-photon interference between two devices spaced by less than 15 μm on the same chip with a postselected visibility of 33%, which is limited by timing resolution of the detectors and background. These results pave the way to integrate multiple quantum light sources on the same chip to develop quantum photonic devices.
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Publication: ACS Energy Lett. 1, 899 (2016)Doi: 10.1021/acsenergylett.6b00331
To date, the performance of all polycrystalline photovoltaics is limited by their open-circuit voltage (Voc), an indicator of charge carrier recombination within the semiconductor layer. Thus, the successful implementation of high-efficiency and low-cost solar cells requires the control and suppression of nonradiative recombination centers within the material. Here, we spectrally and spatially resolve the photovoltage of polycrystalline thin-film Cu(In,Ga)Se2 (CIGS) solar cells. Micro-Raman and energy-dispersive X-ray spectroscopy maps obtained on the same grains showed that the chemical composition of the CIGS layer is very uniform. Surprisingly, we observed concurrent spatial variations in the photovoltage generated across the device, strongly indicating that structural properties are likely responsible for the nonuniform mesoscale behavior reported here. We build a tomography of the photovoltage response at 1 sun global illumination, mimicking the operation conditions of solar cells. Furthermore, we spatially resolve the voltage within the CIGS grains, where we found variations >20%. Our functional characterization could be realized to identify where nonradiative recombination preferentially takes place, enabling the implementation of nonuniform materials for future devices with higher Voc.
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Publication: Phys. Plasmas 23, 120704 (2016)Doi: 10.1063/1.4972082
We present two-dimensional kinetic simulations, with a broad range of initial guide fields, which isolate the role of parallel electric fields (E∥) in energetic electron production during collisionless magnetic reconnection. In the strong guide field regime, E∥ drives essentially all of the electron energy gains, yet fails to generate an energetic component. We suggest that this is due to the weak energy scaling of particle acceleration from E∥ compared to that of a Fermi-type mechanism responsible for energetic electron production in the weak guide-field regime. This result has important implications for energetic electron production in astrophysical systems and reconnection-driven dissipation in turbulence.
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Publication: Physica Scripta 91, 083003 (2016)Doi: 10.1088/0031-8949/91/8/083003
The free electron laser (FEL) has lived up to its promise as given in (Madey 1971 J. Appl. Phys. 42 1906) to wit: 'As shall be seen, finite gain is available ...from the far-infrared through the visible region ...with the further possibility of partially coherent radiation sources in the x-ray region'. In the present paper we review the history of the FEL drawing liberally (and where possible literally) from the original sources. Coauthors, Madey, Scully and Sprangle were involved in the early days of the subject and give a first hand account of the subject with an eye to the future.
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Publication: J. Infrared Milli. Terahz. Waves 37, 111 (2016)Doi: 10.1007/s10762-015-0192-2
For gyrotron applications in plasma installations, one of the most important factors is the gyrotron efficiency. To maximize the interaction efficiency, it is necessary not only to optimize such operating parameters as the magnetic field, beam voltage, and current but also the axial profile of the electromagnetic (EM) field in the interaction space. The present paper describes a study of the effect of the profile of an irregular waveguide serving as a resonator on the axial structure of the EM field. Specific attention is paid to the profile of the uptaper connecting the regular part of a resonator to the output waveguide. Conditions of applicability of the nonuniform string equation, which is widely used in gyrotron designs for finding the axial structure of the EM field, are discussed. Also discussed are the occurrence of reflections from a smooth uptaper and the analogy between the nonuniform string equation and the stationary Schrodinger equation.
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Publication: ACS Photonics 3, 507 (2016)Doi: 10.1021/acsphotonics.5b00586
The fixed optical properties of noble metals currently limit their use in photonic devices that operate at optical frequencies. To achieve metals with tunable optical response, we present noble metal alloyed thin-films formed by the binary mixture of Ag, Au, and Cu. As the dielectric functions (ε) of the alloys cannot be modeled as the linear combination of the pure metals, we combined transmission and reflection measurements of the alloyed thin films with a B-spline model that takes into account the Kromers–Kronig relation and experimentally determined the dielectric functions. We found that in some cases a mixture can provide a material with higher surface plasmon polariton quality factor than the corresponding pure metals. We independently measured the surface plasmon polariton coupling angle for all alloys using the Kretschmann configuration and found excellent agreement between the two methods when determining ε. Our approach paves the way to implement metallic thin films and nanostructures with on-demand optical responses, overcoming the current limitation of the dielectric function of noble metals.
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Publication: Opt. Lett. 41, 3908 (2016)Doi: 10.1364/OL.41.003908
We present spatially resolved measurements of energy deposition into atmospheric air by femtosecond laser filaments. Single filaments formed with varying laser pulse energy and pulsewidth were examined using longitudinal interferometry, sonographic probing, and direct energy loss measurements. We measure peak and average energy absorption of ∼4 μJ/cm∼4 μJ/cm and ∼1 μJ/cm∼1 μJ/cm for input pulse powers up to ∼6∼6 times the critical power for self-focusing.
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Publication: New J. Phys. 18, 113013 (2016)Doi: 10.1088/1367-2630/18/11/113013
We present an all-dielectric photonic crystal structure that supports two-dimensionally confined helical topological edge states. The topological properties of the system are controlled by the crystal parameters. An interface between two regions of differing band topologies gives rise to topological edge states confined in a dielectric slab that propagate around sharp corners without backscattering. Three-dimensional finite-difference time-domain calculations show these edges to be confined in the out-of-plane direction by total internal reflection. Such nanoscale photonic crystal architectures could enable strong interactions between photonic edge states and quantum emitters.
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Publication: Plasma Proc. Polymers 13, 1069 (2016)Doi: 10.1002/ppap.201600043
This study uses photoresist materials in combination with several optical filters as a diagnostic to examine the relative importance of VUV-induced surface modifications for different cold atmospheric pressure plasma (CAPP) sources. The argon fed kHz-driven ring-APPJ showed the largest ratio of VUV surface modification relative to the total modification introduced, whereas the MHz APPJ showed the largest overall surface modification. The MHz APPJ shows increased total thickness reduction and reduced VUV effect as oxygen is added to the feed gas, a condition that is often used for practical applications. We examine the influence of noble gas flow from the APPJ on the local environment. The local environment has a decisive impact on polymer modification from VUV emission as O2 readily absorbs VUV photons.
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Publication: Phys. Plasmas 23, 052306 (2016)Doi: 10.1063/1.4950834
Electron holes are electrostatic non-linear structures widely observed in the space plasma. In the present paper, we analyze the process of energy exchange between electrons trapped within electron hole, untrapped electrons, and an electron hole propagating in a weakly inhomogeneous magnetic field. We show that as the electron hole propagates into the region with stronger magnetic field, trapped electrons are heated due to the conservation of the first adiabatic invariant. At the same time, the electron hole amplitude may increase or decrease in dependence on properties of distribution functions of trapped and untrapped resonant electrons. The energy gain of trapped electrons is due to the energy losses of untrapped electrons and/or decrease of the electron hole energy. We stress that taking into account the energy exchange with untrapped electrons increases the lifetime of electron holes in inhomogeneous magnetic field. We illustrate the suggested mechanism for small-amplitude Schamel's [Phys. Scr. T2, 228–237 (1982)] electron holes and show that during propagation along a positive magnetic field gradient their amplitude should grow. Neglect of the energy exchange with untrapped electrons would result in the electron hole dissipation with only modest heating factor of trapped electrons. The suggested mechanism may contribute to generation of suprathermal electron fluxes in the space plasma.
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Publication: Phys. Plasmas 23, 082107 (2016)Doi: 10.1063/1.4960324
We investigate magnetic reconnection in systems simultaneously containing asymmetric (anti-parallel) magnetic fields, asymmetric plasma densities and temperatures, and arbitrary in-plane bulk flow of plasma in the upstream regions. Such configurations are common in the high-latitudes of Earth's magnetopause and in tokamaks. We investigate the convection speed of the X-line, the scaling of the reconnection rate, and the condition for which the flow suppresses reconnection as a function of upstream flow speeds. We use two-dimensional particle-in-cell simulations to capture the mixing of plasma in the outflow regions better than is possible in fluid modeling. We perform simulations with asymmetric magnetic fields, simulations with asymmetric densities, and simulations with magnetopause-like parameters where both are asymmetric. For flow speeds below the predicted cutoff velocity, we find good scaling agreement with the theory presented in Doss et al. [J. Geophys. Res. 120, 7748 (2015)]. Applications to planetary magnetospheres, tokamaks, and the solar wind are discussed.
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Publication: Plasma Phys. Control. Fusion 58, 004007 (2016)Doi: 10.1088/0741-3335/58/7/074007
The toroidal torque due to the non-resonant interaction with external magnetic perturbations (TF ripple and perturbations from ELM mitigation coils) in ASDEX Upgrade is modelled with help of the NEO-2 and SFINCS codes and compared to semi-analytical models. It is shown that almost all non-axisymmetric transport regimes contributing to neoclassical toroidal viscosity (NTV) are realized within a single discharge at different radial positions. The NTV torque is obtained to be roughly a quarter of the NBI torque. This indicates the presence of other important momentum sources. The role of these momentum sources and possible integral torque balance measurements are briefly discussed.
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Publication: Phys. Plasmas 23, 023114 (2016)Doi: 10.1063/1.4942175
For numerous applications, it is desirable to develop electron beam driven efficient sources of electromagnetic radiation that are capable of producing the required power at beam voltages as low as possible. This trend is limited by space charge effects that cause the reduction of electron kinetic energy and can lead to electron reflection. So far, this effect was analyzed for intense beams propagating in uniform metallic pipes. In the present study, the limiting currents of intense electron beams are analyzed for the case of beam propagation in the tubes with gaps. A general treatment is illustrated by an example evaluating the limiting current in a high-power, tunable 1–10 MHz inductive output tube (IOT), which is currently under development for ionospheric modification. Results of the analytical theory are compared to results of numerical simulations. The results obtained allow one to estimate the interaction efficiency of IOTs.
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Demonstration of Resonance Coupling in Scalable Dielectric Microresonator Coatings for Photovoltaics
Publication: ACS Appl. Mater. Interfaces 8, 24536 (2016)Doi: 10.1021/acsami.6b05734To increase the power conversion efficiency of solar cells, improved antireflection coatings are needed to couple light into the cell with minimal parasitic loss. Here, we present measurements and simulations of an antireflection coating based on silicon dioxide (SiO2) nanospheres that improve solar cell absorption by coupling light from free space into the absorbing layer through excitation of modes within the nanospheres. The deposited monolayer of nanospheres leads to a significant increase in light absorption within an underlying semiconductor on the order of 15–20%. When the periodicity and spacing between the nanospheres are varied, whispering gallery-like modes can be excited and tuned throughout the visible spectrum. The coating was applied to a Si solar cell containing a Si3N4 antireflection layer, and an additional increase in the spectral current density of ∼5% was found. The fabrication process, involving Meyer rod rolling, is scalable and inexpensive and could enable large-scale manufacturability of microresonator-based photovoltaics.
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Publication: Radiophys. Quantum Electron. 58, 905 (2016)Doi: 10.1007/s11141-016-9664-z
The dynamics of the microwave breakdown in a gas is simulated numerically within a simple 1D model which takes into account such processes as the impact ionization of gas molecules, the attachment of electrons to neutral molecules, and plasma diffusion. Calculations are carried out for different spatial distributions of seed electrons with account for reflection of the incident electromagnetic wave from the plasma. The results reveal considerable dependence of the ionization wave evolution on the relation between the field frequency and gas pressure, as well as on the existence of extended rarefied halo of seed electrons. At relatively low gas pressures (or high field frequencies), the breakdown process is accompanied by the stationary ionization wave moving towards the incident electromagnetic wave. In the case of a high gas pressure (or a relatively low field frequency), the peculiarities of the breakdown are associated with the formation of repetitive jumps of the ionization front.
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Publication: The European Phys. J. D 70, 25 (2016)Doi: 10.1140/epjd/e2015-60446-3
Cold atmospheric plasma (CAP) sources are attractive sources of reactive species with promising industrial and biomedical applications, but an understanding of underlying surface mechanisms is lacking. A kHz-powered surface microdischarge (SMD) operating with N2/O2 mixtures was used to study the biological deactivation of two immune-stimulating biomolecules: lipopolysaccharide (LPS) and peptidoglycan (PGN), found in bacteria such as Escherichia coli and Staphylococcus aureus, respectively. Model polymers were also studied to isolate specific functional groups. Changes in the surface chemistry were measured to understand which plasma-generated species and surface modifications are important for biological deactivation. The overall goal of this work is to determine which effects of CAP treatment are generic and which bonds are susceptible to attack. CAP treatment deactivated biomolecules, oxidized surfaces, and introduced surface bound NO3. These effects can be controlled by the N2 fraction in O2 and applied voltage and vary among different target surfaces. The SMD was compared with an Ar/O2/N2-fed kHz-powered atmospheric pressure plasma jet and showed much higher surface modifications and surface chemistry tunability compared to the jet. Possible mechanisms are discussed and findings are compared with recent computational investigations. Our results demonstrate the importance of long-lived plasma-generated species and advance an atomistic understanding of CAP-surface interactions.
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Publication: Appl. Phys. Lett. 108, 121106 (2016)Doi: 10.1063/1.4944843
We demonstrate scalable terahertz (THz) generation by focusing terawatt, two-color laser pulses in air with a cylindrical lens. This focusing geometry creates a two-dimensional air plasma sheet, which yields two diverging THz lobe profiles in the far field. This setup can avoid plasma-induced laser defocusing and subsequent THz saturation, previously observed with spherical lens focusing of high-power laser pulses. By expanding the plasma source into a two-dimensional sheet, cylindrical focusing can lead to scalable THz generation. This scheme provides an energy conversion efficiency of 7 × 10−4, ∼7 times better than spherical lens focusing. The diverging THz lobes are refocused with a combination of cylindrical and parabolic mirrors to produce strong THz fields (>21 MV/cm) at the focal point.
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Publication: J. Vac. Sci. Technol. A 34, 01B101 (2016)Doi: 10.1116/1.4935462
The need for atomic layer etching (ALE) is steadily increasing as smaller critical dimensions and pitches are required in device patterning. A flux-control based cyclic Ar/C4F8 ALE based on steady-state Ar plasma in conjunction with periodic, precise C4F8 injection and synchronized plasma-based low energy Ar+ ion bombardment has been established for SiO2 [Metzler et al., J. Vac. Sci. Technol. A 32, 020603 (2014)]. In this work, the cyclic process is further characterized and extended to ALE of silicon under similar process conditions. The use of CHF3 as a precursor is examined and compared to C4F8. CHF3 is shown to enable selective SiO2/Si etching using a fluorocarbon (FC) film build up. Other critical process parameters investigated are the FC film thickness deposited per cycle, the ion energy, and the etch step length. Etching behavior and mechanisms are studied using in situ real time ellipsometry and x-ray photoelectron spectroscopy. Silicon ALE shows less self-limitation than silicon oxide due to higher physical sputtering rates for the maximum ion energies used in this work, ranged from 20 to 30 eV. The surface chemistry is found to contain fluorinated silicon oxide during the etching of silicon. Plasma parameters during ALE are studied using a Langmuir probe and establish the impact of precursor addition on plasma properties.
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Publication: Phys. Rev. E 94, 042310 (2016)Doi: 10.1103/PhysRevE.94.042310
Learning and memory are acquired through long-lasting changes in synapses. In the simplest models, such synaptic potentiation typically leads to runaway excitation, but in reality there must exist processes that robustly preserve overall stability of the neural system dynamics. How is this accomplished? Various approaches to this basic question have been considered. Here we propose a particularly compelling and natural mechanism for preserving stability of learning neural systems. This mechanism is based on the global processes by which metabolic resources are distributed to the neurons by glial cells. Specifically, we introduce and study a model composed of two interacting networks: a model neural network interconnected by synapses that undergo spike-timing-dependent plasticity; and a model glial network interconnected by gap junctions that diffusively transport metabolic resources among the glia and, ultimately, to neural synapses where they are consumed. Our main result is that the biophysical constraints imposed by diffusive transport of metabolic resources through the glial network can prevent runaway growth of synaptic strength, both during ongoing activity and during learning. Our findings suggest a previously unappreciated role for glial transport of metabolites in the feedback control stabilization of neural network dynamics during learning.
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Publication: Phys. Plasmas 23, 083125 (2016)Doi: 10.1063/1.4961962
For a long time, the gyrotron theory was developed assuming that the transit time of electrons through the interaction space is much shorter than the cavity fill time. Correspondingly, it was assumed that during this transit time, the amplitude of microwave oscillations remains constant. A recent interest to such additional effects as the after-cavity interaction between electrons and the outgoing wave in the output waveguide had stimulated some studies of the beam-wave interaction processes over much longer distances than a regular part of the waveguide which serves as a cavity in gyrotrons. Correspondingly, it turned out that the gyrotron theory free from the assumption about constant amplitude of microwave oscillations during the electron transit time should be developed. The present paper contains some results obtained in the framework of such theory. The main attention is paid to modification of the boundary between the regions of oscillations with constant amplitude and automodulation in the plane of normalized parameters characterizing the external magnetic field and the beam current. It is shown that the theory free from the assumption about the frozen wave amplitude during the electron transit time predicts some widening of the region of automodulation.
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Publication: Phys. Plasmas 23, 053111 (2016)Doi: 10.1063/1.4949762
The effect of the electron beam on the gyrotron operating frequency (the frequency pulling) is studied analytically in the framework of the linear (or small-signal) theory. The theory is applicable for gyrotrons operating at any cyclotron harmonics and in modes with arbitrary axial structures. The present consideration is limited to cases of operation at the fundamental cyclotron resonance and the second harmonic; also two specific axial profiles of the resonator modes are analyzed: the constant and the sinusoidal distributions. In the case of the sinusoidal distribution, we considered the operation in modes with one, two, and three axial variations. It is shown how to use the theory developed for analyzing the frequency tunability due to the frequency pulling effect in a gyrotron with specified parameters of the electron beam.
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Publication: Nature Mater. 16, 572 (2016)Doi: 10.1038/nmat4821
Garnet-type solid-state electrolytes have attracted extensive attention due to their high ionic conductivity, approaching 1 mS cm−1, excellent environmental stability, and wide electrochemical stability window, from lithium metal to ∼6 V. However, to date, there has been little success in the development of high-performance solid-state batteries using these exceptional materials, the major challenge being the high solid–solid interfacial impedance between the garnet electrolyte and electrode materials. In this work, we effectively address the large interfacial impedance between a lithium metal anode and the garnet electrolyte using ultrathin aluminium oxide (Al2O3) by atomic layer deposition. Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZN) is the garnet composition of choice in this work due to its reduced sintering temperature and increased lithium ion conductivity. A significant decrease of interfacial impedance, from 1,710 Ω cm2 to 1 Ω cm2, was observed at room temperature, effectively negating the lithium metal/garnet interfacial impedance. Experimental and computational results reveal that the oxide coating enables wetting of metallic lithium in contact with the garnet electrolyte surface and the lithiated-alumina interface allows effective lithium ion transport between the lithium metal anode and garnet electrolyte. We also demonstrate a working cell with a lithium metal anode, garnet electrolyte and a high-voltage cathode by applying the newly developed interface chemistry.
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Publication: Geophys. Res. Lett. 43, 4145 (2016)Doi: 10.1002/2016GL069034
Kinetic particle-in-cell simulations are used to identify signatures of the electron diffusion region (EDR) and its surroundings during asymmetric magnetic reconnection. A “shoulder” in the sunward pointing normal electric field (EN > 0) at the reconnection magnetic field reversal is a good indicator of the EDR and is caused by magnetosheath electron meandering orbits in the vicinity of the X line. Earthward of the X line, electrons accelerated by EN form strong currents and crescent-shaped distribution functions in the plane perpendicular to B. Just downstream of the X line, parallel electric fields create field-aligned crescent electron distribution functions. In the immediate upstream magnetosheath, magnetic field strength, plasma density, and perpendicular electron temperatures are lower than the asymptotic state. In the magnetosphere inflow region, magnetosheath ions intrude resulting in an Earthward pointing electric field and parallel heating of magnetospheric particles. Many of the above properties persist with a guide field of at least unity.
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Publication: Plasma Processes Polymers 13, 410 (2016)Doi: 10.1002/ppap.201500072
Cold atmospheric plasma (CAP) treatment of biological surfaces results in important changes of biological functions, but little knowledge on specific surface-chemical changes is available. We measured surface-bound NO3 on polymer and biomolecular films after CAP treatment. An O2/N2-based surface microdischarge was used to deactivate lipopolysaccharide (LPS), an immune-stimulating biomolecule found in Gram negative bacteria. The observed LPS biodeactivation was highest for low N2 concentrations in O2, increased roughly linearly with surface NO3, and then saturated. NO3 was also observed after treatment by a very different source: an atmospheric pressure plasma jet operating with an Ar carrier gas. Thus, NO3 formation is a generic surface chemical modification of these materials by CAP sources.
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Publication: Phys. Plasmas 23, 032506 (2016)Doi: 10.1063/1.4943201
The magnetic field that supports tokamak and stellarator plasmas must be produced by coils well separated from the plasma. However, the larger the separation, the more difficult it is to produce a given magnetic field in the plasma region, so plasma configurations should be chosen that can be supported as efficiently as possible by distant coils. The efficiency of an externally generated magnetic field is a measure of the field's shaping component magnitude at the plasma compared to the magnitude near the coils; the efficiency of a plasma equilibrium can be measured using the efficiency of the required external shaping field. Counterintuitively, plasma shapes with low curvature and spectral width may have low efficiency, whereas plasma shapes with sharp edges may have high efficiency. Two precise measures of magnetic field efficiency, which correctly identify such differences in difficulty, will be examined. These measures, which can be expressed as matrices, relate the externally produced normal magnetic field on the plasma surface to the either the normal field or current on a distant control surface. A singular value decomposition (SVD) of either matrix yields an efficiency ordered basis for the magnetic field distributions. Calculations are carried out for both tokamak and stellarator cases. For axisymmetric surfaces with circular cross-section, the SVD is calculated analytically, and the range of poloidal and toroidal mode numbers that can be controlled to a given desired level is determined. If formulated properly, these efficiency measures are independent of the coordinates used to parameterize the surfaces.
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Publication: J. Vac. Sci. Technol. B 34, 041604 (2016)Doi: 10.1116/1.4949274
Previous work on 193 nm photoresist (PR) material has shown that a significant improvement of pattern transfer performance can be obtained by applying a helium plasma pretreatment (PPT) prior to the pattern transfer plasma etching step [Weilnboeck et al., Appl. Phys. Lett. 99, 261501 (2011)]. This work explores whether this PPT is applicable to other organic masking materials commonly employed in resist multilayer masking schemes. The materials investigated include an antireflection coating, a thermally activated hard mask, a near frictionless carbon similar to a 248 nm PR, and an extreme ultraviolet resist. These materials have substantially different ultraviolet/vacuum ultraviolet sensitivity among each other and relative to 193 nm PR. The authors find that the surface roughness seen after a combination of helium PPT and Ar plasma main etching step is either the same or increased slightly relative to a single Ar main etching step, in contrast to 193 nm PR materials. The fragile adamantane group removed during PPT from 193 nm PR is absent for these materials. This indicates that the He PPT efficacy and improved pattern transfer performance is specific to adamantane containing resists.
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Publication: J. Opt. Soc. Am. B - Opt. Phys. 33, 1476 (2016)Doi: 10.1364/JOSAB.33.001476
Single-shot supercontinuum spectral interferometry is a powerful technique for measuring transient refractive index changes. In principle, its time resolution is limited only by the available probe bandwidth. However, this assumes that the phase extraction has sufficient spectral resolution and that the probe spectral phase is exactly known. Using an analytical model for the spectral phase and amplitude modulation of a chirped probe pulse by a weak transient phase perturbation, we show how the probe chirp and spectral resolution determine the achievable time resolution. A simple, practical technique for precise in situ measurement of the probe spectral phase is described in detail, and the sensitivity of the extracted temporal phase profile to uncertainty in the probe spectral phase is demonstrated in numerical simulations.
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Publication: IEEE J. Quantum Electron. 52 [6] (2016)Doi: 10.1109/JQE.2016.2557068
This paper analyzes a mechanism for the remote optical measurements of magnetic field variations above the surface of seawater. This magnetometry mechanism is based on the polarization rotation of reflected polarized laser light, in the presence of the earth's magnetic field. Here the laser light is reflected off the surface of the water and off an underwater object. Two mechanisms responsible for the polarization rotation are the surface magneto-optical kerr effect (SMOKE) and the Faraday effect. In both the mechanisms, the degree of polarization rotation is proportional to the earth's local magnetic field. Variations in the earth's magnetic field due to an underwater object will result in variations in the polarization rotation of the laser light reflected off the water's surface (SMOKE) and off the underwater object (Faraday effect). An analytical expression is obtained for the polarization-rotated field when the incident plane wave is at arbitrary angle and polarization with respect to the water's surface. We find that the polarization rotated field due to SMOKE is small compared with that due to the Faraday effect.
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Publication: Geophys. Res. Lett. 43, 5536 (2016)Doi: 10.1002/2016GL069166
We report evidence for reconnection between colliding reconnection jets in a compressed current sheet at the center of a magnetic flux rope at Earth's magnetopause. The reconnection involved nearly symmetric inflow boundary conditions with a strong guide field of two. The thin (2.5 ion-skin depth (di) width) current sheet (at ~12 di downstream of the X line) was well resolved by MMS, which revealed large asymmetries in plasma and field structures in the exhaust. Ion perpendicular heating, electron parallel heating, and density compression occurred on one side of the exhaust, while ion parallel heating and density depression were shifted to the other side. The normal electric field and double out-of-plane (bifurcated) currents spanned almost the entire exhaust. These observations are in good agreement with a kinetic simulation for similar boundary conditions, demonstrating in new detail that the structure of large guide field symmetric reconnection is distinctly different from antiparallel reconnection.
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Publication: Chaos 26, 094801 (2016)Doi: 10.1063/1.4953662
A “chimera state” is a dynamical pattern that occurs in a network of coupled identical oscillators when the symmetry of the oscillator population is broken into synchronous and asynchronous parts. We report the experimental observation of chimera and cluster states in a network of four globally coupled chaotic opto-electronic oscillators. This is the minimal network that can support chimera states, and our study provides new insight into the fundamental mechanisms underlying their formation. We use a unified approach to determine the stability of all the observed partially synchronous patterns, highlighting the close relationship between chimera and cluster states as belonging to the broader phenomenon of partial synchronization. Our approach is general in terms of network size and connectivity. We also find that chimera states often appear in regions of multistability between global, cluster, and desynchronized states.
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Publication: Sci. Adv. 2 (2016)Doi: 10.1126/sciadv.1501737
Synchronization is an important and prevalent phenomenon in natural and engineered systems. In many dynamical networks, the coupling is balanced or adjusted to admit global synchronization, a condition called Laplacian coupling. Many networks exhibit incomplete synchronization, where two or more clusters of synchronization persist, and computational group theory has recently proved to be valuable in discovering these cluster states based on the topology of the network. In the important case of Laplacian coupling, additional synchronization patterns can exist that would not be predicted from the group theory analysis alone. Understanding how and when clusters form, merge, and persist is essential for understanding collective dynamics, synchronization, and failure mechanisms of complex networks such as electric power grids, distributed control networks, and autonomous swarming vehicles. We describe a method to find and analyze all of the possible cluster synchronization patterns in a Laplacian-coupled network, by applying methods of computational group theory to dynamically equivalent networks. We present a general technique to evaluate the stability of each of the dynamically valid cluster synchronization patterns. Our results are validated in an optoelectronic experiment on a five-node network that confirms the synchronization patterns predicted by the theory.
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Publication: Plasma Chem. Plasma Proc. 36, 121 (2016)Doi: 10.1007/s11090-015-9673-2
We review studies of surface-interaction mechanisms for a surface microdischarge (SMD) and an atmospheric pressure plasma jet (APPJ) with model polymers and biomolecules in our laboratory. We discuss the influence of plasma source type, operating parameters, and gaseous environments on surface modifications and biological deactivation. We focus on mild, remote conditions where the visible plasma plume does not contact the surface. For an APPJ fed with Ar, the interaction of the plasma plume with O2 and/or N2 gaseous environments leads to oxidation and surface-bound NOx even on materials containing neither oxygen nor nitrogen. The APPJ also modifies photo-sensitive polymers. Using optical filters, these modifications were shown to result in part from irradiation with vacuum ultraviolet (VUV) photons in a spectral range corresponding to Ar excimer emission. No VUV-induced effects were seen for the SMD source operated with O2/N2. SMD treatments using O2/N2 mixtures result in surface oxidation and nitridation. A new surface-bound species, NO3, has been measured on the polymers and biomolecules. Depending on the gas chemistry and film molecular structure, the NO3 surface concentration can reach 10 %. Both surface NO3 on plasma-treated films of lipopolysaccharide (LPS), an immune stimulating biomolecule found in bacteria such as E. c oli, and overall surface oxidation correlate with LPS biological deactivation as evaluated using an enzyme-linked immunosorbent assay. Ambient humidity was studied using the SMD and was found to decrease overall surface modifications including NO3 and biodeactivation for O2-rich conditions. Lastly, we discuss possible mechanisms and compare our results with published simulation studies.
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Publication: J. Vac. Sci. Technol. A 34, 041307 (2016)Doi: 10.1116/1.4954961
Angstrom-level plasma etching precision is required for semiconductor manufacturing of sub-10 nm critical dimension features. Atomic layer etching (ALE), achieved by a series of self-limited cycles, can precisely control etching depths by limiting the amount of chemical reactant available at the surface. Recently, SiO2 ALE has been achieved by deposition of a thin (several Angstroms) reactive fluorocarbon (FC) layer on the material surface using controlled FC precursor flow and subsequent low energy Ar+ ion bombardment in a cyclic fashion. Low energy ion bombardment is used to remove the FC layer along with a limited amount of SiO2 from the surface. In the present article, the authors describe controlled etching of Si3N4 and SiO2 layers of one to several Angstroms using this cyclic ALE approach. Si3N4 etching and etching selectivity of SiO2 over Si3N4 were studied and evaluated with regard to the dependence on maximum ion energy, etching step length (ESL), FC surface coverage, and precursor selection. Surface chemistries of Si3N4 were investigated by x-ray photoelectron spectroscopy (XPS) after vacuum transfer at each stage of the ALE process. Since Si3N4 has a lower physical sputtering energy threshold than SiO2, Si3N4 physical sputtering can take place after removal of chemical etchant at the end of each cycle for relatively high ion energies. Si3N4 to SiO2 ALE etching selectivity was observed for these FC depleted conditions. By optimization of the ALE process parameters, e.g., low ion energies, short ESLs, and/or high FC film deposition per cycle, highly selective SiO2 to Si3N4 etching can be achieved for FC accumulation conditions, where FC can be selectively accumulated on Si3N4 surfaces. This highly selective etching is explained by a lower carbon consumption of Si3N4 as compared to SiO2. The comparison of C4F8 and CHF3 only showed a difference in etching selectivity for FC depleted conditions. For FC accumulation conditions, precursor chemistry has a weak impact on etching selectivity. Surface chemistry analysis shows that surface fluorination and FC reduction take place during a single ALE cycle for FC depleted conditions. A fluorine rich carbon layer was observed on the Si3N4 surface after ALE processes for which FC accumulation takes place. The angle resolved-XPS thickness calculations confirmed the results of the ellipsometry measurements in all cases.
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Publication: J. Vac. Sci. Technol. A 34, 01B102 (2016)Doi: 10.1116/1.4935460
With increasing demands on device patterning to achieve smaller critical dimensions and pitches for the 5 nm node and beyond, the need for atomic layer etching (ALE) is steadily increasing. In this work, a cyclic fluorocarbon/Ar plasma is successfully used for ALE patterning in a manufacturing scale reactor. Self-limited etching of silicon oxide is observed. The impact of various process parameters on the etch performance is established. The substrate temperature has been shown to play an especially significant role, with lower temperatures leading to higher selectivity and lower etch rates, but worse pattern fidelity. The cyclic ALE approach established with this work is shown to have great potential for small scale device patterning, showing self-limited etching, improved uniformity and resist mask performance.
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Publication: Phys. Plasmas 23, 060703 (2016)Doi: 10.1063/1.4953420
The velocity-space distribution of alpha particles born in fusion devices is subject to modification at moderate energies due to turbulent transport. Therefore, one must calculate the evolution of an equilibrium distribution whose functional form is not known a priori. Using a novel technique, applicable to any trace impurity, we have made this calculation for fully nonlinear gyrokinetic simulations not only possible but also particularly efficient. We demonstrate a microturbulence-induced departure from the local slowing-down distribution, an inversion of the energy distribution, and associated modifications to the alpha heating and pressure profiles in an ITER-like scenario.
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Publication: Phys. Rev. Lett. 116, 235102 (2016)Doi: 10.1103/PhysRevLett.116.235102
We report observations from the Magnetospheric Multiscale satellites of parallel electric fields (E∥) associated with magnetic reconnection in the subsolar region of the Earth’s magnetopause. E∥ events near the electron diffusion region have amplitudes on the order of 100 mV/m, which are significantly larger than those predicted for an antiparallel reconnection electric field. This Letter addresses specific types of E∥ events, which appear as large-amplitude, near unipolar spikes that are associated with tangled, reconnected magnetic fields. These E∥ events are primarily in or near a current layer near the separatrix and are interpreted to be double layers that may be responsible for secondary reconnection in tangled magnetic fields or flux ropes. These results are telling of the three-dimensional nature of magnetopause reconnection and indicate that magnetopause reconnection may be often patchy and/or drive turbulence along the separatrix that results in flux ropes and/or tangled magnetic fields.
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Publication: Phys. Plasmas 23, 056501 (2016)Doi: 10.1063/1.4943526
As the solar system moves through the interstellar medium, the solar wind is deflected forming the heliosphere. The standard picture of the heliosphere is a comet-shape like structure with the tail extending for 1000s of astronomical units. This standard picture stems from a view where magnetic forces are negligible and the solar magnetic field is convected passively down the tail. Recently, we showed that the magnetic tension of the solar magnetic field plays a crucial role on organizing the solar wind in the heliosheath into two jet-like structures. The two jets are separated by the interstellar medium that flows between them. The heliosphere then has a “croissant”-like shape where the distance to the heliopause downtail is almost the same as towards the nose. This new view of the heliosphere is in agreement with the energetic neutral atoms maps taken by the Interstellar Boundary Explorer and INCA/CASSINI. We developed as well an analytic model of the heliosheath in the axisymmetric limit that shows how the magnetic tension force is the driver for the north and south jets. We confirmed that the formation of these jets with magnetohydrodynamic (MHD) simulations. The main reason why previous global MHD simulations did not see these jets is due to spurious magnetic dissipation that was present at the heliospheric current sheet. We instead kept the same polarity for the interplanetary (solar) magnetic field in both the northern and southern hemispheres, eliminating spurious magnetic dissipation effects at the heliospheric current sheet. In this paper, we extend these previous results to include additional cases where we used: (a) weaker solar magnetic field; (b) solar magnetic field that reverses polarity at the solar equator in the axisymmetric limit; and (c) slower motion through the interstellar system. We discuss as well future challenges regarding the structure of the heliosphere.
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Publication: Space Sci. Rev. 199, 577 (2016)Doi: 10.1007/s11214-014-0078-y
The Magnetospheric Multiscale (MMS) mission will provide measurement capabilities, which will exceed those of earlier and even contemporary missions by orders of magnitude. MMS will, for the first time, be able to measure directly and with sufficient resolution key features of the magnetic reconnection process, down to the critical electron scales, which need to be resolved to understand how reconnection works. Owing to the complexity and extremely high spatial resolution required, no prior measurements exist, which could be employed to guide the definition of measurement requirements, and consequently set essential parameters for mission planning and execution. Insight into expected details of the reconnection process could hence only been obtained from theory and modern kinetic modeling. This situation was recognized early on by MMS leadership, which supported the formation of a fully integrated Theory and Modeling Team (TMT). The TMT participated in all aspects of mission planning, from the proposal stage to individual aspects of instrument performance characteristics. It provided and continues to provide to the mission the latest insights regarding the kinetic physics of magnetic reconnection, as well as associated particle acceleration and turbulence, assuring that, to the best of modern knowledge, the mission is prepared to resolve the inner workings of the magnetic reconnection process. The present paper provides a summary of key recent results or reconnection research by TMT members.
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Publication: Adv. Energy Mater. 7, 1601526 (2016)Doi: 10.1002/aenm.201601526
A stable Na metal anodeis is reported by forming an artificial solid electrolyte interphase via a low-temperature plasma-enhanced atomic layer deposition technology. It is discovered that an ultrathin layer of Al2O3 layer (2.8 nm) can protect Na metal from electrolyte decomposition, prevent 3D dendrite formation, and significantly enhance its cycle stability in a carbonate-based electrolyte.
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Publication: Nucl. Fusion 56, 112009 (2016)Doi: 10.1088/0029-5515/56/11/112009
Improved understanding of runaway-electron formation and decay processes are of prime interest for the safe operation of large tokamaks, and the dynamics of the runaway electrons during dynamical scenarios such as disruptions are of particular concern. In this paper, we present kinetic modelling of scenarios with time-dependent plasma parameters; in particular, we investigate hot-tail runaway generation during a rapid drop in plasma temperature. With the goal of studying runaway-electron generation with a self-consistent electric-field evolution, we also discuss the implementation of a collision operator that conserves momentum and energy and demonstrate its properties. An operator for avalanche runaway-electron generation, which takes the energy dependence of the scattering cross section and the runaway distribution into account, is investigated. We show that the simplified avalanche model of Rosenbluth and Putvinskii (1997 Nucl. Fusion 37 1355) can give inaccurate results for the avalanche growth rate (either lower or higher) for many parameters, especially when the average runaway energy is modest, such as during the initial phase of the avalanche multiplication. The developments presented pave the way for improved modelling of runaway-electron dynamics during disruptions or other dynamic events.
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Publication: Phys. Plasmas 23, 056701 (2016)Doi: 10.1063/1.4943522
Intense charged particle beams are nonneutral plasmas as they can support a host of plasma waves and instabilities. The longitudinal physics, for a long beam, can often be reasonably described by a 1-D cold-fluid model with a geometry factor to account for the transverse effects. The plasma physics of such beams has been extensively studied theoretically and computationally for decades, but until recently, the only experimental measurements were carried out on relatively short linacs. This work reviews experimental studies over the past five years on the University of Maryland Electron Ring, investigating longitudinal phenomena over time scales of thousands of plasma periods, illustrating good agreement with simulations.
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Publication: J. Vac. Sci. Technol. A 34, 031306 (2016)Doi: 10.1116/1.4944609
The authors report a systematic study aimed at evaluating the impact of molecular structure parameters of hydrofluorocarbon (HFC) precursors on plasma deposition of fluorocarbon (FC) films and etching performance of a representative ultra-low-k material, along with amorphous carbon. The precursor gases studied included fluorocarbon and hydrofluorocarbon gases whose molecular weights and chemical structures were systematically varied. Gases with three different degrees of unsaturation (DU) were examined. Trifluoromethane (CHF3) is the only fully saturated gas that was tested. The gases with a DU value of one are 3,3,3-trifluoropropene (C3H3F3), hexafluoropropene (C3F6), 1,1,3,3,3-pentafluoro-1-propene (C3HF5), (E)-1,2,3,3,3-pentafluoropropene (C3HF5 isomer), heptafluoropropyl trifluorovinyl ether (C5F10O), octafluorocyclobutane (C4F8), and octafluoro-2-butene (C4F8 isomer). The gases with a DU value of two includes hexafluoro-1,3-butadiene (C4F6), hexafluoro-2-butyne (C4F6 isomer), octafluorocyclopentene (C5F8), and decafluorocyclohexene (C6F10). The work was performed in a dual frequency capacitively coupled plasma reactor. Real-time characterization of deposition and etching was performed using in situ ellipsometry, and optical emission spectroscopy was used for characterization of CF2 radicals in the gas phase. The chemical composition of the deposited FC films was examined by x-ray photoelectron spectroscopy. The authors found that the CF2 fraction, defined as the number of CF2 groups in a precursor molecule divided by the total number of carbon atoms in the molecule, determines the CF2 optical emission intensity of the plasma. CF2 optical emission, however, is not the dominant factor that determines HFC film deposition rates. Rather, HFC film deposition rates are determined by the number of weak bonds in the precursor molecule, which include a ring structure, C=C, C≡C, and C–H bonds. These bonds are broken preferentially in the plasma, and/or at the surface and fragments arriving at the substrate surface presumably provide dangling bonds that efficiently bond to the substrate or other fragments. Upon application of a radio-frequency bias to the substrate, substrate etching is induced. Highly polymerizing gases show decreased substrate etching rates as compared to HFC gases characterized by a lower HFC film deposition rate. This can be explained by a competition between deposition and etching reactions, and an increased energy and etchant dissipation in relatively thicker steady state FC films that form on the substrate surface. Deposited HFC films exhibit typically a high CF2 density at the film surface, which correlates with both the CF2 fractions in the precursor molecular structure and the deposition rate. The FC films deposited using hydrogen-containing precursors show higher degrees of crosslinking and lower F/C ratios than precursors without hydrogen, and exhibit a lower etch rate of substrate material. A small gap structure that blocks direct ion bombardment was used to simulate the sidewall plasma environment of a feature and was employed for in situ ellipsometry measurements. It is shown that highly polymerizing precursors with a DU of two enable protection of low-k sidewalls during plasma exposure from oxygen-related damage by protective film deposition. Dielectric film modifications are seen for precursors with a lower DU.
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Publication: Phys. Plasmas 23, 063103 (2016)Doi: 10.1063/1.4953098
An intense, short laser pulse propagating across a plasma boundary ponderomotively drives THz radiation. Full format PIC simulations and theoretical analysis are conducted to investigate the properties of this radiation. Simulation results which show the THz emission originates in regions of varying density and covers a broad spectrum with maximum frequency close to the maximum plasma frequency. In the case of a sharp vacuum-plasma boundary, the radiation is generated symmetrically at the plasma entrance and exit, and its properties are independent of plasma density when the density exceeds a characteristic value determined by the product of the plasma frequency and the laser pulse duration. For a diffuse vacuum-plasma boundary, the emission from the plasma entrance and exit is asymmetric: increasing and decreasing density ramps enhance and diminish the radiated energy, respectively. Enhancements by a factor of 50 are found and simulations show that a 1.66 J, 50 fs driver pulse can generate ∼400 μJ of THz radiation in a 1.2 mm increasing density ramp. We present a model that attributes this effect to a plasma resonance process in the density ramp. The results from the model match those of the simulations for ramp lengths less than 600 μm. For longer ramps for which simulations are too time consuming, the model shows that the amount of radiation reaches a maximum at a ramp length determined by collisional absorption.
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Publication: Biomicrofluidics 10, 061301 (2016)Doi: 10.1063/1.4967777
Chemotaxis is a phenomenon which enables cells to sense concentrations of certain chemical species in their microenvironment and move towards chemically favorable regions. Recent advances in microbiology have engineered the chemotactic properties of bacteria to perform novel functions, but traditional methods of characterizing chemotaxis do not fully capture the associated cell motion, making it difficult to infer mechanisms that link the motion to the microbiology which induces it. Microfluidics offers a potential solution in the form of gradient generators. Many of the gradient generators studied to date for this application are flow-based, where a chemical species diffuses across the laminar flow interface between two solutions moving through a microchannel. Despite significant research efforts, flow-based gradient generators have achieved mixed success at accurately capturing the highly subtle chemotactic responses exhibited by bacteria. Here we present an analysis encompassing previously published versions of flow-based gradient generators, the theories that govern their gradient-generating properties, and new, more practical considerations that result from experimental factors. We conclude that flow-based gradient generators present a challenge inherent to their design in that the residence time and gradient decay must be finely balanced, and that this significantly narrows the window for reliable observation and quantification of chemotactic motion. This challenge is compounded by the effects of shear on an ellipsoidal bacterium that causes it to preferentially align with the direction of flow and subsequently suppresses the cross-flow chemotactic response. These problems suggest that a static, non-flowing gradient generator may be a more suitable platform for chemotaxis studies in the long run, despite posing greater difficulties in design and fabrication.
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Publication: Phys. Rev. Lett. 117, 015001 (2016)Doi: 10.1103/PhysRevLett.117.015001
We report observations from the Magnetospheric Multiscale (MMS) satellites of a large guide field magnetic reconnection event. The observations suggest that two of the four MMS spacecraft sampled the electron diffusion region, whereas the other two spacecraft detected the exhaust jet from the event. The guide magnetic field amplitude is approximately 4 times that of the reconnecting field. The event is accompanied by a significant parallel electric field (E∥) that is larger than predicted by simulations. The high-speed (∼300 km/s) crossing of the electron diffusion region limited the data set to one complete electron distribution inside of the electron diffusion region, which shows significant parallel heating. The data suggest that E∥ is balanced by a combination of electron inertia and a parallel gradient of the gyrotropic electron pressure.
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Publication: Phys. Rev. A 94, 023816 (2016)Doi: 10.1103/PhysRevA.94.023816
Ultrashort long-wave infrared (LWIR) laser pulses can resonantly excite vibrations in N2 and O2 through a two-photon transition. The absorptive vibrational component of the ultrafast optical nonlinearity grows in time, starting smaller than but quickly surpassing the electronic, rotational, and vibrational refractive components. The growth of the vibrational component results in a novel mechanism of third-harmonic generation, providing an additional two-photon excitation channel, fundamental + third harmonic. The original and emergent two-photon excitations drive the resonance exactly out of phase, causing spatial decay of the absorptive vibrational nonlinearity. This nearly eliminates two-photon vibrational absorption. Here we present simulations and analytical calculations demonstrating how these processes modify the ultrafast optical nonlinearity in air. The results reveal nonlinear optical phenomena unique to the LWIR regime of ultrashort pulse propagation in the atmosphere.
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Publication: Opt. Lett. 41, 3427 (2016)Doi: 10.1364/OL.41.003427
We demonstrate the generation of axially modulated plasma waveguides using spatially patterned high-energy laser pulses. A spatial light modulator (SLM) imposes transverse phase front modulations on a low-energy (10 mJ) laser pulse which is interferometrically combined with a high-energy (130–450 mJ) pulse, sculpting its intensity profile. This enables dynamic and programmable shaping of the laser profile limited only by the resolution of the SLM and the intensity ratio of the two pulses. The plasma density profile formed by focusing the patterned pulse with an axicon lens is likewise dynamic and programmable. Centimeter-scale, axially modulated plasmas of varying shape and periodicity are demonstrated.
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Publication: ACS Photonics 3, 1069 (2016)Doi: 10.1021/acsphotonics.6b00141
Understanding the optical response of graphene at terahertz frequencies is of critical importance for designing graphene-based devices that operate in this frequency range. Here we present a terahertz pump–probe measurement that simultaneously measures both the transmitted and reflected probe radiation from multilayer epitaxial graphene, allowing for an unambiguous determination of the pump-induced absorption change in the graphene layers. The photon energy in the experiment (30 meV) is on the order of the doping level in the graphene, which enables the exploration of the transition from interband to intraband processes, depending on the amount of pump-induced heating. Our findings establish the presence of a large, photoinduced reflection that contributes to the change in sign of the relative transmitted terahertz radiation, which can be purely positive or predominantly negative depending on the pump fluence, while the change in absorption is found to be negative at all fluences. We develop a hot carrier model that confirms the sign-reversible nature of the relative transmitted terahertz radiation through the graphene multilayer and determine that this behavior originates from either an absorption-bleached or reflection-dominated regime. The theoretical results are incorporated into a model utilizing an energy balance equation that reproduces the measured pump–probe data. These findings, which extend to mid- and far-infrared frequencies, show the importance of considering reflection in graphene–light interactions and have implications for the design of future terahertz photonic components.
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Publication: Phys. Plasmas 23, 103107 (2016)Doi: 10.1063/1.4964722
With ever increasing demands for intensities in modern accelerators, the understanding of space-charge effects becomes crucial. Herein are presented measurements of optically shaped picosecond-long electron beams in a superconducting L-band linac over a wide range of charges, from 0.2 nC to 3.4 nC. At low charges, the shape of the electron beam is preserved, while at higher charge densities, modulations on the beam convert to energy modulations. Energy profile measurements using a spectrometer and time profile measurements using a streak camera reveal the dynamics of longitudinal space-charge on MeV-scale electron beams.
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Publication: Nano Lett. 16, 5415 (2016)Doi: 10.1021/acs.nanolett.6b01708
Single-emitter microscopy has emerged as a promising method of imaging nanostructures with nanoscale resolution. This technique uses the centroid position of an emitter’s far-field radiation pattern to infer its position to a precision that is far below the diffraction limit. However, nanostructures composed of high-dielectric materials such as noble metals can distort the far-field radiation pattern. Previous work has shown that these distortions can significantly degrade the imaging of the local density of states in metallic nanowires using polarization-resolved imaging. But unlike nanowires, nanoparticles do not have a well-defined axis of symmetry, which makes polarization-resolved imaging difficult to apply. Nanoparticles also exhibit a more complex range of distortions, because in addition to introducing a high dielectric surface, they also act as efficient scatterers. Thus, the distortion effects of nanoparticles in single-emitter microscopy remains poorly understood. Here we demonstrate that metallic nanoparticles can significantly distort the accuracy of single-emitter imaging at distances exceeding 300 nm. We use a single quantum dot to probe both the magnitude and the direction of the metallic nanoparticle-induced imaging distortion and show that the diffraction spot of the quantum dot can shift by more than 35 nm. The centroid position of the emitter generally shifts away from the nanoparticle position, which is in contradiction to the conventional wisdom that the nanoparticle is a scattering object that will pull in the diffraction spot of the emitter toward its center. These results suggest that dielectric distortion of the emission pattern dominates over scattering. We also show that by monitoring the distortion of the quantum dot diffraction spot we can obtain high-resolution spatial images of the nanoparticle, providing a new method for performing highly precise, subdiffraction spatial imaging. These results provide a better understanding of the complex near-field coupling between emitters and nanostructures and open up new opportunities to perform super-resolution microscopy with higher accuracy.
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Publication: J. Appl. Phys. 119, 023104 (2016)Doi: 10.1063/1.4939646
Many photovoltaic technologies are shifting toward thin-film devices to simultaneously reduce costs and improve carrier collection efficiencies; however, the need for nearly complete light absorption within the semiconductor to achieve large short-circuit currents constrains this thickness reduction. Light trapping strategies can be employed to increase absorption in thinner devices. Random scattering coatings offer a simple, cost-effective way to increase solar cell absorption without the drawback of increased surface recombination or reduced bandwidth that occurs when using surface texturing or gratings. However, coatings that show excellent performance as scatterers in free space generally do not enhance device absorption as much as an ideal Lambertian scatterer. Here, we present an experimental technique and theoretical model that accurately describes the absorption improvement that is achievable with coatings based on random ensembles of dielectric scatterers. We find that the ideal Lambertian model substantially overestimates the experimental scattering results, but significant path length enhancements are still achievable. The experimental techniques presented here should enable the testing of various optical models that attempt to surpass the ray optics light trapping limit, which have in many cases been hindered by the experimental difficulty of coupling the incident light into the optical modes of the absorber.
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Focusing Waves at Arbitrary Locations in a Ray-Chaotic Enclosure Using Time-Reversed Synthetic Sonas
Publication: Phys. Rev. E 93, 052205 (2016)Doi: 10.1103/PhysRevE.93.052205Time-reversal methods are widely used to achieve wave focusing in acoustics and electromagnetics. Past time-reversal experiments typically require that a transmitter be initially present at the target focusing point, which limits the application of this technique. In this paper, we propose a method to focus waves at an arbitrary location inside a complex enclosure using a numerically calculated wave excitation signal. We use a semiclassical ray algorithm to calculate the signal that would be received at a transceiver port resulting from the injection of a short pulse at the desired target location. The time-reversed version of this signal is then injected into the transceiver port, and an approximate reconstruction of the short pulse is created at the target. The quality of the pulse reconstruction is quantified in three different ways, and the values of these metrics are shown to be predicted by the statistics of the scattering parameter ∣∣S21∣∣2 between the transceiver and target points in the enclosure over the bandwidth of the pulse. We demonstrate the method experimentally using a flat microwave billiard, and we quantify the reconstruction quality as a function of enclosure loss, port coupling, and other considerations.
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Publication: Rev. Sci. Instrum. 87, 025106 (2016)Doi: 10.1063/1.4941337
The dynamics of quantized vortices in superfluids has received increased attention recently because of novel techniques developed to visualize them directly. One of these techniques [G. P. Bewley et al., Nature 441, 588 (2006)] visualized quantized vortices and their reconnections in superfluid flows of 4He by using solid hydrogen tracers of micron-size or larger. The present work improves upon the previous technique by using substantially smaller particles created by injecting atmospheric air diluted in helium gas. These smaller particles are detectable thanks to the higher index of refraction of nitrogen compared to hydrogen and thanks to an improved visualization setup. The optical counting estimate, which agrees with terminal velocity estimates, suggests that the tracer diameter is typically 400 ± 200 nm and could be as small as 200 nm; being smaller, but not so small as to be influenced by thermal motion, the particles get trapped on the vortices faster, perturb the vortices less, possess smaller Stokes drag, and stay trapped on fast-moving vortices, as also on vortices generated closer to the superfluid transition temperature. Unlike the past, the ability to create particles in the superfluid state directly (instead of creating them above the λ-point and cooling the fluid subsequently), ensures greater temperature stability for longer periods, and enables the tracking of long and isolated vortices. These advantages have also led to the direct visualization of Kelvin waves. The use of other seed gases could lead to the visualization of even smaller tracers for quantized vortices. We discuss the visualization setup and provide suggestions for further improvement.
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Publication: Phys. Chem. Chem. Phys. 18 [28], (2016)Doi: 10.1039/C6CP03271K
orphologically complex electrochemical systems such as composite or nanostructured lithium ion battery electrodes exhibit spatially inhomogeneous internal current distributions, particularly when driven at high total currents, due to resistances in the electrodes and electrolyte, distributions of diffusion path lengths, and nonlinear current–voltage characteristics. Measuring and controlling these distributions is interesting from both an engineering standpoint, as nonhomogenous currents lead to lower utilization of electrode material, as well as from a fundamental standpoint, as comparisons between theory and experiment are relatively scarce. Here we describe a new approach using a deliberately simple model battery electrode to examine the current distribution in a electrode material limited by poor electronic conductivity. We utilize quantitative spatially resolved X-ray photoelectron spectroscopy to measure the spatial distribution of the state-of-charge of a V2O5 model electrode as a proxy measure for the current distribution on electrodes discharged at varying current densities. We show that the current at the electrode–electrolyte interface falls off with distance from the current collector, and that the current distribution is a strong function of total current. We compare the observed distributions with a simple analytical model which reproduces the dependence of the distribution on total current, but fails to predict the correct length scale. A more complete numerical simulation suggests that dynamic changes in the electronic conductivity of the V2O5 concurrent with lithium insertion may contribute to the differences between theory and experiment. Our observations should help inform design criteria for future electrode architectures.
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Publication: Phys. Plasmas 23, 033507 (2016)Doi: 10.1063/1.4943404
his paper analyzes and evaluates a concept for remotely detecting the presence of radioactivity using electromagnetic signatures. The detection concept is based on the use of laser beams and the resulting electromagnetic signatures near the radioactive material. Free electrons, generated from ionizing radiation associated with the radioactive material, cascade down to low energies and attach to molecular oxygen. The resulting ion density depends on the level of radioactivity and can be readily photo-ionized by a low-intensity laser beam. This process provides a controllable source of seed electrons for the further collisional ionization (breakdown) of the air using a high-power, focused, CO2CO2 laser pulse. When the air breakdown process saturates, the ionizing CO2CO2 radiation reflects off the plasma region and can be detected. The time required for this to occur is a function of the level of radioactivity. This monostatic detection arrangement has the advantage that both the photo-ionizing and avalanche laser beams and the detector can be co-located.
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Publication: 2D Mater. 3, 041006 (2016)Doi: 10.1088/2053-1583/3/4/041006
Black phosphorus has attracted interest as a material for use in optoelectronic devices due to many favorable properties such as a high carrier mobility, field-effect, and a direct bandgap that can range from 0.3 eV in its bulk crystalline form to 1.7–2.0 eV for a single atomic layer. The low bandgap energy for multilayer black phosphorus consisting of more than approximately five atomic layers allows for direct transition photoabsorption that enables detection of light out to mid-infrared frequencies. In this work, we characterize the room temperature optical response of a black phosphorus photoconductive detector at wavelengths ranging from 1.56 to 3.75 μm. Pulsed autocorrelation measurements in the near-infrared regime reveal a strong, sub-linear photocurrent nonlinearity with a response time of 1 ns, indicating that gigahertz electrical bandwidth is feasible. Time resolved photoconduction measurements covering near- and mid-infrared frequencies show a fast 65 ps rise time, followed by a carrier relaxation with a time scale that matches the intrinsic limit determined by autocorrelation. The sublinear photoresponse is shown to be caused by a reduction in the carrier relaxation time as more energy is absorbed in the black phosphorus flake and is well described by a carrier recombination model that is nonlinear with excess carrier density. The device exhibits a measured noise-equivalent power of 530 pW Hz−1/2, which is the value expected for Johnson noise limited performance. The fast and sensitive room temperature photoresponse demonstrates that black phosphorus is a promising new material for mid-infrared optoelectronics.
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Publication: Science 352, 629 (2016)Doi: 10.1126/science.aaf2939
Magnetic reconnection occurs when the magnetic field permeating a conductive plasma rapidly rearranges itself, releasing energy and accelerating particles. Reconnection is important in a wide variety of physical systems, but the details of how it occurs are poorly understood. Burch et al. used NASA's Magnetospheric Multiscale mission to probe the plasma properties within a reconnection event in Earth's magnetosphere (see the Perspective by Coates). They find that the process is driven by the electron-scale dynamics. The results will aid our understanding of magnetized plasmas, including those in fusion reactors, the solar atmosphere, solar wind, and the magnetospheres of Earth and other planets.
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Publication: ACS Nano 10, 2693 (2016)Doi: 10.1021/acsnano.5b07757
Materials that undergo conversion reactions to form different materials upon lithiation typically offer high specific capacity for energy storage applications such as Li ion batteries. However, since the reaction products often involve complex mixtures of electrically insulating and conducting particles and significant changes in volume and phase, the reversibility of conversion reactions is poor, preventing their use in rechargeable (secondary) batteries. In this paper, we fabricate and protect 3D conversion electrodes by first coating multiwalled carbon nanotubes (MWCNT) with a model conversion material, RuO2, and subsequently protecting them with conformal thin-film lithium phosphous oxynitride (LiPON), a well-known solid-state electrolyte. Atomic layer deposition is used to deposit the RuO2 and the LiPON, thus forming core double-shell MWCNT@RuO2@LiPON electrodes as a model system. We find that the LiPON protection layer enhances cyclability of the conversion electrode, which we attribute to two factors. (1) The LiPON layer provides high Li ion conductivity at the interface between the electrolyte and the electrode. (2) By constraining the electrode materials mechanically, the LiPON protection layer ensures electronic connectivity and thus conductivity during lithiation/delithiation cycles. These two mechanisms are striking in their ability to preserve capacity despite the profound changes in structure and composition intrinsic to conversion electrode materials. This LiPON-protected structure exhibits superior cycling stability and reversibility as well as decreased overpotentials compared to the unprotected core–shell structure. Furthermore, even at very low lithiation potential (0.05 V), the LiPON-protected electrode largely reduces the formation of a solid electrolyte interphase.
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Publication: Appl. Opt. 55, 1757 (2016)Doi: 10.1364/AO.55.001757
In this paper, we analyze beam combining and atmospheric propagation of high-power lasers for directed-energy (DE) applications. The large linewidths inherent in high-power fiber and slab lasers cause random phase and intensity fluctuations that occur on subnanosecond time scales. Coherently combining these high-power lasers would involve instruments capable of precise phase control and operation at rates greater than ∼10 GHz∼10 GHz. To the best of our knowledge, this technology does not currently exist. This presents a challenging problem when attempting to phase lock high-power lasers that is not encountered when phase locking low-power lasers, for example, at milliwatt power levels. Regardless, we demonstrate that even if instruments are developed that can precisely control the phase of high-power lasers, coherent combining is problematic for DE applications. The dephasing effects of atmospheric turbulence typically encountered in DE applications will degrade the coherent properties of the beam before it reaches the target. Through simulations, we find that coherent beam combining in moderate turbulence and over multikilometer propagation distances has little advantage over incoherent combining. Additionally, in cases of strong turbulence and multikilometer propagation ranges, we find nearly indistinguishable intensity profiles and virtually no difference in the energy on the target between coherently and incoherently combined laser beams. Consequently, we find that coherent beam combining at the transmitter plane is ineffective under typical atmospheric conditions.
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Publication: Thin Solid Films 620, 65 (2016)Doi: 10.1016/j.tsf.2016.07.075
Thin film oxides have attracted attention in implementations of gas sensors, notably NO2, owing to their unique physical, optical, and chemical properties. While the properties are presumed to be strongly dependent on the surface conditions of the thin films, it is not yet clear how surface properties of the thin film gas sensor affect its analyte sensing response. Here, we report the influence of surface carbon contamination and roughness on the NO2 sensing properties of TiO2 thin film sensors. The TiO2 thin films were prepared by rf-sputtering. The surface of the films were intentionally contaminated and damaged with organic polymers (photolithography resist) and microwave plasma, respectively. The surface chemistry of the films was assessed by high resolution X-ray photoelectron spectroscopy, and atomic force microscopy was exploited to obtain the morphology of the fabricated sensors. The work aims to improve the long-term efficacy of gas sensors by studying the reasons for degradation in performance. Our results indicate that the carbon residue and surface roughness of the TiO2 based sensor prolong the NO2 response time by roughly threefold and fivefold, respectively. The recovery rate of the sensor is deteriorated by the poor surface conditions as well.
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Publication: Phys. Rev. B 94, 045426 (2016)Doi: 10.1103/PhysRevB.94.045426
We study the dark exciton's behavior as a coherent physical two-level spin system (qubit) using an external magnetic field in the Faraday configuration. Our studies are based on polarization-sensitive intensity autocorrelation measurements of the optical transition resulting from the recombination of a spin-blockaded biexciton state, which heralds the dark exciton and its spin state. We demonstrate control over the dark exciton eigenstates without degrading its decoherence time. Our observations agree well with computational predictions based on a master equation model.
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Publication: Geoophys. Res. Lett. 43, 6060 (2016)Doi: 10.1002/2016GL069212
We report Magnetospheric Multiscale observations of macroscopic and electron-scale current layers in asymmetric reconnection. By intercomparing plasma, magnetic, and electric field data at multiple crossings of a reconnecting magnetopause on 22 October 2015, when the average interspacecraft separation was ~10 km, we demonstrate that the ion and electron moments are sufficiently accurate to provide reliable current density measurements at 30 ms cadence. These measurements, which resolve current layers narrower than the interspacecraft separation, reveal electron-scale filamentary Hall currents and electron vorticity within the reconnection exhaust far downstream of the X line and even in the magnetosheath. Slightly downstream of the X line, intense (up to 3 μA/m2) electron currents, a super-Alfvénic outflowing electron jet, and nongyrotropic crescent shape electron distributions were observed deep inside the ion-scale magnetopause current sheet and embedded in the ion diffusion region. These characteristics are similar to those attributed to the electron dissipation/diffusion region around the X line.
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Publication: Nano Lett. 16, 2734 (2016)Doi: 10.1021/acs.nanolett.6b00405
Subwavelength graphene structures support localized plasmonic resonances in the terahertz and mid-infrared spectral regimes. The strong field confinement at the resonant frequency is predicted to significantly enhance the light-graphene interaction, which could enable nonlinear optics at low intensity in atomically thin, subwavelength devices. To date, the nonlinear response of graphene plasmons and their energy loss dynamics have not been experimentally studied. We measure and theoretically model the terahertz nonlinear response and energy relaxation dynamics of plasmons in graphene nanoribbons. We employ a terahertz pump–terahertz probe technique at the plasmon frequency and observe a strong saturation of plasmon absorption followed by a 10 ps relaxation time. The observed nonlinearity is enhanced by 2 orders of magnitude compared to unpatterned graphene with no plasmon resonance. We further present a thermal model for the nonlinear plasmonic absorption that supports the experimental results. The model shows that the observed strong linearity is caused by an unexpected red shift of plasmon resonance together with a broadening and weakening of the resonance caused by the transient increase in electron temperature. The model further predicts that even greater resonant enhancement of the nonlinear response can be expected in high-mobility graphene, suggesting that nonlinear graphene plasmonic devices could be promising candidates for nonlinear optical processing.
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Publication: Phys. Rev. A 94, 012307 (2016)Doi: 10.1103/PhysRevA.94.012307
We propose a method to perform single-shot optical readout of a quantum bit (qubit) using cavity quantum electrodynamics. We selectively couple the optical transitions associated with different qubit basis states to the cavity and utilize the change in cavity transmissivity to generate a qubit readout signal composed of many photons. We show that this approach enables single-shot optical readout even when the qubit does not have a good cycling transition, which is required for standard resonance fluorescence measurements. We calculate the probability that the measurement detects the correct qubit state using the example of a quantum-dot spin under various experimental conditions and demonstrate that it can exceed 0.99.
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Publication: Biomater. 104, 104 (2016)Doi: 10.1016/j.biomaterials.2016.06.040
Cell morphology has been identified as a potential indicator of stem cell response to biomaterials. However, determination of cell shape phenotype in biomaterials is complicated by heterogeneous cell populations, microenvironment heterogeneity, and multi-parametric definitions of cell morphology. To associate cell morphology with cell-material interactions, we developed a shape phenotyping framework based on support vector machines. A feature selection procedure was implemented to select the most significant combination of cell shape metrics to build classifiers with both accuracy and stability to identify and predict microenvironment-driven morphological differences in heterogeneous cell populations. The analysis was conducted at a multi-cell level, where a “supercell” method used average shape measurements of small groups of single cells to account for heterogeneous populations and microenvironment. A subsampling validation algorithm revealed the range of supercell sizes and sample sizes needed for classifier stability and generalization capability. As an example, the responses of human bone marrow stromal cells (hBMSCs) to fibrous vs flat microenvironments were compared on day 1. Our analysis showed that 57 cells (grouped into supercells of size 4) are the minimum needed for phenotyping. The analysis identified that a combination of minor axis length, solidity, and mean negative curvature were the strongest early shape-based indicator of hBMSCs response to fibrous microenvironment.
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Publication: Adv. Mater. Interfaces 3, 1600426 (2016)Doi: 10.1002/admi.201600426
Chemical and electrochemical instability of the Li metal interface with organic solvent has been a major impediment to use of Li-metal anodes for next-generation batteries. Here the character of Li surface degradation and the application of atomic layer deposition (ALD) as a protection layer to suppress the degradation are addressed. Using standard Li foil samples in organic solvent with and without in situ deposited ALD Al2O3 protective layers, results from in situ atomic force microscopy, mass spectrometry (including differential electrochemical mass spectrometry), X-ray Photoelectron Spectroscopy (XPS), and ex situ scanning electron microscopy/energy dispersive X-ray spectroscopy are reported. Despite the presence of a thin oxide/hydroxide/carbonate layer on the Li foil surface, degradation readily occurs in organic solvent, particularly at surface features such as ridges. Introduction of the ALD protective layer – deposited directly on this Li foil surface – dramatically suppresses the degradation.
2015
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Publication: Microelectron. J. 46, 1364 (2015)Doi: 10.1016/j.mejo.2015.09.015
We report on a random number generator whose randomness derives from a Boolean chaotic oscillator, designed and fabricated as an integrated circuit. The underlying physics of chaotic dynamics in the Boolean chaotic oscillator is verified by time domain analysis, frequency domain analysis, and computation of Lyapunov exponents. To test the hypothesis physically, a discrete Boolean chaotic oscillator is implemented using commercial chips on printed circuit boards. Using a CMOS 0.35 μm process, we build a CMOS Boolean chaotic oscillator, which consists of a core chaotic oscillator and a source follower buffer. The quality of the measured bit sequences makes a suitable randomness source, and its property is analyzed using NIST standard statistical tests after subsequent post-processings. This circuit requires 0.087 nJ for a generation of single random bit with the chip area of 0.057 mm2.
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Publication: Phys. Rev. Lett. 115, 124801 (2015)Doi: 10.1103/PhysRevLett.115.124801
Applying the Briggs-Bers “pole-pinch” criterion to the exact transcendental dispersion relation of a dielectric traveling wave tube (TWT), we find that there is no absolute instability regardless of the beam current. We extend this analysis to the circuit band edges of a linear beam TWT by approximating the circuit mode as a hyperbola in the frequency-wave-number (ω−k) plane and consider the weak coupling limit. For an operating mode whose group velocity is in the same direction as the beam mode, we find that the lower band edge is not subjected to absolute instability. At the upper band edge, we find a threshold beam current beyond which absolute instability is excited. The nonexistence of absolute instability in a linear beam TWT and the existence in a gyrotron TWT, both at the lower band edge, is contrasted. The general study given here is applicable to some contemporary TWTs such as metamaterial-based and advanced Smith-Purcell TWTs.
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Publication: Tellus Ser. A - Dynamic Meteorology Oceanography 67, 26617 (2015)Doi: 10.3402/tellusa.v67.26617
Ensemble data assimilation methods are potentially attractive because they provide a computationally affordable (and computationally parallel) means of obtaining flow-dependent background-error statistics. However, a limitation of these methods is that the rank of their flow-dependent background-error covariance estimate, and hence the space of possible analysis increments, is limited by the number of forecast ensemble members. To overcome this deficiency ensemble methods typically use empirical localisation, which allows more degrees of freedom for the analysis increment by suppressing spatially distant background correlations. The method presented here improves the performance of an Ensemble Kalman filter by increasing the size of the ensemble at analysis time in order to boost the rank of its background-error covariance estimate. The additional ensemble members added to the forecast ensemble at analysis time are created by adding a collection of ‘climatological’ perturbations to the forecast ensemble mean. These perturbations are constant in time and provide state space directions, possibly missed by the dynamically forecasted background ensemble, in which the analysis increment can correct the forecast mean based on observations. As the climatological perturbations are calculated once, there is negligible computational cost in obtaining the additional ensemble members at each analysis cycle. Included here are a formulation of the method, results of numerical experiments conducted with a spatiotemporally chaotic model in one spatial dimension and discussion of possible future extensions and applications. The numerical tests indicate that the method presented here has significant potential for improving analyses and forecasts.
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Publication: Phys. Rev. A 91, 032520 (2015)Doi: 10.1103/PhysRevA.91.032520
We present a calculation of the Casimir torque acting on a liquid crystal near a birefringent crystal. In this system, a liquid crystal bulk is uniformly aligned at one surface and is twisted at the other surface by a birefringent crystal, e.g., barium titanate. The liquid crystal is separated from the solid crystal by an isotropic, transparent material such as SiO2. By varying the thickness of the deposited layer, we can observe the effect of retardation on the torque (which differentiates it from the close-range van der Waals torque). We find that a barium titanate slab would cause 5CB (4−cyano−4'−pentylbiphenyl) liquid crystal to rotate by 10∘ through its bulk when separated by 35 nm of SiO2. The optical technique for measuring this twist is also outlined.
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Publication: Phys. Plasmas 22, 045104 (2015)Doi: 10.1063/1.4918353
Measurements of impurities in Alcator C-Mod indicate that in the pedestal region, significant poloidal asymmetries can exist in the impurity density, ion temperature, and main ion density. In light of the observation that ion temperature and electrostatic potential are not constant on a flux surface [Theiler et al., Nucl. Fusion 54, 083017 (2014)], a technique based on total pressure conservation to align profiles measured at separate poloidal locations is presented and applied. Gyrokinetic neoclassical simulations with XGCa support the observed large poloidal variations in ion temperature and density, and that the total pressure is approximately constant on a flux surface. With the updated alignment technique, the observed in-out asymmetry in impurity density is reduced from previous publishing [Churchill et al., Nucl. Fusion 53, 122002 (2013)], but remains substantial ( nz,H/nz,L ∼ 6). Candidate asymmetry drivers are explored, showing that neither non-uniform impurity sources nor localized fluctuation-driven transport are able to explain satisfactorily the impurity density asymmetry. Since impurity density asymmetries are only present in plasmas with strong electron density gradients, and radial transport timescales become comparable to parallel transport timescales in the pedestal region, it is suggested that global transport effects relating to the strong electron density gradients in the pedestal are the main driver for the pedestal in-out impurity density asymmetry.
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Publication: J. Vac. Sci. Technol. B 33, 051601 (2015)Doi: 10.1116/1.4928493
The authors show that extended He plasma pretreatment (PPT) of methacrylate-based 193 nm photoresist (PR) material in conjunction with a subsequent biased Ar plasma treatment can lead to blister formation at the polymer surface due to delamination of an ultrathin, ion-induced, dense, amorphous carbon (DAC) layer formed by low energy ion bombardment. For our experimental conditions, the delaminated layer is 1–2 nm thick and primarily composed of sp2-hybrized amorphous carbon. A He or Ar plasma process alone will not lead to this phenomenon, and so far the authors have only observed it for a methacrylate polymer. A possible mechanism of the formation of the ultrathin layer that is consistent with all observations is as follows: During He plasma pretreatment, volatile species are produced by ultraviolet/vacuum ultraviolet radiation-induced photolysis of the polymer pendant groups, e.g., adamantyl and chain-scissioning of the polymer backbone to a depth of greater than 100 nm. While volatile products formed close to the polymer surface can diffuse out during He PPT, those formed deep within the polymer bulk cannot and their concentration will become significant for extended He PPT. During the biased Ar plasma treatment step, a DAC surface layer is generated by Ar+ ion bombardment within the first seconds of plasma exposure. The thickness is dependent on ion energy and in the range of one to several nanometers. This layer appears to be impermeable to gaseous products formed in the PR material. Thus, volatile species diffusing to the surface can accumulate underneath the DAC layer, causing a loss of adhesion and subsequent delamination of this layer from the PR bulk film. The authors also report surface and electrical characterizations of the ultrathin DAC layer using optical microscopy, transmission electron microscopy, Raman and x-ray photoemission spectroscopy, and two-point probe techniques.
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Publication: ACS Appl. Mater. Interfaces 7, 26007 (2015)Doi: 10.1021/acsami.5b07058
The further development of all-solid-state batteries is still limited by the understanding/engineering of the interfaces formed upon cycling. Here, we correlate the morphological, chemical, and electrical changes of the surface of thin-film devices with Al negative electrodes. The stable Al–Li–O alloy formed at the stress-free surface of the electrode causes rapid capacity fade, from 48.0 to 41.5 μAh/cm2 in two cycles. Surprisingly, the addition of a Cu capping layer is insufficient to prevent the device degradation. Nevertheless, Si electrodes present extremely stable cycling, maintaining >92% of its capacity after 100 cycles, with average Coulombic efficiency of 98%.
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Publication: Nucl. Fusion 55, 033005 (2015)Doi: 10.1088/0029-5515/55/3/033005
A stellarator is said to be omnigeneous if all particles have vanishing average radial drifts. In omnigeneous stellarators, particles are perfectly confined in the absence of turbulence and collisions, whereas in non-omnigeneous configurations, particle can drift large radial distances. One of the consequences of omnigeneity is that the unfavourable inverse scaling with collisionality of the stellarator neoclassical fluxes disappears. In the pioneering and influential article by Cary and Shasharina (1997 Phys. Plasmas 4 3323), the conditions that the magnetic field of a stellarator must satisfy to be omnigeneous are derived. However, Cary and Shasharina (1997 Phys. Plasmas 4 3323) only considered omnigeneous stellarators in which all the minima of the magnetic field strength on a flux surface must have the same value. The same is assumed for the maxima. We show that omnigeneous magnetic fields can have local minima and maxima with different values. Thus, the parameter space in which omnigeneous stellarators are possible is larger than previously expected. The analysis presented in this article is only valid for orbits with vanishing radial width, and in principle it is not applicable to energetic particles. However, one would expect that improving neoclassical confinement would improve energetic particle confinement.
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Publication: Chaos 25, 097618 (2015)Doi: 10.1063/1.4922973
In this paper, we propose, discuss, and illustrate a computationally feasible definition of chaos which can be applied very generally to situations that are commonly encountered, including attractors, repellers, and non-periodically forced systems. This definition is based on an entropy-like quantity, which we call “expansion entropy,” and we define chaos as occurring when this quantity is positive. We relate and compare expansion entropy to the well-known concept of topological entropy to which it is equivalent under appropriate conditions. We also present example illustrations, discuss computational implementations, and point out issues arising from attempts at giving definitions of chaos that are not entropy-based.
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Publication: Tellus Ser. A - Dynamic Meteorology Oceanography 67, 26495 (2015)Doi: 10.3402/tellusa.v67.26495
Limited-area models (LAMs) allow high-resolution forecasts to be made for geographic regions of interest when resources are limited. Typically, boundary conditions for these models are provided through one-way boundary coupling from a coarser resolution global model. Here, data assimilation is considered in a situation in which a global model supplies boundary conditions to multiple LAMs. The data assimilation method presented combines information from all of the models to construct a single ‘composite state’, on which data assimilation is subsequently performed. The analysis composite state is then used to form the initial conditions of the global model and all of the LAMs for the next forecast cycle. The method is tested by using numerical experiments with simple, chaotic models. The results of the experiments show that there is a clear forecast benefit to allowing LAM states to influence one another during the analysis. In addition, adding LAM information at analysis time has a strong positive impact on global model forecast performance, even at points not covered by the LAMs.
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Publication: Appl. Phys. Lett. 107, 081103 (2015)Doi: 10.1063/1.4929403
We present polarization-resolved transient transmission measurements on multi-layer black phosphorus. Background free two-color pump-probe spectroscopy measurements are carried out on mechanically exfoliated black phosphorus flakes that have been transferred to a large-bandgap, silicon carbide substrate. The blue-shifted pump pulse (780 nm) induces an increased transmission of the probe pulse (1560 nm) over a time scale commensurate with the measurement resolution (hundreds of fs). After the initial pump-induced transparency, the sign of the transient flips and a slower enhanced absorption is observed. This extended absorption is characterized by two relaxation time scales of 180 ps and 1.3 ns. The saturation peak is attributed to Pauli blocking while the extended absorption is ascribed to a Drude response of the pump-induced carriers. The anisotropic carrier mobility in the black phosphorus leads to different weights of the Drude absorption, depending on the probe polarization, which is readily observed in the amplitude of the pump-probe signals.
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Publication: Phys. Plasmas 22, 100704 (2015)Doi: 10.1063/1.4933212
Kinetic simulations of 3D collisionless magnetic reconnection with a guide field show a dramatic enhancement of energetic electron production when compared with 2D systems. In the 2D systems, electrons are trapped in magnetic islands that limit their energy gain, whereas in the 3D systems the filamentation of the current layer leads to a stochastic magnetic field that enables the electrons to access volume-filling acceleration regions. The dominant accelerator of the most energetic electrons is a Fermi-like mechanism associated with reflection of charged particles from contracting field lines.
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Publication: Opt. Exp. 23, 28728 (2015)Doi: 10.1364/OE.23.028728
Graphene has unique optical and electronic properties that make it attractive as an active material for broadband ultrafast detection. We present here a graphene-based detector that shows 40-picosecond electrical rise time over a spectral range that spans nearly three orders of magnitude, from the visible to the far-infrared. The detector employs a large area graphene active region with interdigitated electrodes that are connected to a log-periodic antenna to improve the long-wavelength collection efficiency, and a silicon carbide substrate that is transparent throughout the visible regime. The detector exhibits a noise-equivalent power of approximately 100 µW·Hz–½ and is characterized at wavelengths from 780 nm to 500 µm.
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Publication: Opt. Mater. Exp. 5, 2501 (2015)Doi: 10.1364/OME.5.002501
While the field of plasmonics has grown significantly in recent years, the relatively high losses and limited material choices have remained a challenge for the development of many device concepts. The decay of plasmons into hot carrier excitations is one of the main loss mechanisms; however, this process offers an opportunity for the direct utilization of loss if excited carriers can be collected prior to thermalization. From a materials point-of-view, noble metals (especially gold and silver) are almost exclusively employed in these hot carrier plasmonic devices; nevertheless, many other materials may offer advantages for collecting these hot carriers. In this manuscript, we present results for 16 materials ranging from pure metals and alloys to nanowires and graphene and show their potential applicability for hot carrier excitation and extraction. By considering the expected hot carrier distributions based on the electron density of states for the materials, we predict the preferred hot carrier type for collection and their expected performance under different illumination conditions. By considering materials not traditionally used in plasmonics, we find many promising alternative materials for the emerging field of hot carrier plasmonics.
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Publication: Nature Commun. 6, 6558 (2015)Doi: 10.1038/ncomms7558
An emitter near a surface induces an image dipole that can modify the observed emission intensity and radiation pattern. These image-dipole effects are generally not taken into account in single-emitter tracking and super-resolved imaging applications. Here we show that the interference between an emitter and its image dipole induces a strong polarization anisotropy and a large spatial displacement of the observed emission pattern. We demonstrate these effects by tracking the emission of a single quantum dot along two orthogonal polarizations as it is deterministically positioned near a silver nanowire. The two orthogonally polarized diffraction spots can be displaced by up to 50 nm, which arises from a Young’s interference effect between the quantum dot and its induced image dipole. We show that the observed spatially varying interference fringe provides a useful measure for correcting image-dipole-induced distortions. These results provide a pathway towards probing and correcting image-dipole effects in near-field imaging applications.
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Publication: Nano Lett. 15, 7099 (2015)Doi: 10.1021/acs.nanolett.5b03191
We report here a new type of plasmon resonance that occurs when graphene is connected to a metal. These new plasmon modes offer the potential to incorporate a tunable plasmonic channel into a device with electrical contacts, a critical step toward practical graphene terahertz optoelectronics. Through theory and experiments, we demonstrate, for example, anomalously high resonant absorption or transmission when subwavelength graphene-filled apertures are introduced into an otherwise conductive layer. These tunable plasmon resonances are essential yet missing ingredients needed for terahertz filters, oscillators, detectors, and modulators.
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Publication: Tellus Ser. A - Dynamic Meteorology Oceanography 67, 28040 (2015)Doi: 10.3402/tellusa.v67.28040
Idealised perfect model experiments suggest that performing data assimilation on a ‘composite’ state vector, constructed from global and limited-area model states, can be beneficial to both model states. Here, an illustrative scheme is implemented to account for systematic forecast errors attributed to the imperfect model dynamics. Results from numerical experiments suggest that even simple bias correction schemes can correct forecast errors in composite states.
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Publication: Adv. Energy Mater. 5, 1501142 (201)Doi: 10.1002/aenm.201501142
A novel imaging platform to determine the open-circuit voltage of solar cells with nanoscale spatial resolution is presented. Here, a variant of illuminated Kelvin probe force microscopy can be implemented to quantify local variations in the voltage of different solar cells. The new metrology can be applied to any optoelectronic device, and works in ambient environments.
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Publication: Entropy 17, 1 (2015)Doi: 10.3390/e17010001
A popular approach in the investigation of the short-term behavior of a non-stationary time series is to assume that the time series decomposes additively into a long-term trend and short-term fluctuations. A first step towards investigating the short-term behavior requires estimation of the trend, typically via smoothing in the time domain. We propose a method for time-domain smoothing, called complexity-regularized regression (CRR). This method extends recent work, which infers a regression function that makes residuals from a model “look random”. Our approach operationalizes non-randomness in the residuals by applying ideas from computational mechanics, in particular the statistical complexity of the residual process. The method is compared to generalized cross-validation (GCV), a standard approach for inferring regression functions, and shown to outperform GCV when the error terms are serially correlated. Regression under serially-correlated residuals has applications to time series analysis, where the residuals may represent short timescale activity. We apply CRR to a time series drawn from the Dow Jones Industrial Average and examine how both the long-term and short-term behavior of the market have changed over time.
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Publication: Phys. Plasmas 22, 112508 (2015)Doi: 10.1063/1.4935901
Impurities cause radiation losses and plasma dilution, and in stellarator plasmas the neoclassical ambipolar radial electric field is often unfavorable for avoiding strong impurity peaking. In this work we use a new continuum drift-kinetic solver, the SFINCS code (the Stellarator Fokker-Planck Iterative Neoclassical Conservative Solver) [M. Landreman et al., Phys. Plasmas 21, 042503 (2014)] which employs the full linearized Fokker-Planck-Landau operator, to calculate neoclassical impurity transport coefficients for a Wendelstein 7-X (W7-X) magnetic configuration. We compare SFINCS calculations with theoretical asymptotes in the high collisionality limit. We observe and explain a 1/ν-scaling of the inter-species radial transport coefficient at low collisionality, arising due to the field term in the inter-species collision operator, and which is not found with simplified collision models even when momentum correction is applied. However, this type of scaling disappears if a radial electric field is present. We also use SFINCS to analyze how the impurity content affects the neoclassical impurity dynamics and the bootstrap current. We show that a change in plasma effective charge Zeff of order unity can affect the bootstrap current enough to cause a deviation in the divertor strike point locations.
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Publication: Nano Lett. 15, 147 (2015)Doi: 10.1021/nl503246h
When high-energy photons are absorbed in a semiconductor or metal, electrons and holes are generated with excess kinetic energy, so-called hot carriers. This extra energy is dissipated, for example, by phonon emission, which results in sample heating. Recovery of hot carriers is important for detectors, sensors, and power convertors; however, the design and implementation of these devices is difficult due to strict requirements on the device geometry, angle of illumination, and incident photon wavelength. Here, we present for the first time a simple, angle-independent device based on transparent conducting electrodes that allows for the generation and collection of hot carriers. We show experimental photocurrent generation from both monochromatic and broadband light sources, show uniform absorption for incident illumination at up to 60° from the surface normal, and find an expected open-circuit voltage in the range 1.5–3.0 V. Under solar illumination, the device is 1 order of magnitude more efficient than previous metal–insulator–metal designs, and power conversion efficiencies >10% are predicted with optimized structures. This approach opens the door to new hot carrier collection devices and detectors based on transparent conducting electrodes.
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Publication: J. Phys. B - Atomic Molec. Opt. Phys. 48, 094011 (2015)Doi: 10.1088/0953-4075/48/9/094011
The axial dependence of femtosecond filamentation in air is measured under conditions of varying laser pulsewidth, energy, and focusing f-number. Filaments are characterized by the ultrafast z-dependent absorption of energy from the laser pulse and diagnosed by measuring the local single cycle acoustic wave generated. Results are compared to 2D + 1 simulations of pulse propagation, whose results are highly sensitive to the instantaneous (electronic) part of the nonlinear response of N2 and O2. We find that recent measurements of the nonlinear refractive index (n2) in Wahlstrand et al (2012 Phys. Rev. A 85 043820) provide the best match and an excellent fit between experiments and simulations.
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Publication: Prog. Earth Planetary Sci. 2, 29 (2015)Doi: 10.1186/s40645-015-0058-1
Our understanding of the dynamics of the Earth’s core can be advanced by a combination of observation, experiments, and simulations. A crucial aspect of the core is the interplay between the flow of the conducting liquid and the magnetic field this flow sustains via dynamo action. This non-linear interaction, and the presence of turbulence in the flow, precludes direct numerical simulation of the system with realistic turbulence. Thus, in addition to simulations and observations (both seismological and geomagnetic), experiments can contribute insight into the core dynamics. Liquid sodium laboratory experiments can serve as models of the Earth’s core with the key ingredients of conducting fluid, turbulent flow, and overall rotation, and can also approximate the geometry of the core. By accessing regions of parameter space inaccessible to numerical studies, experiments can benchmark simulations and reveal phenomena relevant to the Earth’s core and other planetary cores. This review focuses on the particular contribution of liquid sodium spherical Couette devices to this subject matter.
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Publication: Phys. Rev. Lett. 115, 195001 (2015)Doi: 10.1103/PhysRevLett.115.195001
A scheme for positron plasma wakefield acceleration using hollow or donut-shaped electron driver beams is studied. An annular-shaped, electron-free region forms around the hollow driver beam, creating a favorable region (longitudinal field is accelerating and transverse field is focusing) for positron acceleration. For Facility for Advanced Accelerator Experimental Tests (FACET)-like parameters, the hollow beam driver produces accelerating gradients on the order of 10 GV/m. The accelerating gradient increases linearly with the total charge in the driver beam. Simulations show acceleration of a 23-GeV positron beam to 35.4 GeV with a maximum energy spread of 0.4% and very small emittance over a plasma length of 140 cm is possible.
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Publication: Chaos 26, 123122 (2015)Doi: 10.1063/1.4938534
In this paper, we study dynamical systems in which a large number N of identical Landau-Stuart oscillators are globally coupled via a mean-field. Previously, it has been observed that this type of system can exhibit a variety of different dynamical behaviors. These behaviors include time periodic cluster states in which each oscillator is in one of a small number of groups for which all oscillators in each group have the same state which is different from group to group, as well as a behavior in which all oscillators have different states and the macroscopic dynamics of the mean field is chaotic. We argue that this second type of behavior is “extensive” in the sense that the chaotic attractor in the full phase space of the system has a fractal dimension that scales linearly with N and that the number of positive Lyapunov exponents of the attractor also scales linearly with N. An important focus of this paper is the transition between cluster states and extensive chaos as the system is subjected to slow adiabatic parameter change. We observe discontinuous transitions between the cluster states (which correspond to low dimensional dynamics) and the extensively chaotic states. Furthermore, examining the cluster state, as the system approaches the discontinuous transition to extensive chaos, we find that the oscillator population distribution between the clusters continually evolves so that the cluster state is always marginally stable. This behavior is used to reveal the mechanism of the discontinuous transition. We also apply the Kaplan-Yorke formula to study the fractal structure of the extensively chaotic attractors.
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Publication: J. Geophys. Res. - Space Phys. 120, 7255 (2015)Doi: 10.1002/2015JA021464
Magnetic reconnection at the Earth's magnetopause plays an important role in magnetospheric dynamics. Understanding the dynamics requires theory and observations. Previous theoretical work suggests that for no guide field, ions in the exhaust region on the magnetosheath side of the boundary counterstream with a velocity separation that is twice the upstream Alfvén speed (vA) and that the counterstreaming velocity decreases with increasing guide field. These theoretical predictions are tested for reconnection at the Earth's magnetopause using observations from the Cluster spacecraft. The difference between the incident and reflected ion velocities (vsep) in the magnetosheath boundary layer ion populations is used to determine the exhaust velocity. The ratio of vsep over twice the Alfvén speed (RV = vsep/2vA,L) is predicted to approach 1 for reconnection with shear angles near 180° (no guide field) but is observed to reach a value of approximately 0.84 for the magnetopause crossings analyzed with shear angles near 180°. This value is consistent with previous observations of ion velocities from reconnection at the magnetopause investigated using the Walén relation. While magnetic shear angle can contribute to the disagreement between observations and the Walén relation, it does not play a large role, given the reduced ratio for the events near 180° in this study.
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Publication: J. Geophys. Res. - Space Phys. 120, 7748 (2015)Doi: 10.1002/2015JA021489
We perform a systematic theoretical and numerical study of antiparallel two-dimensional magnetic reconnection with asymmetries in the plasma density and reconnecting magnetic field strength in addition to a bulk flow shear across the reconnection site in the plane of the reconnecting fields, which commonly occurs at planetary magnetospheres. We analytically predict the speed at which an isolated X line is convected by the flow, the reconnection rate, and the critical flow speed at which reconnection no longer takes place for arbitrary reconnecting magnetic field strengths, densities, and upstream flow speeds, and we confirm the results with two-fluid numerical simulations. The predictions and simulation results counter the prevailing model of reconnection at Earth's dayside magnetopause which says reconnection occurs with a stationary X line for sub-Alfvénic magnetosheath flow, reconnection occurs but the X line convects for magnetosheath flows between the Alfvén speed and double the Alfvén speed, and reconnection does not occur for magnetosheath flows greater than double the Alfvén speed. In particular, we find that X line motion is governed by momentum conservation from the upstream flows, which are weighted differently in asymmetric systems, so the X line convects for generic conditions including sub-Alfvénic upstream speeds. For the reconnection rate, as with symmetric reconnection, it drops with increasing flow shear and there is a cutoff speed above which reconnection is not predominant. However, while the cutoff condition for symmetric reconnection is that the difference in flows on the two sides of the reconnection site is twice the Alfvén speed, we find asymmetries cause the cutoff speed for asymmetric reconnection to be higher than twice the asymmetric form of the Alfvén speed. The stronger the asymmetries, the more the cutoff exceeds double the asymmetric Alfvén speed. This is due to the fact that in asymmetric reconnection, the plasma with the smaller mass flux into the dissipation region contributes a smaller mass to the dissipation region, so the effect of its flow on opposing the release of energy by the reconnected magnetic fields is diminished and the reconnection is not suppressed to the extent previously thought. The results compare favorably with an observation of reconnection at Earth's polar cusps during a period of northward interplanetary magnetic field, where reconnection occurs despite the magnetosheath flow speed being more than twice the magnetosheath Alfvén speed, the previously proposed suppression condition. These results are expected to be of broad importance for magnetospheric physics of Earth and other planets; particular applications are discussed.
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Publication: Geophys. Res. Lett. 42, 3627 (2015)Doi: 10.1002/2015GL063946
Time domain structures (TDS) (electrostatic or electromagnetic electron holes, solitary waves, double layers, etc.) are ≥1 ms pulses having significant parallel (to the background magnetic field) electric fields. They are abundant through space and occur in packets of hundreds in the outer Van Allen radiation belts where they produce magnetic-field-aligned electron pitch angle distributions at energies up to a hundred keV. TDS can provide the seed electrons that are later accelerated to relativistic energies by whistlers and they also produce field-aligned electrons that may be responsible for some types of auroras. These field-aligned electron distributions result from at least three processes. The first process is parallel acceleration by Landau trapping in the TDS parallel electric field. The second process is Fermi acceleration due to reflection of electrons by the TDS. The third process is an effective and rapid pitch angle scattering resulting from electron interactions with the perpendicular and parallel electric and magnetic fields of many TDS. TDS are created by current-driven and beam-related instabilities and by whistler-related processes such as parametric decay of whistlers and nonlinear evolution from oblique whistlers. New results on the temporal relationship of TDS and particle injections, types of field-aligned electron pitch angle distributions produced by TDS, the mechanisms for generation of field-aligned distributions by TDS, the maximum energies of field-aligned electrons created by TDS in the absence of whistler mode waves, TDS generation by oblique whistlers and three-wave-parametric decay, and the correlation between TDS and auroral particle precipitation, are presented.
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Publication: Radio Sci. 50, 678 (2015)Doi: 10.1002/2014RS005577
In this work, we extend results on the coupling of radiation through apertures inside wave chaotic electromagnetic cavities to the short-wavelength limit. The aperture is assumed to have irregular and smooth boundary geometry. A statistical approach is then used to model both fields in the aperture and in the plane of the aperture. Particular emphasis is devoted to the calculation of the free-space or radiation aperture admittance matrix, whose ensemble averaged element takes a simple closed-form expression. The radiation admittance matrix is found to be purely diagonal at relatively short wavelength, and it exhibits unusual frequency behavior. The extreme scenario of an irregular aperture radiating inside an irregular cavity is represented through the random coupling model. This mathematical framework uses a limited number of details of both the aperture and the environment in those problems involving the coupling of an external radiation. The universal fluctuation of the effective area of an electrically large aperture is found for a wave chaotic cavity at variable losses. Results are expected to be useful for the physical understanding of scattering in extremely complicated environments, mode-stirred reverberation chambers, wireless channels, radar traces, and statistical optics.
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Publication: Current Opinion Solid State Mater. Sci. 19, 227 (2015)Doi: 10.1016/j.cossms.2014.12.004
We examine the scientific challenges and opportunities presented at the mesoscale in the context of employing nanostructures for electrical energy storage. In order to capitalize on the power–energy and charge/discharge cycling stability that nanostructures offer, massive assemblies of nanostructures in networks must be organized into dense mesoscale architectures. With a fairly wide variety of architectures already demonstrated and more expected, the essential questions are whether regular or random 3-D arrangements are favorable, which embodiments should show best performance, and at what dimensional scaling? Dense packing raises challenging new questions about ion available and transport in highly confined electrolyte nanoenvironments, as well as designs to balance ion transport in electrolyte and electron transport in electrodes over distances long compared to nanostructure characteristic dimensions. Architectures and dimensional scaling present important issues of defects, statistical outliers, and their dynamic evolution, which in turn control degradation and failure phenomena. These considerations promise a rich set of mesoscale scientific challenges crucial to exploiting storage nanostructures in mesoscale architectures for energy storage.
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Publication: Phys. Plasmas 22, 023103 (2015)Doi: 10.1063/1.4907159
The quasi-static code WAKE [P. Mora and T. Antonsen, Phys. Plasmas 4, 217 (1997)] is upgraded to model the propagation of an ultra-relativistic charged particle beam through a warm background plasma in plasma wakefield acceleration. The upgraded code is benchmarked against the full particle-in-cell code OSIRIS [Hemker et al., Phys. Rev. Spec. Top. Accel. Beams 3, 061301 (2000)] and the quasi-static code QuickPIC [Huang et al., J. Comput. Phys. 217, 658 (2006)]. The effect of non-zero plasma temperature on the peak accelerating electric field is studied for a two bunch electron beam driver with parameters corresponding to the plasma wakefield acceleration experiments at Facilities for Accelerator Science and Experimental Test Beams. It is shown that plasma temperature does not affect the energy gain and spread of the accelerated particles despite suppressing the peak accelerating electric field. The role of plasma temperature in improving the numerical convergence of the electric field with the grid resolution is discussed.
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Publication: J. Plasma Phys. 81, 905810501 (2015)Doi: 10.1017/S0022377815000495
In this work we numerically demonstrate both significant transient (i.e. non-modal) linear amplification and sustained nonlinear turbulence in a kinetic plasma system with no unstable eigenmodes. The particular system considered is an electrostatic slab with magnetic shear, kinetic electrons and ions, weak collisions and a density gradient, but with no temperature gradient. In contrast to hydrodynamic examples of non-modal growth and subcritical turbulence, here there is no sheared flow in the equilibrium. Significant transient linear amplification is found when the magnetic shear and collisionality are weak. It is also demonstrated that nonlinear turbulence can be sustained if initialized at sufficient amplitude. We prove that these two phenomena are related: when sustained turbulence occurs without unstable eigenmodes, states that are typical of the turbulence must yield transient linear amplification of the gyrokinetic free energy.
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Absolute Measurement of the Ultrafast Nonlinear Electronic and Rovibrational Response in H-2 and D-2
Publication: Phys. Rev. A 92, 063828 (2015)Doi: 10.1103/PhysRevA.92.063828The electronic, rotational, and vibrational components of the ultrafast optical nonlinearity in H2 and D2 are measured directly and absolutely at intensities up to the ionization threshold of ∼1014W/cm2. As the most basic nonlinear interactions of the simplest molecules exposed to high fields, these results constitute a benchmark for high-field laser-matter theory and simulation.
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Publication: Geophys. Res. Lett. 42, 2563 (2015)Doi: 10.1002/2015GL063528
The dynamics and structure of whistler turbulence relevant to electron acceleration in the Earth's outer radiation belt are explored with simulations and comparisons with observations. An initial state with an electron temperature anisotropy in a spatially localized domain drives whistlers which scatter electrons. An outward propagating front of whistlers and hot electrons nonlinearly evolves to form regions of intense parallel electric field with structure similar to observations. The precipitating hot electrons propagate away from the source region in intense bunches rather than as a smooth flux.
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Publication: Photochem. Photobiol. 91, 862 (2015)Doi: 10.1111/php.12452
This article presents the design, construction and characterization of a novel type of light probe for measuring the angular radiance distribution of light fields. The differential acceptance angle (DAA) probe can resolve the directionality of a light field in environments with steep light gradients, such as microbial mats, without the need to remove, reorient, and reinsert the probe, a clear advantage over prior techniques. The probe consists of an inner irradiance sensor inside a concentric, moveable light-absorbing sheath. The radiative intensity in a specific zenith direction can be calculated by comparing the irradiance onto the sensor at different acceptance angles. We used this probe to measure the angular radiance distribution of two sample light fields, and observed good agreement with a conventional radiance probe. The DAA probe will aid researchers in understanding light transfer physics in dense microbial communities and expedite validation of numerical radiative transfer models for these environments.
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Publication: IEEE Trans. Electromagnetic Compatibility 57, 1049 (2015)Doi: 10.1109/TEMC.2015.2421346
In this paper, a statistical model for the coupling of electromagnetic radiation into enclosures through apertures is presented. The model gives a unified picture bridging deterministic theories of aperture radiation, and statistical models necessary for capturing the properties of irregular shaped enclosures. A Monte Carlo technique based on random matrix theory is used to predict and study the power transmitted through the aperture into the enclosure. Universal behavior of the net power entering the aperture is found. Results are of interest for predicting the coupling of external radiation through openings in irregular enclosures and reverberation chambers.
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Publication: Opt. Exp. 23, A1120 (2015)Doi: 10.1364/OE.23.0A1120
The method of detailed balance, introduced by Shockley and Queisser, is often used to find an upper theoretical limit for the efficiency of semiconductor pn-junction based photovoltaics. Typically the solar cell is assumed to be at an ambient temperature of 300 K. In this paper, we describe and analyze the use of radiative cooling techniques to lower the solar cell temperature below the ambient to surpass the detailed balance limit for a cell in contact with an ideal heat sink. We show that by combining specifically designed radiative cooling structures with solar cells, efficiencies higher than the limiting efficiency achievable at 300 K can be obtained for solar cells in both terrestrial and extraterrestrial environments. We show that our proposed structure yields an efficiency 0.87% higher than a typical PV module at operating temperatures in a terrestrial application. We also demonstrate an efficiency advantage of 0.4-2.6% for solar cells in an extraterrestrial environment in near-earth orbit.
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Publication: Nonlinearity 38, 3803 (2015)Doi: 10.1088/0951-7715/28/11/3803
Attracting chaotic behaviour in dynamical systems is often sensitive to small changes in parameters. If a perturbation in the parameter by a tiny amount epsilon can change the asymptoti behaviour of the system from being chaotic to being periodic, we call it parameter value epsilon-uncertain. Here, using a self-similar model of the intricate, intertwined parameter-space structure of the chaotic and periodic attractors, we investigate the scaling of this uncertainty with epsilon. We show that as epsilon approaches 0, the great majority of epsilon-uncertain parameters lie in high order windows, that is, windows within windows within windows .... The expected value of the order of the highest order window containing this parameter approaches infinity as epsilon goes to zero.
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Publication: Phys. Rev. Lett. 114, 095003 (2015)Doi: 10.1103/PhysRevLett.114.095003
We demonstrate that the universal mode driven by the density gradient in a plasma slab can be absolutely unstable even in the presence of reasonable magnetic shear. Previous studies from the 1970s that reached the opposite conclusion used an eigenmode equation limited to Lx ≫ ρi, where Lx is the scale length of the mode in the radial direction, and ρi is the ion Larmor radius. Here we instead use a gyrokinetic approach which does not have this same limitation. Instability is found for perpendicular wave numbers ky in the range 0.7 ≲ kyρi ≲ 100, and for sufficiently weak magnetic shear: Ls/Ln ≳ 17, where Ls and Ln are the scale lengths of magnetic shear and density. Thus, the plasma drift wave in a sheared magnetic field may be unstable even with no temperature gradients, no trapped particles, and no magnetic curvature.
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Publication: J. Comput. Phys. 294, 58 (2015)Doi: 10.1016/j.jcp.2015.03.039
We examine the merits of using a family of polynomials that are orthogonal with respect to a non-classical weight function to discretize the speed variable in continuum kinetic calculations. We consider a model one-dimensional partial differential equation describing energy diffusion in velocity space due to Fokker–Planck collisions. This relatively simple case allows us to compare the results of the projected dynamics with an expensive but highly accurate spectral transform approach. It also allows us to integrate in time exactly, and to focus entirely on the effectiveness of the discretization of the speed variable. We show that for a fixed number of modes or grid points, the non-classical polynomials can be many orders of magnitude more accurate than classical Hermite polynomials or finite-difference solvers for kinetic equations in plasma physics. We provide a detailed analysis of the difference in behavior and accuracy of the two families of polynomials. For the non-classical polynomials, if the initial condition is not smooth at the origin when interpreted as a three-dimensional radial function, the exact solution leaves the polynomial subspace for a time, but returns (up to roundoff accuracy) to the same point evolved to by the projected dynamics in that time. By contrast, using classical polynomials, the exact solution differs significantly from the projected dynamics solution when it returns to the subspace. We also explore the connection between eigenfunctions of the projected evolution operator and (non-normalizable) eigenfunctions of the full evolution operator, as well as the effect of truncating the computational domain.
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Publication: Astrophys. J. Lett. 808, L44 (2015)Doi: 10.1088/2041-8205/808/2/L44
An analytic model of the heliosheath (HS) between the termination shock (TS) and the heliopause (HP) is developed in the limit in which the interstellar flow and magnetic field are neglected. The heliosphere in this limit is axisymmetric and the overall structure of the HS and HP is controlled by the solar magnetic field even in the limit in which the ratio of the plasma to magnetic field pressure, β = 8πP/B2, in the HS is large. The tension of the solar magnetic field produces a drop in the total pressure between the TS and the HP. This same pressure drop accelerates the plasma flow downstream of the TS into the north and south directions to form two collimated jets. The radii of these jets are controlled by the flow through the TS and the acceleration of this flow by the magnetic field—a stronger solar magnetic field boosts the velocity of the jets and reduces the radii of the jets and the HP. MHD simulations of the global heliosphere embedded in a stationary interstellar medium match well with the analytic model. The results suggest that mechanisms that reduce the HS plasma pressure downstream of the TS can enhance the jet outflow velocity and reduce the HP radius to values more consistent with the Voyager 1 observations than in current global models.
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Publication: J. Fluid Mech. 774, 342 (2015)Doi: 10.1017/jfm.2015.275
We present azimuthal velocity profiles measured in a Taylor–Couette apparatus, which has been used as a model of stellar and planetary accretion disks. The apparatus has a cylinder radius ratio of η = 0.716, an aspect ratio of Γ = 11.74, and the plates closing the cylinders in the axial direction are attached to the outer cylinder. We investigate angular momentum transport and Ekman pumping in the Rayleigh-stable regime. This regime is linearly stable and is characterized by radially increasing specific angular momentum. We present several Rayleigh-stable profiles for shear Reynolds numbers ReS ∼ O(105), for both Ωi > Ωo > 0 (quasi-Keplerian regime) and Ωo > Ωi > 0 (sub-rotating regime), where Ωi,o is the inner/outer cylinder rotation rate. None of the velocity profiles match the non-vortical laminar Taylor–Couette profile. The deviation from that profile increases as solid-body rotation is approached at fixed ReS. Flow super-rotation, an angular velocity greater than those of both cylinders, is observed in the sub-rotating regime. The velocity profiles give lower bounds for the torques required to rotate the inner cylinder that are larger than the torques for the case of laminar Taylor–Couette flow. The quasi-Keplerian profiles are composed of a well-mixed inner region, having approximately constant angular momentum, connected to an outer region in solid-body rotation with the outer cylinder and attached axial boundaries. These regions suggest that the angular momentum is transported axially to the axial boundaries. Therefore, Taylor–Couette flow with closing plates attached to the outer cylinder is an imperfect model for accretion disk flows, especially with regard to their stability.
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Publication: ACS Nano 9, 464 (2015)Doi: 10.1021/nn505644q
Pushing lithium-ion battery (LIB) technology forward to its fundamental scaling limits requires the ability to create designer heterostructured materials and architectures. Atomic layer deposition (ALD) has recently been applied to advanced nanostructured energy storage devices due to the wide range of available materials, angstrom thickness control, and extreme conformality over high aspect ratio nanostructures. A class of materials referred to as conversion electrodes has recently been proposed as high capacity electrodes. RuO2 is considered an ideal conversion material due to its high combined electronic and ionic conductivity and high gravimetric capacity, and as such is an excellent material to explore the behavior of conversion electrodes at nanoscale thicknesses. We report here a fully characterized atomic layer deposition process for RuO2, electrochemical cycling data for ALD RuO2, and the application of the RuO2 to a composite carbon nanotube electrode scaffold with nucleation-controlled RuO2 growth. A growth rate of 0.4 Å/cycle is found between ∼210–240 °C. In a planar configuration, the resulting RuO2 films show high first cycle electrochemical capacities of ∼1400 mAh/g, but the capacity rapidly degrades with charge/discharge cycling. We also fabricated core/shell MWCNT/RuO2 heterostructured 3D electrodes, which show a 50× increase in the areal capacity over their planar counterparts, with an areal lithium capacity of 1.6 mAh/cm2.
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Publication: ACS Appl. Mater. Interfaces 7, 11402 (2015)Doi: 10.1021/acsami.5b01969
One of the greatest obstacles for the realization of the nonaqueous Li–O2 battery is finding a solvent that is chemically and electrochemically stable under cell operating conditions. Dimethyl sulfoxide (DMSO) is an attractive candidate for rechargeable Li–O2 battery studies; however, there is still significant controversy regarding its stability on the Li–O2 cathode surface. We performed multiple experiments (in situ XPS, FTIR, Raman, and XRD) which assess the stability of the DMSO–Li2O2 interface and report perspectives on previously published studies. Our electrochemical experiments show long-term stable cycling of a DMSO-based operating Li–O2 cell with a platinum@carbon nanotube core–shell cathode fabricated via atomic layer deposition, specifically with >45 cycles of 40 h of discharge per cycle. This work is complemented by density functional theory calculations of DMSO degradation pathways on Li2O2. Both experimental and theoretical evidence strongly suggests that DMSO is chemically and electrochemically stable on the surface of Li2O2 under the reported operating conditions.
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Publication: Biointerphases 10, 029512 (2015)Doi: 10.1116/1.4919410
An atmospheric pressure plasma jet (APPJ) was used to treat polystyrene (PS) films under remote conditions where neither the plume nor visible afterglow interacts with the film surface. Carefully controlled conditions were achieved by mounting the APPJ inside a vacuum chamber interfaced to a UHV surface analysis system. PS was chosen as a model system as it contains neither oxygen nor nitrogen, has been extensively studied, and provides insight into how the aromatic structures widespread in biological systems are modified by atmospheric plasma. These remote treatments cause negligible etching and surface roughening, which is promising for treatment of sensitive materials. The surface chemistry was measured by X-ray photoelectron spectroscopy to evaluate how ambient chemistry, feed gas chemistry, and plasma–ambient interaction impact the formation of specific moieties. A variety of oxidized carbon species and low concentrations of NOx species were measured after APPJ treatment. In the remote conditions used in this work, modifications are not attributed to short-lived species, e.g., O atoms. It was found that O3 does not correlate with modifications, suggesting that other long-lived species such as singlet delta oxygen or NOx are important. Indeed, surface-bound NO3 was observed after treatment, which must originate from gas phase NOx as neither N nor O are found in the pristine film. By varying the ambient and feed gas chemistry to produce O-rich and O-poor conditions, a possible correlation between the oxygen and nitrogen composition was established. When oxygen is present in the feed gas or ambient, high levels of oxidation with low concentrations of NO3 on the surface were observed. For O-poor conditions, NO and NO2 were measured, suggesting that these species contribute to the oxidation process, but are easily oxidized when oxygen is present. That is, surface oxidation limits and competes with surface nitridation. Overall, surface oxidation takes place easily, but nitridation only occurs under specific conditions with the overall nitrogen content never exceeding 3%. Possible mechanisms for these processes are discussed. This work demonstrates the need to control plasma–ambient interactions and indicates a potential to take advantage of plasma–ambient interactions to fine-tune the reactive species output of APP sources, which is required for specialized applications, including polymer surface modifications and plasma medicine.
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Publication: J. Appl. Phys. 118, 123301 (2015)Doi: 10.1063/1.4931144
We investigate the feasibility of using optically generated channels of reduced air density to provide trajectory correction (guiding) for a supersonic projectile. It is shown that the projectile experiences a force perpendicular to its direction of motion as one side of the projectile passes through a channel of reduced air density. A single channel of reduced air density can be generated by the energy deposited from filamentation of an intense laser pulse. We propose changing the laser pulse energy from shot-to-shot to build longer effective channels. Current femtosecond laser systems with multi-millijoule pulses could provide trajectory correction of several meters on 5 km trajectories for sub-kilogram projectiles traveling at Mach 3.
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Publication: Optica 2, 553 (2015)Doi: 10.1364/OPTICA.2.000553
The interaction of terahertz waves with silicon is usually explained using a linear model of conduction in which free carriers respond to the oscillating electric field, leading to absorption. Here we employ a silicon dielectric waveguide to confine and concentrate terahertz pulses, and observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, predicts a comparable field-induced transparency and elucidates the physical mechanism underlying this nonlinear effect: velocity saturation—an effect that fundamentally limits the speed of most semiconductor devices. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation. The results reported here could have significance in understanding and designing a variety of emerging and future terahertz devices, such as waveguides, mixers, detectors, and oscillators.
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Publication: Sci. Rep. 5, 13536 (2015)Doi: 10.1038/srep13536
The Shockley-Queisser limit describes the maximum solar energy conversion efficiency achievable for a particular material and is the standard by which new photovoltaic technologies are compared. This limit is based on the principle of detailed balance, which equates the photon flux into a device to the particle flux (photons or electrons) out of that device. Nanostructured solar cells represent a novel class of photovoltaic devices and questions have been raised about whether or not they can exceed the Shockley-Queisser limit. Here we show that single-junction nanostructured solar cells have a theoretical maximum efficiency of ∼42% under AM 1.5 solar illumination. While this exceeds the efficiency of a non-concentrating planar device, it does not exceed the Shockley-Queisser limit for a planar device with optical concentration. We consider the effect of diffuse illumination and find that with optical concentration from the nanostructures of only × 1,000, an efficiency of 35.5% is achievable even with 25% diffuse illumination. We conclude that nanostructured solar cells offer an important route towards higher efficiency photovoltaic devices through a built-in optical concentration.
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Publication: Cytoskeleton 72, 268 (2015)Doi: 10.1002/cm.21229
We investigate the dynamics of cell shape and analyze the actin and myosin distributions of cells exhibiting cortical density traveling waves. These waves propagate by repeated cycles of cortical compression (folding) and dilation (unfolding) that lead to periodic protrusions (oscillations) of the cell boundary. The focus of our detailed analysis is the remarkable periodicity of this phenotype, in which both the overall shape transformation and distribution of actomyosin density are repeated from cycle to cycle even though the characteristics of the shape transformation vary significantly for different regions of the cell. We show, using correlation analysis, that during traveling wave propagation cortical actin and plasma membrane densities are tightly coupled at each point along the cell periphery. We also demonstrate that the major protrusion appears at the wave trailing edge just after the actin cortex density has reached a maximum. Making use of the extraordinary periodicity, we employ latrunculin to demonstrate that sequestering actin monomers can have two distinct effects: low latrunculin concentrations can trigger and enhance traveling waves but higher concentrations of this drug retard the waves. The fundamental mechanism underlying this periodically protruding phenotype, involving folding and unfolding of the cortex-membrane couple, is likely to hold important clues for diverse phenomena including cell division and amoeboid-type migration.
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Publication: Appl. Phys. Lett. 106, 013502 (2015)Doi: 10.1063/1.4905508
It is shown that gyrotrons operating at cyclotron harmonics can be designed for operation in symmetric TE0,p-modes. Such operation in fundamental harmonic gyrotrons is possible only at small radial indices (p ≤ 3) because of the severe mode competition with TE2,p-modes, which are equally coupled to annular beams as the symmetric modes. At cyclotron harmonics, however, this “degeneracy” of coupling is absent, and there is a region in the parameter space where harmonic gyrotrons can steadily operate in symmetric modes. This fact is especially important for sub-THz and THz-range gyrotrons where ohmic losses limit the power achievable in continuous-wave and high duty cycle regimes.
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Publication: IEEE J. Photovoltaics 5, 577 (2015)Doi: 10.1109/JPHOTOV.2015.2392940
In order to absorb a significant fraction of the incident sunlight, a solar cell must incorporate light management techniques, including high-quality antireflection coatings. Such coatings are typically produced via high-temperature deposition methods that are costly. Here, we present a new technique based on light scattering within a transparent paper that is placed on top of a solar cell. The optical and electronic responses of both Si and GaAs solar cells are tested with these coatings, and a broadband angle-insensitive response is achieved. A comparison between textured and untextured solar cells is also made, and the power conversion efficiency is improved for untextured surfaces, while the increased path length within the paper resulting from textured cells increases absorption within the paper. The process is simple and inexpensive, and the paper coatings are made from a recyclable material, leading to less environmental impact.
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Publication: J. Electrochem. Soc. 162, A2372 (2015)Doi: 10.1149/2.0881512jes
Assembling nanostructured materials into rationally designed mesoscale arrays for use as electrodes in electrochemical systems is anticipated to reveal new challenges, particularly concerning new synthesis modes, architecture-related performance limitations, and degradation mechanisms. In this work, we focus on characterizing the degradation of densely packed vertically aligned carbon nanotubes (VACNTs) grown directly on a metallic foam to form a self-supporting, hierarchically porous 3D electrode architecture with an integrated current collector. The degradation pathways of this electrode, observed with microscopy and semi in-situ XPS after cycling as a redox scaffold in aprotic Li—O2 and Li—S batteries, shed new light on important design, performance, and degradation considerations for integrated mesoscale electrode architectures.
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Publication: Nano Lett. 15, 4295 (2015)Doi: 10.1021/acs.nanolett.5b00137
We report a large area terahertz detector utilizing a tunable plasmonic resonance in subwavelength graphene microribbons on SiC(0001) to increase the absorption efficiency. By tailoring the orientation of the graphene ribbons with respect to an array of subwavelength bimetallic electrodes, we achieve a condition in which the plasmonic mode can be efficiently excited by an incident wave polarized perpendicular to the electrode array, while the resulting photothermal voltage can be observed between the outermost electrodes.
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Publication: Polymers 7, 1 (2015)Doi: 10.3390/polym7010001
The polymorphic phase transformation in the cocrystallization of adefovir dipivoxil (AD) and succinic acid (SUC) was investigated. Inspired by biological and biomimetic crystallization, polymeric additives were utilized to control the phase transformation. With addition of poly(acrylic acid), the metastable phase newly identified through the analysis of X-ray diffraction was clearly isolated from the previously reported stable form. Without additives, mixed phases were obtained even at the early stage of cocrystallization. Also, infrared spectroscopy analysis verified the alteration of the hydrogen bonding that was mainly responsible for the cocrystal formation between AD and SUC. The hydrogen bonding in the metastable phase was relatively stronger than that in the stable form, which indicated the locally strong AD/SUC coupling in the initial stage of cocrystallization followed by the overall stabilization during the phase transformation. The stronger hydrogen bonding could be responsible for the faster nucleation of the initially observed metastable phase. The present study demonstrated that the polymeric additives could function as effective regulators for the polymorph-selective cocrystallization.
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Publication: Nano Lett. 15, 1481 (2015)Doi: 10.1021/nl503280u
Nitrogen vacancy (NV) color centers in diamond enable local magnetic field sensing with high sensitivity by optical detection of electron spin resonance (ESR). The integration of this capability with microfluidic technology has a broad range of applications in chemical and biological sensing. We demonstrate a method to perform localized magnetometry in a microfluidic device with a 48 nm spatial precision. The device manipulates individual magnetic particles in three dimensions using a combination of flow control and magnetic actuation. We map out the local field distribution of the magnetic particle by manipulating it in the vicinity of a single NV center and optically detecting the induced Zeeman shift with a magnetic field sensitivity of 17.5 μT Hz–1/2. Our results enable accurate nanoscale mapping of the magnetic field distribution of a broad range of target objects in a microfluidic device.
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Publication: Phys. Plasmas 22, 20705 (2015)Doi: 10.1063/1.4908545
A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks.
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Publication: J. Amer. Chem. Soc. 137, 12388 (2015)Doi: 10.1021/jacs.5b07820
Mg metal is a promising anode material for next generation rechargeable battery due to its dendrite-free deposition and high capacity. However, the best cathode for rechargeable Mg battery was based on high molecular weight MgxMo3S4, thus rendering full cell energetically uncompetitive. To increase energy density, high capacity cathode material like sulfur is proposed. However, to date, only limited work has been reported on Mg/S system, all plagued by poor reversibility attributed to the formation of electrochemically inactive MgSx species. Here, we report a new strategy, based on the effect of Li+ in activating MgSx species, to conjugate a dendrite-free Mg anode with a reversible polysulfide cathode and present a truly reversible Mg/S battery with capacity up to 1000 mAh/gs for more than 30 cycles. Mechanistic insights supported by spectroscopic and microscopic characterization strongly suggest that the reversibility arises from chemical reactivation of MgSx by Li+.
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Publication: Appl. Phys. Lett. 107, 013501 (2015)Doi: 10.1063/1.4926410
In recent years, there was an active development of high-power, sub-terahertz (sub-THz) gyrotrons for numerous applications. For example, a 0.67 THz gyrotron delivering more than 200 kW with about 20% efficiency was developed. This record high efficiency was achieved because the gyrotron operated in a high-order TE31,8-mode with the power of ohmic losses less than 10% of the power of outgoing radiation. That gyrotron operated at the fundamental cyclotron resonance, and a high magnetic field of about 27 T was created by a pulse solenoid. For numerous applications, it is beneficial to use gyrotrons at cyclotron harmonics which can operate in available cryomagnets with fields not exceeding 15 T. However, typically, the gyrotron operation at harmonics faces severe competition from parasitic modes at the fundamental resonance. In the present paper, we consider a similar 0.67 THz gyrotron designed for operation in the same TE31,8-mode, but at the second harmonic. We focus on two nonlinear effects typical for interaction between the fundamental and second harmonic modes, viz., the mode suppression and the nonlinear excitation of the mode at the fundamental harmonic by the second harmonic oscillations. Our study includes both the analytical theory and numerical simulations performed with the self-consistent code MAGY. The simulations show that stable second harmonic operation in the TE31,8 mode is possible with only modest sacrifice of efficiency and power.
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Publication: Geophys. Res. Lett. 42, 9657 (2015)Doi: 10.1002/2015GL065961
The physical processes that control the partition of released magnetic energy between electrons and ions during reconnection is explored through particle-in-cell simulations and analytical techniques. We demonstrate that the development of a large-scale parallel electric field and its associated potential controls the relative heating of electrons and ions. The potential develops to restrain heated exhaust electrons and enhances their heating by confining electrons in the region where magnetic energy is released. Simultaneously, the potential slows ions entering the exhaust below the Alfvénic speed expected from the traditional counterstreaming picture of ion heating. Unexpectedly, the magnitude of the potential and therefore the relative partition of energy between electrons and ions is not a constant but rather depends on the upstream parameters and specifically the upstream electron normalized temperature (electron beta). These findings suggest that the fraction of magnetic energy converted into the total thermal energy may be independent of upstream parameters.
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Publication: Phys. Rev. E 92, 022903 (2015)Doi: 10.1103/PhysRevE.92.022903
Stimulated Brillouin scattering (SBS) is a noise-driven nonlinear interaction between acoustical and optical waves. In optical fibers, SBS can be observed at relatively low optical powers and can severely limit signal transmission. Although SBS is initiated by high dimensional noise, it also exhibits many of the hallmarks of a complex nonlinear dynamical system. We report here a comprehensive experimental and numerical study of the fluctuations in the reflected Stokes wave produced by SBS in optical fibers. Using time series analysis, we demonstrate a reduction of dimensionality and dynamical filtering of the Stokes wave. We begin with a careful comparison of the measured average transmitted and reflected intensities from below the SBS threshold to saturation of the transmitted power. Initially the power spectra and correlation functions of the time series of the reflected wave fluctuations at the SBS threshold and above are measured and simulated. Much greater dynamical insight is provided when we study the scaling behavior of the intensity fluctuations using Hurst exponents and detrended fluctuation analysis for time scales extending over six orders of magnitude. At the highest input powers, we notice the emergence of three distinct dynamical scaling regimes: persistent, Brownian, and antipersistent. Next, we explore the Hilbert phase fluctuations of the intensity time series and amplitude-phase coupling. Finally, time-delay embedding techniques reveal a gradual reduction in dimensionality of the spatiotemporal dynamics as the laser input is increased toward saturation of the transmitted power. Through all of these techniques, we find a transition from noisier to smoother dynamics with increasing input power. We find excellent agreement between our experimental measurements and simulations.
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Publication: Sci. Rep. 5, 14803 (2015)Doi: 10.1038/srep14803
We use an ultra-fast near-infrared pulse coincidence technique to study the time, temperature and power dependence of the photoresponse of a bi-metal contacted graphene photodetector. We observe two components of the photovoltage signal. One component is gate-voltage dependent, linear in power at room temperature and sub-linear at low temperature-consistent with the hot-electron photothermoelectric effect due to absorption in the graphene. The power dependence is consistent with supercollision-dominated cooling in graphene. The other component is gate-voltage independent and linear in temperature and power, which we interpret as due to thermoelectricity of the metal electrodes due to differential light absorption.
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Publication: J. Appl. Phys. 118, 154304 (2015)Doi: 10.1063/1.4933176
Nanoporous silicon is known to have a thermal conductivity that is orders of magnitude smaller than the bulk crystalline silicon from which it is formed. Even though the strong columnar microscopic structure of porous silicon indicates the possibility of highly anisotropic thermal properties, there have been no measurements. We report here an experimental investigation of this anisotropy. An analytical heat spreading model with 3ω thermal conductivity measurement method was used to derive both in-plane and cross-plane conductivities. Additionally, we describe a finite element analysis that supports the experimental measurements. Our measurements reveal that because of the nanoscale columnar nature of the material, the in-plane thermal conductivity of nanoporous silicon is 1–2 orders of magnitude smaller than the cross-plane thermal conductivity and 2–3 orders of magnitude smaller than that of crystalline silicon, making it comparable to the best thermal insulators available.
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Publication: Phys. Rev. E 91, 052913 (2015)Doi: 10.1103/PhysRevE.91.052913
In this paper we consider the motion of point particles in a particular type of one-degree-of-freedom, slowly changing, temporally periodic Hamiltonian. Through most of the time cycle, the particles conserve their action, but when a separatrix is approached and crossed, the conservation of action breaks down, as shown in previous theoretical studies. These crossings have the effect that the numerical solution shows an apparent contradiction. Specifically we consider two initial constant energy phase space curves H = EA and H = EB at time t = 0, where H is the Hamiltonian and EA and EB are the two initial energies. The curve H = EA encircles the curve H = EB. We then sprinkle many initial conditions (particles) on these curves and numerically follow their orbits from t = 0 forward in time by one cycle period. At the end of the cycle the vast majority of points initially on the curves H = EA and H = EB now appear to lie on two new constant energy curves H = E′A and H = E′B, where the B′ curve now encircles the A′ curve (as opposed to the initial case where the A curve encircles the B curve). Due to the uniqueness of Hamilton dynamics, curves evolved under the dynamics cannot cross each other. Thus the apparent curves H = E′A and H = E′B must be only approximate representations of the true situation that respects the topological exclusion of curve crossing. In this paper we resolve this apparent paradox and study its consequences. For this purpose we introduce a “robust” numerical simulation technique for studying the complex time evolution of a phase space curve in a Hamiltonian system. We also consider how a very tiny amount of friction can have a major consequence, as well as what happens when a very large number of cycles is followed. We also discuss how this phenomenon might extend to chaotic motion in higher dimensional Hamiltonian systems.
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Publication: Phys. Chem. Chem. Phys. 17, 3873 (2015)Doi: 10.1039/c4cp04211e
Here we report a strategy for combining techniques for pore branching and barrier layer thinning to produce 3D porous anodized aluminum oxide films with direct ohmic contact to the native aluminum. This method provides an example of a rationally designed template which need not be removed from the aluminum, but which is also not constrained to traditional 2D pore geometry. We first demonstrate the barrier layer removal and pore branching techniques independently, and then combine them to produce free standing arrays of interconnected Ni nanostructures. Nickel nanostructures are deposited directly onto the aluminum to demonstrate the success of the structural modification, and showcase the potential for these films to be used as templates. This approach is the first to demonstrate the design and execution of multiple pore modification techniques in the same membrane, and demonstrates the first directly deposited 3D structures on aluminum substrates.
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Publication: J. Physics - Condensed Matter 27, 214012 (2015)Doi: 10.1088/0953-8984/27/21/214012
Measurements of the Casimir force require the elimination of the electrostatic force between the surfaces. However, due to electrostatic patch potentials, the voltage required to minimize the total force may not be sufficient to completely nullify the electrostatic interaction. Thus, these surface potential variations cause an additional force, which can obscure the Casimir force signal. In this paper, we inspect the spatially varying surface potential of e-beamed, sputtered, sputtered and annealed, and template stripped gold surfaces with Heterodyne amplitude modulated Kelvin probe force microscopy (HAM-KPFM). It is demonstrated that HAM-KPFM improves the spatial resolution of surface potential measurements compared to amplitude modulated Kelvin probe force microscopy. We find that patch potentials vary depending on sample preparation, and that the calculated pressure can be similar to the pressure difference between Casimir force calculations employing the plasma and Drude models.
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Publication: Astrophys. J. Lett. 800, L28 (2015)Doi: 10.1088/2041-8205/800/2/L28
The classic accepted view of the heliosphere is a quiescent, comet-like shape aligned in the direction of the Sun's travel through the interstellar medium (ISM) extending for thousands of astronomical units (AUs). Here, we show, based on magnetohydrodynamic (MHD) simulations, that the tension (hoop) force of the twisted magnetic field of the Sun confines the solar wind plasma beyond the termination shock and drives jets to the north and south very much like astrophysical jets. These jets are deflected into the tail region by the motion of the Sun through the ISM similar to bent galactic jets moving through the intergalactic medium. The interstellar wind blows the two jets into the tail but is not strong enough to force the lobes into a single comet-like tail, as happens to some astrophysical jets. Instead, the interstellar wind flows around the heliosphere and into the equatorial region between the two jets. As in some astrophysical jets that are kink unstable, we show here that the heliospheric jets are turbulent (due to large-scale MHD instabilities and reconnection) and strongly mix the solar wind with the ISM beyond 400 AU. The resulting turbulence has important implications for particle acceleration in the heliosphere. The two-lobe structure is consistent with the energetic neutral atom (ENA) images of the heliotail from IBEX where two lobes are visible in the north and south and the suggestion from the Cassini ENAs that the heliosphere is "tailless."
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Publication: Phys. Rev. E 92, 022804 (2015)Doi: 10.1103/PhysRevE.92.022804
We provide experimental evidence that adding links to a network's structure can hinder synchronization. Our experiments and theoretical analysis of networks of time-delayed optoelectronic oscillators uncover the scenario of loss of identical synchronization upon connectivity modifications. This counterintuitive loss of synchronization can occur even when the network structure is improved from a connectivity perspective. Utilizing a master stability function approach, we show that a time delay in the coupling of nodes plays a crucial role in determining a network's synchronization properties and that this effect is more prominent in directed networks than in undirected networks, especially for large networks. Our results provide insight into the impact of structural modifications in networks with equal coupling delays and open the path to design changes to the network connectivity to sustain and control the performance of real-world networks.
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Publication: ACS Nano 9, 5884 (2015)Doi: 10.1021/acsnano.5b02166
Lithium metal is considered to be the most promising anode for next-generation batteries due to its high energy density of 3840 mAh g–1. However, the extreme reactivity of the Li surface can induce parasitic reactions with solvents, contamination, and shuttled active species in the electrolyte, reducing the performance of batteries employing Li metal anodes. One promising solution to this issue is application of thin chemical protection layers to the Li metal surface. Using a custom-made ultrahigh vacuum integrated deposition and characterization system, we demonstrate atomic layer deposition (ALD) of protection layers directly on Li metal with exquisite thickness control. We demonstrate as a proof-of-concept that a 14 nm thick ALD Al2O3 layer can protect the Li surface from corrosion due to atmosphere, sulfur, and electrolyte exposure. Using Li–S battery cells as a test system, we demonstrate an improved capacity retention using ALD-protected anodes over cells assembled with bare Li metal anodes for up to 100 cycles.
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Publication: Phys. Rev. E 91, 060902 (2015)Doi: 10.1103/PhysRevE.91.060902
We investigate the effect of preferentially connecting oscillators with similar frequency to each other in networks of coupled phase oscillators (i.e., frequency assortativity). Using the network Kuramoto model as an example, we find that frequency assortativity can induce chaos in the macroscopic dynamics. By applying a mean-field approximation in combination with the dimension reduction method of Ott and Antonsen, we show that the dynamics can be described by a low dimensional system of equations. We use the reduced system to characterize the macroscopic chaos using Lyapunov exponents, bifurcation diagrams, and time-delay embeddings. Finally, we show that the emergence of chaos stems from the formation of multiple groups of synchronized oscillators, i.e., meta-oscillators.
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Publication: Convergent Sci. Phys. Oncology 1, 25002 (2015)Doi: 10.1088/2057-1739/1/2/025002
Complex phenotypic differences among different acute leukemias cannot be fully captured by analyzing the expression levels of one single molecule, such as a miR, at a time, but requires systematic analysis of large sets of miRs. While a popular approach for analysis of such datasets is principal component analysis (PCA), this method is not designed to optimally discriminate different phenotypes. Moreover, PCA and other low-dimensional representation methods yield linear or non-linear combinations of all measured miRs. Global human miR expression was measured in AML, B-ALL, and T-ALL cell lines and patient RNA samples. By systematically applying support vector machines to all measured miRs taken in dyad and triad groups, we built miR networks using cell line data and validated our findings with primary patient samples. All the coordinately transcribed members of the miR-23a cluster (which includes also miR-24 and miR-27a), known to function as tumor suppressors of acute leukemias, appeared in the AML, B-ALL and T-ALL centric networks. Subsequent qRT-PCR analysis showed that the most connected miR in the B-ALL-centric network, miR-708, is highly and specifically expressed in B-ALLs, suggesting that miR-708 might serve as a biomarker for B-ALL. This approach is systematic, quantitative, scalable, and unbiased. Rather than a single signature, our approach yields a network of signatures reflecting the redundant nature of intracellular pathways. The network representation allows for visual analysis of all signatures by an expert and for future integration of additional information. Furthermore, each signature involves only small sets of miRs, such as dyads and triads, which are well suited for in depth validation through laboratory experiments. In particular, loss- and gain-of-function assays designed to drive changes in leukemia cell survival, proliferation and differentiation will benefit from the identification of multi-miR signatures that characterize leukemia subtypes and their normal counterpart cells of origin.
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Publication: Appl. Phys. Lett. 106, 091107 (2015)Doi: 10.1063/1.4914003
Light reflected off a material or absorbed within it exerts radiation pressure through the transfer of momentum. Micro/nano-mechanical transducers have become sensitive enough that radiation pressure can influence these systems. However, photothermal effects often accompany and overwhelm the radiation pressure, complicating its measurement. In this letter, we investigate the radiation force on an uncoated silicon nitride microcantilever in ambient conditions. We identify and separate the radiation pressure and photothermal forces through an analysis of the cantilever's frequency response. Further, by working in a regime where radiation pressure is dominant, we are able to accurately measure the radiation pressure. Experimental results are compared to theory and found to agree within the measured and calculated uncertainties.
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Publication: Opt. Lett. 40, 5794 (2015)Doi: 10.1364/OL.40.005794
We measure the nonlinear refractive index coefficients in N2, O2, and Ar from visible through Mid-infrared wavelengths (λ = 0.4 - 2.4 μm). The wavelengths investigated correspond to transparency windows in the atmosphere. Good agreement is found with theoretical models of c(3). Our results are essential for accurately simulating the propagation of ultrashort mid-infrared pulses in the atmosphere.
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Publication: PLOS Comput. Biol. 11, 31004565 (2015)Doi: 10.1371/journal.pcbi.1004565
The Gene Ontology (GO) provides biologists with a controlled terminology that describes how genes are associated with functions and how functional terms are related to one another. These term-term relationships encode how scientists conceive the organization of biological functions, and they take the form of a directed acyclic graph (DAG). Here, we propose that the network structure of gene-term annotations made using GO can be employed to establish an alternative approach for grouping functional terms that captures intrinsic functional relationships that are not evident in the hierarchical structure established in the GO DAG. Instead of relying on an externally defined organization for biological functions, our approach connects biological functions together if they are performed by the same genes, as indicated in a compendium of gene annotation data from numerous different sources. We show that grouping terms by this alternate scheme provides a new framework with which to describe and predict the functions of experimentally identified sets of genes.
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Publication: Opt. Lett. 40.2481 (2015)Doi: 10.1364/OL.40.002481
We describe and demonstrate an electro-optic technique to simultaneously downconvert and demodulate vector-modulated millimeter-wave signals. The system uses electro-optic phase modulation and optical filtering to perform harmonic downconversion of the RF signal to an intermediate frequency (IF) or to baseband. We demonstrate downconversion of RF signals between 7 and 70-GHz to IFs below 20-GHz. Furthermore, we show harmonic downconversion and vector demodulation of 2.5-Gb/s QPSK and 5-Gb/s 16-QAM signals at carrier frequencies of 40-GHz to baseband.
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Publication: Geophys. Res. Lett. 42, 7239 (2015)Doi: 10.1002/2015GL065168
A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by antiparallel reconnection in conjunction with particle-in-cell simulations, showing spatial variations in the anisotropy across the outflow far (>100di) X line. A consistent pattern is found in both the spacecraft data and the simulations: While the total temperature across the exhaust is rather constant, near the downstream of the boundaries Ti,|| dominates. The plasma is well above the firehose threshold within patchy spatial regions at |BX|∈[0.1,0.5]B0, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the midplane (| Bx| <~ 0.1 B0), Ti,⊥>Ti,|| and ions undergo Speiser-like motion despite the large distance from the X line.
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Publication: Chem. Mater. 27, 5324 (2015)Doi: 10.1021/acs.chemmater.5b01654
We demonstrate an atomic layer deposition (ALD) process for the solid electrolyte lithium phosphorousoxynitride (LiPON) using lithium tert-butoxide (LiOtBu), H2O, trimethylphosphate (TMP), and plasma N2 (PN2) as precursors. We use in-situ spectroscopic ellipsometry to determine growth rates for process optimization to design a rational, quaternary precursor ALD process where only certain substrate–precursor chemical reactions are favorable. We demonstrate via in-situ XPS tunable nitrogen incorporation into the films by variation of the PN2 dose and find that ALD films over approximately 4.5% nitrogen are amorphous, whereas LiPON ALD films with less than 4.5% nitrogen are polycrystalline. Finally, we characterize the ionic conductivity of the ALD films as a function of nitrogen content and demonstrate their functionality on a model battery electrode—a Si anode on a Cu current collector.
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Publication: Phys. Chem. Chem. Phys. 17, 5256 (2015)Doi: 10.1039/c4cp05591h
Magnesium batteries have been considered to be one of the promising beyond lithium ion technologies due to magnesium's abundance, safety, and high volumetric capacity. However, very few materials show reversible performance as a cathode in magnesium ion systems. We present herein the best reported cycling performances of MnO2 as a magnesium battery cathode material. We show that the previously reported poor Mg2+ insertion/deinsertion capacities in MnO2 can be greatly improved by synthesizing self-standing nanowires and introducing a small amount of water molecules into the electrolyte. Electrochemical and elemental analysis results revealed that the magnitude of Mg2+ insertion into MnO2 highly depends on the ratio between water molecules and Mg2+ ions present in the electrolyte and the highest Mg2+ insertion capacity was observed at a ratio of 6H2O/Mg2+ in the electrolyte. We demonstrate for the first time, that MnO2 nanowire electrode can be “activated” for Mg2+ insertion/deinsertion by cycling in water containing electrolyte resulting in enhanced reversible Mg2+ insertion/deinsertion even with the absence of water molecules. The MnO2 nanowire electrode cycled in dry Mg electrolyte after activation in water-containing electrolyte showed an initial capacity of 120 mA h g−1 at a rate of 0.4 C and maintained 72% of its initial capacity after 100 cycles.
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Publication: Phys. Rev. E 91, 032807 (2015)Doi: 10.1103/PhysRevE.91.032807
This paper explores the effectiveness of network attack when the attacker has imperfect information about the network. For Erdős-Rényi networks, we observe that dynamical importance and betweenness centrality-based attacks are surprisingly robust to the presence of a moderate amount of imperfect information and are more effective compared with simpler degree-based attacks even at moderate levels of network information error. In contrast, for scale-free networks the effectiveness of attack is much less degraded by a moderate level of information error. Furthermore, in the Erdős-Rényi case the effectiveness of network attack is much more degraded by missing links as compared with the same number of false links.
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Publication: Phys. Rev. Lett. 115, 194802 (2015)Doi: 10.1103/PhysRevLett.115.194802
We demonstrate laser-plasma acceleration of high charge electron beams to the ∼10 MeV scale using ultrashort laser pulses with as little energy as 10 mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ∼0.5 nC is measured for energies >1 MeV. Acceleration is correlated to the presence of a relativistically self-focused laser filament accompanied by an intense coherent broadband light flash, associated with wave breaking, which can radiate more than ∼3% of the laser energy in a ∼1 fs bandwidth consistent with half-cycle optical emission. Our results enable truly portable applications of laser-driven acceleration, such as low dose radiography, ultrafast probing of matter, and isotope production.
2014
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Publication: Phys. Rev. A 90, 042322 (2014)Doi: 10.1103/PhysRevA.90.042322
We propose a method for deterministic generation of entanglement between a quantum-dot spin and a photon. Entanglement is established by an elastic scattering event from a cavity that is strongly coupled to a singly charged quantum dot. Due to the elastic nature of the interaction, the energy of the photon does not become entangled with the quantum dot, eliminating the need for temporal postselection. We derive an analytical expression for fidelity of the generated entangled state and investigate its behavior under realistic experimental conditions. We show that entanglement fidelities exceeding 0.974 can be realized using currently achievable quantum-dot cavity quantum electrodynamics systems.
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Publication: IEEE Trans. Plasma Sci. 42, 1482 (2014)Doi: 10.1109/TPS.2014.2302634
Bob Barker, a strong proponent of high power microwave (HPM) research and education, passed away on December 15, 2013 at the age of 64. His loving wife, Fran, and family were at his bedside at the Halquist Hospice Center. His life was too brief, but his accomplishments were many and impressive.
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Publication: Phys. Rev. Lett. 112, 138103 (2014)Doi: 10.1103/PhysRevLett.112.138103
The collective dynamics of a network of excitable nodes changes dramatically when inhibitory nodes are introduced. We consider inhibitory nodes which may be activated just like excitatory nodes but, upon activating, decrease the probability of activation of network neighbors. We show that, although the direct effect of inhibitory nodes is to decrease activity, the collective dynamics becomes self-sustaining. We explain this counterintuitive result by defining and analyzing a “branching function” which may be thought of as an activity-dependent branching ratio. The shape of the branching function implies that, for a range of global coupling parameters, dynamics are self-sustaining. Within the self-sustaining region of parameter space lies a critical line along which dynamics take the form of avalanches with universal scaling of size and duration, embedded in a ceaseless time series of activity. Our analyses, confirmed by numerical simulation, suggest that inhibition may play a counterintuitive role in excitable networks.
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Publication: Nature Commun. 5, 4224 (2014)Doi: 10.1038/ncomms5224
Various band structure engineering methods have been studied to improve the performance of graphitic transparent conductors; however, none has demonstrated an increase of optical transmittance in the visible range. Here we measure in situ optical transmittance spectra and electrical transport properties of ultrathin graphite (3–60 graphene layers) simultaneously during electrochemical lithiation/delithiation. On intercalation, we observe an increase of both optical transmittance (up to twofold) and electrical conductivity (up to two orders of magnitude), strikingly different from other materials. Transmission as high as 91.7% with a sheet resistance of 3.0 Ω per square is achieved for 19-layer LiC6, which corresponds to a figure of merit σdc/σopt=1,400, significantly higher than any other continuous transparent electrodes. The unconventional modification of ultrathin graphite optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques enable investigation of other aspects of intercalation in nanostructures.
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Publication: IEEE Trans. Electromagnetic Compatibility 56, 530 (2014)2014Doi: 10.1109/TEMC.2014.2304542
This study demonstrates the presence of chaotic oscillations in standard CMOS circuits. At radio-frequencies, ordinary digital circuits can show unexpected nonlinear responses. We examine a CMOS inverter coupled with electrostatic discharging (ESD) protection circuits, designed with 0.35 μm CMOS technology, for evidence of its chaotic oscillations. As the circuit is directly driven by a radio-frequency signal, the circuit enters a chaotic dynamic regime when the input frequency is higher than the maximum operating frequency of CMOS inverter. We observe an aperiodic signal, a broadband spectrum, and a complex spectrum. We discuss the nonlinear physical effects in the given circuit: ESD diode rectification, dc bias shift due to a nonquasi-static regime operation of the ESD PN-junction diode, and a nonlinear resonant feedback current path. In order to predict these chaotic dynamics, we develop a transistor-based model, and compare it with the experimental results. To verify the presence of chaotic oscillations mathematically, we develop an ordinary differential equation model with the circuit-related nonlinearities. The largest Lyapunov exponents are calculated to verify the chaotic oscillations. The importance of this study derives from investigating chaotic oscillations in standard CMOS circuits as circuit-effects due to high-intensity electromagnetic signals.
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Publication: AIP Adv. 4, 087138 (2014)Doi: 10.1063/1.4894448
Time reversal mirrors work perfectly only for lossless wave propagation; dissipation destroys time-reversal invariance and limits the performance of time-reversal mirrors. Here, a new measure of time-reversal mirror performance is introduced and the adverse effect of dissipation on this performance measure is investigated. The technique of exponential amplification is employed to partially overcome the effect of non-uniform loss distributions, and its success is tested quantitatively using the new performance measure. A numerical model of a star graph is employed to test the applicability of this technique on realizations with various random spatial distributions of loss. A subset of the numerical results are also verified by experimental results from an electromagnetic time-reversal mirror. The exponential amplification technique is a simple way to improve the performance of emerging technologies based on time-reversed wave propagation such as directed communication and wireless power transfer.
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Publication: Wave Motion 51, 606 (2014)Doi: 10.1016/j.wavemoti.2014.02.003
In this review, a model (the random coupling model) that gives a statistical description of the coupling of radiation into and out of large enclosures through localized and/or distributed ports is presented. The random coupling model combines both deterministic and statistical phenomena. The model makes use of wave chaos theory to extend the classical modal description of the cavity fields in the presence of boundaries that lead to chaotic ray trajectories. The model is based on a clear separation between the universal statistical behavior of the closed chaotic system, and the deterministic coupling port characteristics. Moreover, the ability of the random coupling model to describe interconnected cavities, aperture coupling, and the effects of short ray trajectories is discussed. A relation between the random coupling model and other formulations adopted in acoustics, optics, and statistical electromagnetics, is examined. In particular, a rigorous analogy of the random coupling model with the Statistical Energy Analysis used in acoustics is presented.
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Publication: ACS Nano 8, 11833 (2014)Doi: 10.1021/nn5052585
The local collection characteristics of grain interiors and grain boundaries in thin-film CdTe polycrystalline solar cells are investigated using scanning photocurrent microscopy. The carriers are locally generated by light injected through a small aperture (50–300 nm) of a near-field scanning optical microscope in an illumination mode. Possible influence of rough surface topography on light coupling is examined and eliminated by sculpting smooth wedges on the granular CdTe surface. By varying the wavelength of light, nanoscale spatial variations in external quantum efficiency are mapped. We find that the grain boundaries (GBs) are better current collectors than the grain interiors (GIs). The increased collection efficiency is caused by two distinct effects associated with the material composition of GBs. First, GBs are charged, and the corresponding built-in field facilitates the separation and the extraction of the photogenerated carriers. Second, the GB regions generate more photocurrent at long wavelength corresponding to the band edge, which can be caused by a smaller local band gap. Resolving carrier collection with nanoscale resolution in solar cell materials is crucial for optimizing the polycrystalline device performance through appropriate thermal processing and passivation of defects and surfaces.
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Publication: Nucl. Instrum. Methods Phys. Res. Sect. 1 - Accelerators Spectrometers Detecors and Associated Equipment 633, 178 (2014)Doi: 10.1016/j.nima.2013.05.077
The foremost challenge for Heavy-Ion Fusion (HIF) is achieving sufficiently low emittances and small energy spreads in the presence of intense space-charge, to achieve the high deposition densities necessary for pellet ignition. The University of Maryland Electron Ring (UMER) uses intense low-energy electron beams to access the scaled physics of HIF drivers. In particular, the long path-length propagation in UMER presents an opportunity to study, at realistic scales, the longitudinal beam dynamics and manipulations required for such a driver. With the use of induction modules, as in the ion machines such as NDCX-II, the resulting bunch dynamics show evidence of space-charge waves excited by an initial mismatch between the detailed initial beam distribution at the bunch ends and the applied focusing waveforms, persisting with multiple damped reflections propagating along the bunch flat-top. Using the induction module we are able to suppress space-charge waves with great accuracy, at amplitudes that include wave steepening prior to the formation of solitary wave trains. The longitudinal dynamics largely dominates when no containment fields are applied, coupling through the natural chromaticity of the ring even within the first turn. After subsequent turns, the bunch elongates and wraps the circumference of the machine multiple times; eventually reaching a point of instability that has also been shown through simulation, obtaining excellent agreement when the detailed longitudinal dynamics of the experiment are carefully incorporated into the model.
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Publication: Nature Commun. 5, 4079 (2014)Doi: 10.1038/ncomms5079
Synchronization is of central importance in power distribution, telecommunication, neuronal and biological networks. Many networks are observed to produce patterns of synchronized clusters, but it has been difficult to predict these clusters or understand the conditions under which they form. Here we present a new framework and develop techniques for the analysis of network dynamics that shows the connection between network symmetries and cluster formation. The connection between symmetries and cluster synchronization is experimentally confirmed in the context of real networks with heterogeneities and noise using an electro-optic network. We experimentally observe and theoretically predict a surprising phenomenon in which some clusters lose synchrony without disturbing the others. Our analysis shows that such behaviour will occur in a wide variety of networks and node dynamics. The results could guide the design of new power grid systems or lead to new understanding of the dynamical behaviour of networks ranging from neural to social.
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Publication: New J. Phys. 16, 113005 (2014)Doi: 10.1088/1367-2630/16/11/113005
Measurement of the differential rotation of the Sunʼs interior is one of the great achievements of helioseismology, providing important constraints for stellar physics. The technique relies on observing and analyzing rotationally-induced splittings of p-modes in the star. Here, we demonstrate the first use of the technique in a laboratory setting. We apply it in a spherical cavity with a spinning central core (spherical-Couette flow) to determine the mean azimuthal velocity of the air filling the cavity. We excite a number of acoustic resonances (analogous to p-modes in the Sun) using a speaker and record the response with an array of small microphones on the outer sphere. Many observed acoustic modes show rotationally-induced splittings, which allow us to perform an inversion to determine the airʼs azimuthal velocity as a function of both radius and latitude. We validate the method by comparing the velocity field obtained through inversion against the velocity profile measured with a calibrated hot film anemometer. This modal acoustic velocimetry technique has great potential for laboratory setups involving rotating fluids in axisymmetric cavities. It will be useful especially in liquid metals where direct optical methods are unsuitable and ultrasonic techniques very challenging at best.
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Publication: Opt. Exp. 22, 3013 (2014)Doi: 10.1364/OE.22.003013
We propose a method to overcome Auger recombination in nanocrystal quantum dot lasers using cavity-enhanced spontaneous emission. We derive a numerical model for a laser composed of nanocrystal quantum dots coupled to optical nanocavities with small mode-volume. Using this model, we demonstrate that spontaneous emission enhancement of the biexciton transition lowers the lasing threshold by reducing the effect of Auger recombination. We analyze a photonic crystal nanobeam cavity laser as a realistic device structure that implements the proposed approach.
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Publication: J. Mater. Chem. A 2, 20552 (2014)Doi: 10.1039/c4ta03716b
The atomistic mechanism for lithiation/delithiation in all-solid-state batteries is still an open question, and the ‘holy grail’ to engineer devices with extended lifetime. Here, by combining real-time scanning electron microscopy in ultra-high vacuum with electrochemical cycling, we quantify the dynamic degradation of Al anodes in Li-ion all-solid-state batteries, a promising alternative for ultra lightweight devices. We find that AlLi alloy mounds are formed on the top surface of the Al anode and that degradation of battery capacity occurs because of Li trapped in them. Our approach establishes a new platform for probing the real-time degradation of electrodes, and can be expanded to other complex systems, allowing for high throughput characterization of batteries with nanoscale resolution.
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Publication: Phys. Chem. Chem. Phys. 16, 2224 (2014)Doi: 10.1039/c3cp53560f
Surface enhanced Raman spectroscopy (SERS) is a powerful spectroscopic technique capable of detecting trace amounts of chemicals and identifying them based on their unique vibrational characteristics. While there are many complex methods for fabricating SERS substrates, there has been a recent shift towards the development of simple, low cost fabrication methods that can be performed in most labs or even in the field. The potential of SERS for widespread use will likely be realized only with development of cheaper, simpler methods. In this Perspective article we briefly review several of the more popular methods for SERS substrate fabrication, discuss the characteristics of simple SERS substrates, and examine several methods for producing simple SERS substrates. We highlight potential applications and future directions for simple SERS substrates, focusing on highly SERS active three-dimensional nanostructures fabricated by inkjet and screen printing and galvanic displacement for portable SERS analysis – an area that we believe has exciting potential for future research and commercialization.
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Publication: J. Mater. Chem. C 2, 1248 (2014)Doi: 10.1039/c3tc31726a
In this study we report a novel, rationally designed, solution based silver nanowire (Ag NW) paper hybrid that demonstrates a flexible, low cost, and scalable device ready transparent conducting electrode (TCE) with exceptional and stable optoelectronic properties. Its high transmittance (91%) and low sheet resistance (13 Ω sq−1) represent the highest reported figure of merit value for solution based TCEs according to conventional models. We also thoroughly investigate the diffuse light scattering properties of our Ag NW paper with various techniques that elucidate the total optical haze as well as the diffuse scattering angle distribution for this TCE. Through a simulation of the impact the optical properties of TCEs have on the light absorption in the conversion layers for various thin film solar cells, we demonstrate that our Ag NW paper induces greater light absorption than ITO for each simulated thin film solar cell.
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Publication: Opt. Exp. 22, 22730 (2014)Doi: 10.1364/OE.22.022730
We use pump-probe spectroscopy and continuous wave cross-phase and cross-amplitude modulation measurements to study the optical nonlinearity of a hydrogenated amorphous silicon (a-Si:H) nanowire waveguide, and we compare the results to those of a crystalline silicon waveguide of similar dimensions. The a-Si:H nanowire shows essentially zero instantaneous two-photon absorption, but it displays a strong, long-lived non-instantaneous nonlinearity that is both absorptive and refractive. Power scaling measurements show that this non-instantaneous nonlinearity in a-Si:H scales as a third-order nonlinearity, and the refractive component possesses the opposite sign to that expected for free-carrier dispersion.
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Publication: Adv. Energy Mater. 4, 1301804 (2014)Doi: 10.1002/aenm.201301804
A new paper-based anti-reflection coating for solar cells is presented showing a large reduction in the reflection over the entire solar spectrum for a wide range of angles. This process is simple and inexpensive, requiring no high temperature or vacuum-based processing, and is made from renewable cellulose fibers.
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Publication: IEEE Trans. Terahertz Sci. Technol. 4, 454 (2014)Doi: 10.1109/TTHZ.2014.2325739
We experimentally examine anisotropically etched silicon surfaces for terahertz (THz) plasmonic guided wave applications. Highly doped silicon surfaces are anisotropically etched to form a one-dimensional array of subwavelength concave pyramidal troughs. The plasmonic wave guides are found to support highly confined guided modes both in transverse and longitudinal directions. The resonant frequencies of the modes can be controlled by adjusting the geometrical parameters of the troughs. The existence of guided modes in plasmonic wave guides is also established through finite-element-based numerical simulations. These wave guides could provide a silicon-based alternative to metallic wave guides for use in future THz devices.
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Publication: Nature Photon. 8, 858 (2014)Doi: 10.1038/NPHOTON.2014.224
When an atom strongly couples to a cavity, the two systems can coherently exchange a single quantum excitation through the process of vacuum Rabi oscillation. Controlling this process enables precise synthesis of non-classical light, which plays a central role in quantum information and measurement. Although this control has been realized in microwave-frequency devices, it has been difficult to achieve at optical frequencies, which are essential for quantum communication and metrology. Here, we demonstrate coherent control of vacuum Rabi oscillation in an optical frequency device. We use a photonic molecule composed of two coupled nanocavities to simultaneously achieve strong coupling and a cavity-enhanced a.c. Stark shift. The Stark shift tunes a single quantum dot onto resonance with the photonic molecule on picosecond timescales, creating fast coherent transfer of energy between an atomic and photonic excitation. These results enable ultrafast control of light–matter quantum interactions in a nanophotonic device platform.
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Publication: Adv. Functional Mater. 24, 6717 (2014)Doi: 10.1002/adfm.201401284
Solution processing of colloidal nanocrystals into porous architectures using block co-polymer templating offers a simple yet robust route to construct materials with open porosity and high surface area. These features, when realized in materials that show efficient redox activity and good conductivity, should be ideal for electrochemical energy storage because they allow for efficient electrolyte diffusion and ample surface and near-surface redox reactions. Here, a route to synthesize nanoporous pseudocapacitors is presented using preformed ITO nanocrystals to make a conductive scaffold, coated with a conformal layer of vanadia deposited using atomic layer deposition (ALD). Two vanadia thicknesses are deposited, 2 and 7 nm, to examine the kinetics of Li+ diffusion into vanadia in a system where all other chemical and structural parameters are fixed. Porosity measurements show that the internal surface area of 2 nm vanadia samples is fully accessible; whereas for the 7 nm vanadia, there is some pore blockage that limits electrolyte diffusion. Despite this fact, composites with both thick and thin vanadia layers show high levels of pseudocapacitance, indicating fast diffusion of Li+ through even the 7 nm thick vanadia. This work thus sets a minimum accessible length-scale of 7 nm for intercalation pseudocapacitance in orthorhombic V2O5.
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Publication: Opt. Lett. 39, 3161 (2014)Doi: 10.1364/OL.39.003161
We present a side-by-side comparison of the nonlinear behavior of four passive AlGaAs ridge waveguides where the bandgap energy of the core layers ranges from 1.60 to 1.79 eV. By engineering the bandgap to suppress two-photon absorption, minimizing the linear loss, and minimizing the mode area, we achieve efficient wavelength conversion in the C-band via partially degenerate four-wave mixing with a continuous-wave pump. The observed conversion efficiency [Idler(OUT)/Signal(IN)=−6.8 dB][Idler(OUT)/Signal(IN)=−6.8 dB] is among the highest reported in passive semiconductor or glass waveguides.
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Publication: Phys. Rev. Lett. 113, 084101 (2014)Doi: 10.1103/PhysRevLett.113.084101
The character of the time-asymptotic evolution of physical systems can have complex, singular behavior with variation of a system parameter, particularly when chaos is involved. A perturbation of the parameter by a small amount ε can convert an attractor from chaotic to nonchaotic or vice versa. We call a parameter value where this can happen ε uncertain. The probability that a random choice of the parameter is ε uncertain commonly scales like a power law in ε. Surprisingly, two seemingly similar ways of defining this scaling, both of physical interest, yield different numerical values for the scaling exponent. We show why this happens and present a quantitative analysis of this phenomenon.
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Publication: Nature Nanotechnol. 9, 1031 (2014)Doi: 10.1038/NNANO.2014.247
A single nanopore structure that embeds all components of an electrochemical storage device could bring about the ultimate miniaturization in energy storage. Self-alignment of electrodes within each nanopore may enable closer and more controlled spacing between electrodes than in state-of-art batteries. Such an ‘all-in-one’ nanopore battery array would also present an alternative to interdigitated electrode structures that employ complex three-dimensional geometries with greater spatial heterogeneity. Here, we report a battery composed of an array of nanobatteries connected in parallel, each composed of an anode, a cathode and a liquid electrolyte confined within the nanopores of anodic aluminium oxide, as an all-in-one nanosize device. Each nanoelectrode includes an outer Ru nanotube current collector and an inner nanotube of V2O5 storage material, forming a symmetric full nanopore storage cell with anode and cathode separated by an electrolyte region. The V2O5 is prelithiated at one end to serve as the anode, with pristine V2O5 at the other end serving as the cathode, forming a battery that is asymmetrically cycled between 0.2 V and 1.8 V. The capacity retention of this full cell (relative to 1 C values) is 95% at 5 C and 46% at 150 C, with a 1,000-cycle life. From a fundamental point of view, our all-in-one nanopore battery array unveils an electrochemical regime in which ion insertion and surface charge mechanisms for energy storage become indistinguishable, and offers a testbed for studying ion transport limits in dense nanostructured electrode arrays.
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Publication: Nature Nanotechnol. 9, 814 (2014)Doi: 10.1038/NNANO.2014.182
Terahertz radiation has uses in applications ranging from security to medicine. However, sensitive room-temperature detection of terahertz radiation is notoriously difficult. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism; photoexcited carriers rapidly thermalize due to strong electron–electron interactions, but lose energy to the lattice more slowly. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating or dissimilar contact metals produces a net current via the thermoelectric effect. Here, we demonstrate a graphene thermoelectric terahertz photodetector with sensitivity exceeding 10 V W–1 (700 V W–1) at room temperature and noise-equivalent power less than 1,100 pW Hz–1/2 (20 pW Hz–1/2), referenced to the incident (absorbed) power. This implies a performance that is competitive with the best room-temperature terahertz detectors for an optimally coupled device, and time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.
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Publication: Phys. Rev. A 89, 033418 (2014)Doi: 10.1103/PhysRevA.89.033418
A nonlocal quantum-mechanical model is presented for calculating the atomic dielectric response to a strong laser electric field. By replacing the Coulomb potential with a nonlocal potential in the Schrödinger equation, a 3 + 1-dimensional calculation of the time-dependent electric dipole moment can be reformulated as a 0 + 1-dimensional integral equation that retains the three-dimensional dynamics, while offering significant computational savings. The model is benchmarked against an established ionization model and ab initio simulation of the time-dependent Schrödinger equation. The reduced computational overhead makes the model a promising candidate to incorporate full quantum-mechanical time dynamics in laser pulse propagation simulations.
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Publication: IEEE J. Photovoltaics 4, 233 (2014)Doi: 10.1109/JPHOTOV.2013.2286522
The limiting efficiency for photovoltaic energy conversion based on a semiconductor p-n junction is typically determined using the method of detailed balance put forth by Shockley and Queisser. Here, we describe how this theory is altered in the presence of a photonic structure that is capable of modifying the absorption and emission of photons and optimize a device with optical loss. By incorporating specifically designed photonic structures, higher maximum efficiencies can be achieved for low bandgap materials by restricting the absorption and emission of above bandgap photons. Similarly, restriction of the emission angle leads to increased optical concentration. We consider how both of these effects are modified in the presence of a nonideal photonic structure. Further, we find that the energy of the photonic bandgap that is needed for maximum efficiency depends critically on the reflectivity of the photonic crystal.
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Publication: J. Appl. Phys. 116, 064902 (2014)Doi: 10.1063/1.4892568
In this paper, an analysis of a remote atmospheric magnetometry concept is considered, using molecular oxygen as the paramagnetic species. The objective is to use this mechanism for the remote detection of underwater and underground objects. Kerr self-focusing is used to bring a polarized, high-intensity, laser pulse to focus at a remote detection site where the laser pulse induces a ringing in the oxygen magnetization current. This current creates a co-propagating electromagnetic field behind the laser pulse, i.e., the wakefield, which has a rotated polarization that depends on the background magnetic field. The detection signature for underwater and underground objects is the change in the wakefield polarization between different measurement locations. The coupled Maxwell-density matrix equations are used to describe the oxygen magnetization in the presence of an intense laser pulse and ambient magnetic field. The magnetic dipole transition line that is considered is the b1Σ+g−X3Σ−g transition band of oxygen near 762 nm. The major challenges are the collisional dephasing of the atmospheric oxygen transitions and the strength of the effective magnetic dipole interaction.
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Publication: Biochem. Eng. J. 89, 2 (2014)Doi: 10.1016/j.bej.2013.12.013
Membrane functionality in microfluidics is critical for sample separation, concentration, compartmentalization, filtration, pumping, gradient generation, gas–liquid exchange, and other processes. Integration of functional membranes in microfluidics, however, is nontrivial. Here, we report a simple approach for biofabricating freestanding, semi-permeable biopolymer membranes in microfluidics, initiated with intentionally trapped air bubbles caught within specifically designed polydimethylsiloxane (PDMS) apertures. Pressure-driven dissipation of air bubbles through the gas permeable PDMS facilitates local and quiescent contact of two oppositely charged polyelectrolyte polysaccharides forming a layered or sandwiched membrane. This polyelectrolyte complex membrane (PECM) is permeable to ions including hydroxyl ions, which further facilitates layer-by-layer assembly of membrane stratum. Assembled membranes that bridge the 40-μm apertures are sufficiently strong to withstand >1 atmosphere hydrostatic pressure. Further, the semi-permeable membranes allow for programmed generation of small molecule gradients while preventing protein efflux. We envision the simplicity of fabrication, which requires no reagents or complicated valving, when coupled with the functional properties of the membrane polysaccharides, will find utility in cell and tissue studies including preclinical drug screening and toxicity analyses.
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Publication: Solid-State Electron. 101, 106 (2014)Doi: 10.1016/j.sse.2014.06.018
Pulsed laser deposition is suggested as a convenient method for fabrication of Bi2−xSbxTe3 thin films for p-type thermoelectric elements. However, challenges with controlling the stoichiometry and the microstructure of the films need to be addressed. Annealing of the films in an environment of nitrogen and tellurium vapor provided a means to producing Bi2−xSbxTe3 thin films with power factor values similar or greater than bulk materials. Films deposited at 2 mTorr, 375 °C with a laser power of 1.6 W were metal-rich and disordered, with small negative Seebeck coefficients. Upon annealing these films become single phase with a stoichiometry close to 2:3, textured with the basal plane parallel to the substrate, and exhibit excellent p-type thermoelectric characteristics. Interestingly, using this particular deposition and annealing sequence no secondary phases (e.g. crystalline tellurium) are formed.
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Publication: EPS 107, 60006 (2014)Doi: 10.1209/0295-5075/107/60006
Employing the Kuramoto model as an illustrative example, we show how the use of the mean-field approximation can be applied to large networks of phase oscillators with assortativity. We then use the ansatz of Ott and Antonsen (Chaos, 19 (2008) 037113) to reduce the mean-field kinetic equations to a system of ordinary differential equations. The resulting formulation is illustrated by application to a network Kuramoto problem with degree assortativity and correlation between the node degrees and the natural oscillation frequencies. Good agreement is found between the solutions of the reduced set of ordinary differential equations obtained from our theory and full simulations of the system. These results highlight the ability of our method to capture all the phase transitions (bifurcations) and system attractors. One interesting result is that degree assortativity can induce transitions from a steady macroscopic state to a temporally oscillating macroscopic state through both (presumed) Hopf and SNIPER (saddle-node, infinite period) bifurcations. Possible use of these techniques to a broad class of phase oscillator network problems is discussed.
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Publication: Opt. Exp. 22, A259 (2014)Doi: 10.1364/OE.22.00A259
We propose a polymer photovoltaic device with a new scattering mechanism based on photon absorption and re-emission in a quantum dot layer. A matrix of aluminum nanorods with optimized radius and period are used to modify the coupling of light emitted from the quantum dots into the polymer layer. Our analysis shows that this architecture is capable of increasing the absorption of an ordinary polymer photovoltaic device by 28%.
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Publication: Phys. Plasmas 21, 093114 (2014)Doi: 10.1063/1.4896709
This study was motivated by some experiments in which it was found that during the voltage rise, instead of expected excitation of a high-frequency parasitic mode, the excitation of a lower-frequency parasitic mode takes place in a certain range of voltages. For explaining this fact, the dependence of start currents of possible competing modes on the beam voltage was carried out in the cold-cavity approximation and by using the self-consistent approach. It was found that in the case of cavities, which consist of the combination of a section of constant radius waveguide and a slightly uptapered waveguide, these two approaches yield completely different results. Thus, experimentally observed excitation of the low-frequency parasitic mode can be explained by the self-consistent modification of the axial profile of the excited field, which has strong influence on the diffractive quality factor of competing modes. This modification is especially pronounced in the case of excitation of modes with many axial variations which can be excited in the region of beam interaction with the backward-wave component of such modes.
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Publication: Rev. Sci. Instrum. 85, 073705 (2014)Doi: 10.1063/1.4886811
Cryogenic fluid flows including liquid nitrogen and superfluid helium are a rich environment for novel scientific discovery. Flows can be measured optically and dynamically when faithful tracer particles are dispersed in the liquid. We present a reliable technique for dispersing commercially available fluorescent nanoparticles into cryogenic fluids using ultrasound. Five types of fluorescent nanoparticles ranging in size from 5 nm to 1 μm were imaged in liquid nitrogen and superfluid helium, and were tracked at frame rates up to 100 Hz.
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Publication: Phys. Plasmas 21, 122902 (2014)Doi: 10.1063/1.4904203
Electron bulk heating during magnetic reconnection with symmetric inflow conditions is examined using kinetic particle-in-cell simulations. Inflowing plasma parameters are varied over a wide range of conditions, and the increase in electron temperature is measured in the exhaust well downstream of the x-line. The degree of electron heating is well correlated with the inflowing Alfvén speed cAr based on the reconnecting magnetic field through the relation ΔTe=0.033 mi c2Ar, where ΔTe is the increase in electron temperature. For the range of simulations performed, the heating shows almost no correlation with inflow total temperature Ttot=Ti+Te or plasma β. An out-of-plane (guide) magnetic field of similar magnitude to the reconnecting field does not affect the total heating, but it does quench perpendicular heating, with almost all heating being in the parallel direction. These results are qualitatively consistent with a recent statistical survey of electron heating in the dayside magnetopause (Phan et al., Geophys. Res. Lett. 40, 4475, 2013), which also found that ΔTe was proportional to the inflowing Alfvén speed. The net electron heating varies very little with distance downstream of the x-line. The simulations show at most a very weak dependence of electron heating on the ion to electron mass ratio. In the antiparallel reconnection case, the largely parallel heating is eventually isotropized downstream due a scattering mechanism, such as stochastic particle motion or instabilities. The simulation size is large enough to be directly relevant to reconnection in the Earth's magnetosphere, and the present findings may prove to be universal in nature with applications to the solar wind, the solar corona, and other astrophysical plasmas. The study highlights key properties that must be satisfied by an electron heating mechanism: (1) preferential heating in the parallel direction; (2) heating proportional to mi c2Ar; (3) at most a weak dependence on electron mass; and (4) an exhaust electron temperature that varies little with distance from the x-line.
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Publication: Phys. Plasmas 21, 063112 (2014)Doi: 10.1063/1.4886141
Development of MW-class gyrotrons for future controlled fusion reactors requires careful analysis of the stability of high efficiency operation in very high-order modes. In the present paper, this problem is analyzed in the framework of the non-stationary self-consistent theory of gyrotrons. Two approaches are used: the one based on the wave envelope representation of the resonator field and the second one based on representation of this field as a superposition of eigenmodes, whose fields are determined by a self-consistent set of equations. It is shown that at relatively low beam currents, when the maximum efficiency can be realized in the regime of soft self-excitation, the operation in the desired mode is stable even in the case of a very dense spectrum of competing modes. At higher currents, the maximum efficiency can be realized in the regimes with hard self-excitation; here the operation in the desired mode can be unstable because of the presence of some competing modes with low start currents. Two 170 GHz European gyrotrons for the international thermonuclear experimental reactor are considered as examples. In the first one, which is the 2 MW gyrotron with a coaxial resonator, the stability of operation in a chosen TE34,19-mode in the presence of two sideband modes with almost equidistant spectrum is analyzed and the region of magnetic fields in which the oscillations of the central mode are stable is determined. The operation of the second gyrotron, which is the 1 MW gyrotron with a cylindrical cavity currently under development in Europe, is studied by using the wave envelope approach. It is shown that high efficiency operation of this gyrotron in the TE32,9-mode should be stable.
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Publication: J. Geophys. Res. - Solid Earth 119, 4538 (2014)Doi: 10.1002/2013JB010733
We present new experimental results from the University of Maryland Three Meter Geodynamo experiment. We drive a fully turbulent flow in water and also in sodium at magnetic Reynolds number Rm = ΔΩ(ro−ri)2/η, up to 715 (about half design maximum) in a spherical Couette apparatus geometrically similar to Earth's core. We have not yet observed a self-generating dynamo, but we study MHD effects with an externally applied axisymmetric magnetic field. We survey a broad range of Rossby number −68 < Ro = ΔΩ/Ωo< 65 in both purely hydrodynamic water experiments and sodium experiments with weak, nearly passive applied field. We characterize angular momentum transport and substantial generation of internal toroidal magnetic field (the Ω effect) as a function of Ro and find a rich dependence of both angular momentum transport and Ω effect on Ro. Internal azimuthal field generation peaks at Ro = 6 with a gain as high as 9 with weak applied field. At this Rossby number, we also perform experiments with significant Lorentz forces by increasing the applied magnetic field. We observe a reduction of the Ω effect, a large increase in angular momentum transport, and the onset of new dynamical states. The state we reach at maximum applied field shows substantial magnetic field gain in the axial dipole moment, enhancing the applied dipole moment. This intermittent dipole enhancement must come from nonaxisymmetric flow and seems to be a geodynamo-style feedback involving differential rotation and large-scale drifting waves.
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Publication: IEEE Trans. Plasma Sci. 42, 1586 (2014)Doi: 10.1109/TPS.2014.2322674
In this paper, the theory describing the processes in gyrotrons with misaligned electron beams is used for interpreting some experimental results obtained in the FU IV A, 335-GHz gyrotron operating at the fundamental cyclotron resonance. The theory describes the effect of the beam misalignment on the single-mode operation in the small-signal (start currents) and large-signal (efficiency) regimes. It also describes the effect of the beam misalignment on the interaction between two oppositely rotating modes. Comparison of theoretical and experimental results demonstrates good qualitative agreement; in particular, both of them show that 0.3-mm misalignment of the beam axis causes efficiency degradation by a factor of two.
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Publication: IEEE Trans. Electron Devices 61, 1655 (2014)Doi: 10.1109/TED.2014.2304839
A planar sheath-like slow-wave structure with rectangular geometry is considered for use in a traveling-wave amplifier driven by a sheet electron beam. The sheath like structure provides a low-dispersion operating mode and hence potential for broad bandwidth. However, due to the large transverse dimension of the structure multiple backward-wave modes can interact with the beam. Both the operating mode and the parasitic modes are analyzed using field theories with the planar sheath approximation. These solutions are then compared with finite element computations. Suppression of backward waves is then considered by designing the structure to preferentially absorb these waves. The results show a good control of mode competition and high-primary mode gain.
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Publication: Phys. Plasmas 21, 055402 (2014)Doi: 10.1063/1.4878356
This paper presents overviews of a number of processes and applications associated with high-power, high-intensity lasers, and their interactions. These processes and applications include: free electron lasers, backward Raman amplification, atmospheric propagation of laser pulses, laser driven acceleration, atmospheric lasing, and remote detection of radioactivity. The interrelated physical mechanisms in the various processes are discussed.
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Publication: Phys. Rev. ST - Accelertors and Beams 17, 122803 (2014)Doi: 10.1103/PhysRevSTAB.17.122803
We propose a design for a minimally perturbing diagnostic minichicane, which utilizes optical synchrotron radiation (OSR) generated from magnetic bends in the chicane, to measure the rms horizontal and vertical beam sizes, divergences, emittances, Twiss parameters and energy spread of a relativistic electron beam. The beam is externally focused to a waist at the first bend and the OSR generated there can be used to measure the rms beam size. Subsequent pairs of bends produce far field OSR interferences (OSRI) whose visibility depends on the beam energy spread and the angular divergence. Under proper conditions, one of these two effects will dominate the OSRI visibility from a particular pair of bends and can be used to diagnose the dominant effect. The properties of different configuration of bends in the chicane have been analyzed to provide an optimum diagnostic design for a given set of beam parameters to: (1) provide a sufficient number of OSR interferences to allow a measurement of the fringe visibility; (2) minimize the effect of coherent synchrotron radiation and space charge forces on the particles motion; and (3) minimize the effect of compression on the bunch length as the beam passes through the chicane. A design for the chicane has been produced for application to the FERMI free electron laser facility and by extension to similar high brightness linear accelerators. Such a diagnostic promises to greatly improve control of the electron beam optics with a noninvasive measurement of beam parameters and allow on-line optics matching and feedback.
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Publication: Sci. Rep. 4, 7047 (2014)Doi: 10.1038/srep07047
Networks in nature are often formed within a spatial domain in a dynamical manner, gaining links and nodes as they develop over time. Motivated by the growth and development of neuronal networks, we propose a class of spatially-based growing network models and investigate the resulting statistical network properties as a function of the dimension and topology of the space in which the networks are embedded. In particular, we consider two models in which nodes are placed one by one in random locations in space, with each such placement followed by configuration relaxation toward uniform node density and connection of the new node with spatially nearby nodes. We find that such growth processes naturally result in networks with small-world features, including a short characteristic path length and nonzero clustering. We find no qualitative differences in these properties for two different topologies and we suggest that results for these properties may not depend strongly on the topology of the embedding space. The results do depend strongly on dimension and higher-dimensional spaces result in shorter path lengths but less clustering.
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Publication: Phys. Rev. Lett. 113, 027403 (2014)Doi: 10.1103/PhysRevLett.113.027403
We demonstrate resonant coupling of a Mollow triplet sideband to an optical cavity in the strong coupling regime. We show that, in this regime, the resonant sideband is strongly enhanced relative to the detuned sideband. Furthermore, the linewidth of the Mollow sidebands exhibits a highly nonlinear pump power dependence when tuned across the cavity resonance due to strong resonant interactions with the cavity mode. We compare our results to calculations using the effective phonon master equation and show that the nonlinear linewidth behavior is caused by strong coherent interaction with the cavity mode that exists only when the Mollow sideband is near cavity resonance.
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Publication: Chaos 24, 033107 (2014)Doi: 10.1063/1.4887275
Slow parameter drift is common in many systems (e.g., the amount of greenhouse gases in the terrestrial atmosphere is increasing). In such situations, the attractor on which the system trajectory lies can be destroyed, and the trajectory will then go to another attractor of the system. We consider the case where there are more than one of these possible final attractors, and we ask whether we can control the outcome (i.e., the attractor that ultimately captures the trajectory) using only small controlling perturbations. Specifically, we consider the problem of controlling a noisy system whose parameter slowly drifts through a saddle-node bifurcation taking place on a fractal boundary between the basins of multiple attractors. We show that, when the noise level is low, a small perturbation of size comparable to the noise amplitude applied at a single point in time can ensure that the system will evolve toward a target attracting state with high probability. For a range of noise levels, we find that the minimum size of perturbation required for control is much smaller within a time period that starts some time after the bifurcation, providing a “window of opportunity” for driving the system toward a desirable state. We refer to this procedure as tipping point control.
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Publication: Phys. Rev. E 90, 022814 (2014)Doi: 10.1103/PhysRevE.90.022814
We study the stability of orbits in large Boolean networks. We treat the case in which the network has a given complex topology, and we do not assume a specific form for the update rules, which may be correlated with local topological properties of the network. While recent past work has addressed the separate effects of complex network topology and certain classes of update rules on stability, only crude results exist about how these effects interact. We present a widely applicable solution to this problem. Numerical simulations confirm our theory and show that local correlations between topology and update rules can have profound effects on the qualitative behavior of these systems.
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Publication: J. Vac. Sci. Technol. B 32, 041206Doi: 10.1116/1.4889858
The authors studied the influence of isotopes on the Ar/H2 and Ar/D2 plasmas using Langmuir probe and ion mass analyzer measurements at several pressures relevant to low temperature plasma surface processing. As up to 50% H2 is added to Ar plasma, electron energy distribution functions show an increase in electron temperature (from 2.5 eV to 3 eV for 30 mTorr with 50% addition) and a decrease in electron density (2.5 × 1011 cm−3 → 2.5 × 1010 cm−3 at 30 mTorr with 50% addition). At lower pressures (5 and 10 mTorr), these effects are not as pronounced. This change in electron properties is very similar for Ar/D2 plasmas due to similar electron cross-sections for H2 and D2. Ion types transition from predominantly Ar+ to molecular ions ArH+/H3+ and ArD+/D3+ with the addition of H2 and D2 to Ar, respectively. At high pressures and for the heavier isotope addition, this transition to molecular ions is much faster. Higher pressures increase the ion–molecules collision induced formation of the diatomic and triatomic molecular ions due to a decrease in gaseous mean-free paths. The latter changes are more pronounced for D2 addition to Ar plasma due to lower wall-loss of ions and an increased reaction rate for ion–molecular interactions as compared to Ar/H2. Differences in plasma species are also seen in the etching behavior of amorphous hydrocarbon films in both Ar/H2 and Ar/D2 plasma chemistries. D2 addition to Ar plasma shows a larger increase in etch rate than H2 addition.
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Publication: Plasma Phys. Control. Fusion 56, 045005 (2014)Doi: 10.48550/arXiv.1312.2148
Conventional radially-local neoclassical calculations become inadequate if the radial gradient scale lengths of the H-mode pedestal become as small as the poloidal ion gyroradius. Here, we describe a radially global δf continuum code that generalizes neoclassical calculations to allow stronger gradients. As with conventional neoclassical calculations, the formulation is time-independent and requires only the solution of a single sparse linear system. We demonstrate precise agreement with an asymptotic analytic solution of the radially global kinetic equation in the appropriate limits of aspect ratio and collisionality. This agreement depends crucially on accurate treatment of finite orbit width effects.
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Publication: Opt. Mater. Exp. 4, 300 (2014)Doi: 10.1364/OME.4.000300
We have investigated the broadband terahertz (THz) optical properties of nanoporous silicon samples with different porosities and the ultrafast carrier dynamics of photogenerated charge carriers in these materials. Following photoexcitation, we observe a fast carrier recovery time consisting of two dominant recombination processes with decay constants below ~10 ps. All samples exhibit initially low THz absorption that increases at higher frequencies, and is likely due to contributions from phonon bands and oxidation of the porous surface. The refractive index depends on porosity but shows little frequency dependence. These properties indicate that nanoporous silicon is a useful material for fast, ultrabroadband THz applications (e.g. intensity modulation).
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Publication: J. Infrared Millim. Terahertz Waves 35, 325 (201)Doi: 10.1007/s10762-014-0050-7
Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.
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Publication: Opt. Exp. 22, 17466 (2014)Doi: 10.1364/OE.22.017466
The nonlinear response of nanoporous silicon optical waveguides is investigated using a novel pump-probe method. In this approach we use a two-frequency heterodyne technique to measure the pump-induced transient change in phase and intensity in a single measurement. We measure a 100 picosecond material response time and report behavior matching a physical model dominated by free-carrier effects significantly stronger than those observed in traditional silicon-based waveguides.
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Publication: IEEE Trans. Plasma Sci. 42, 1321 (2014)Doi: 10.1109/TPS.2014.2308546
The Warp code (and its framework of associated tools) was initially developed for particle-in-cell simulations of space-charge-dominated ion beams in accelerators, for heavy-ion-driven inertial fusion energy, and related experiments. It has found a broad range of applications, including nonneutral plasmas in traps, stray electron clouds in accelerators, laser-based acceleration, and the focusing of ion beams produced when short-pulse lasers irradiate foil targets. We summarize novel methods used in Warp, including: time-stepping conducive to diagnosis and particle injection; an interactive Python-Fortran-C structure that enables scripted and interactive user steering of runs; a variety of geometries (3-D x, y, z; 2-D r, z; 2-D x, y); electrostatic and electromagnetic field solvers; a cut-cell representation for internal boundaries; the use of warped coordinates for bent beam lines; adaptive mesh refinement, including a capability for time-dependent space-charge-limited flow from curved surfaces; models for accelerator lattice elements (magnetic or electrostatic quadrupole lenses, accelerating gaps, etc.) at user-selectable levels of detail; models for particle interactions with gas and walls; moment/envelope models that support sophisticated particle loading; a drift-Lorentz mover for rapid tracking through regions of strong and weak magnetic field; a Lorentz-boosted frame formulation with a Lorentz-invariant modification of the Boris mover; an electromagnetic solver with tunable dispersion and stride-based digital filtering; and a pseudospectral electromagnetic solver. Warp has proven useful for a wide range of applications, described very briefly herein. It is available as an open-source code under a BSD license. This paper describes material presented during the Prof. Charles K. (Ned) Birdsall Memorial Session of the 2013 IEEE Pulsed Power and Plasma Science Conference. In addition to our overview of the computational methods used in Warp, we summarize a few aspects of Ned's contributions to plasma simulation and to the careers of those he mentored.
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Publication: J. Phys. Chem. C 118, 27749 (2014)Doi: 10.1021/jp509298r
We demonstrate the ultraclean atomic layer deposition (ALD) of Li2O and LiOH using lithium tert-butoxide (LiOtBu) precursor with H2O and plasma O2 as oxidants, along with conversion of Li2O and LiOH products to Li2CO3 upon CO2 dosing. Using LiOtBu and H2O results in LiOH below 240 °C and Li2O above 240 °C for otherwise identical process parameters. Substituting plasma O2 as the oxidation precursor results in a combination of Li2CO3 and Li2O products, indicating modification of the ALD reaction preventing volatilization of the C from the Li precursor. The chemistry of the films is definitively characterized for the first time with XPS utilizing an all-UHV transfer procedure from the ALD reactor. We use in situ UHV gas dosing to investigate the reaction mechanisms of ALD Li2O and LiOH with H2O and CO2 to simulate reactions upon air exposure. Lastly, we employ in situ spectroscopic ellipsometry to determine the reaction kinetics of thermal LiOH decomposition, and we report an activation energy of 112.7 ± 0.6 kJ/mol.
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Publication: Opt. Exp. 22, 26820 (2014)Doi: 10.1364/OE.22.026814
Enhancements of the continuous-wave four-wave mixing conversion efficiency and bandwidth are accomplished through the application of plasma-assisted photoresist reflow to reduce the sidewall roughness of sub-square-micron-modal area waveguides. Nonlinear AlGaAs optical waveguides with a propagation loss of 0.56 dB/cm demonstrate continuous-wave four-wave mixing conversion efficiency of −7.8 dB. Narrow waveguides that are fabricated with engineered processing produce waveguides with uncoated sidewalls and anti-reflection coatings that show group velocity dispersion of +0.22 ps2/m. Waveguides that are 5-mm long demonstrate broadband four-wave mixing conversion efficiencies with a half-width 3-dB bandwidth of 63.8-nm.
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Publication: Phys. Plasmas 21, 013108 (2014)Doi: 10.1063/1.4862779
This study was motivated by a new concept of remote detection of concealed radioactive materials by using a high power terahertz (THz) wave beam, which can be focused in a small spot where the wave electric field exceeds the breakdown threshold. In the presence of seed electrons in such a volume, this focusing can initiate the avalanche breakdown. Typically, an ambient density of free electrons is assumed to be at the level of one particle per cubic centimeter. So, when a breakdown-prone volume is smaller than 1 cm3, there should be significant difference between the breakdown rates in the case of presence of additional sources of ionization versus its absence. Since the flux density of gamma rays emitted by radioactive materials rapidly falls with the distance from the source, while the intensity of THz waves also decreases with the distance due to wave attenuation in the atmosphere, it is important to find an optimal location of the breakdown to be initiated for a given distance between a radioactive material and a THz antenna. This problem is analyzed in a given paper with the account for not only atmospheric attenuation of THz waves but also the air turbulence.
2013
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Publication: Phys. Rev. E 87, 052127 (2013)Doi: 10.1103/PhysRevE.87.052127
Percolation, the formation of a macroscopic connected component, is a key feature in the description of complex networks. The dynamical properties of a variety of systems can be understood in terms of percolation, including the robustness of power grids and information networks, the spreading of epidemics and forest fires, and the stability of gene regulatory networks. Recent studies have shown that if network edges are added “competitively” in undirected networks, the onset of percolation is abrupt or “explosive.” The unusual qualitative features of this phase transition have been the subject of much recent attention. Here we generalize this previously studied network growth process from undirected networks to directed networks and use finite-size scaling theory to find several scaling exponents. We find that this process is also characterized by a very rapid growth in the giant component, but that this growth is not as sudden as in undirected networks.
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Publication: Molecular Systems Biiology 9, 636 (2013)Doi: 10.1038/msb.2012.71
Escherichia coli were genetically modified to enable programmed motility, sensing, and actuation based on the density of features on nearby surfaces. Then, based on calculated feature density, these cells expressed marker proteins to indicate phenotypic response. Specifically, site-specific synthesis of bacterial quorum sensing autoinducer-2 (AI-2) is used to initiate and recruit motile cells. In our model system, we rewired E. coli's AI-2 signaling pathway to direct bacteria to a squamous cancer cell line of head and neck (SCCHN), where they initiate synthesis of a reporter (drug surrogate) based on a threshold density of epidermal growth factor receptor (EGFR). This represents a new type of controller for targeted drug delivery as actuation (synthesis and delivery) depends on a receptor density marking the diseased cell. The ability to survey local surfaces and initiate gene expression based on feature density represents a new area-based switch in synthetic biology that will find use beyond the proposed cancer model here.
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Publication: Astrophys. J. Lett. 775, L14 (2013)Doi: 10.1088/2041-8205/775/1/L14
A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.
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Publication: IEEE Trans. Plasma Sci. 41, 70 (2013)Doi: 10.1109/TPS.2012.2225113
In the circuits of high-power microwave (HPM) devices, such as HPM sources or high-gradient accelerating structures, small quantities of metallic dust may exist. These metallic particles of micrometer size immersed in high-RF fields can absorb enough energy for significant heating and melting. The heating effect of an RF magnetic field was analyzed in our previous papers. In this paper, we analyze the role of the RF electric field which may cause field emission from a microparticle. Our consideration is restricted by ellipsoidal metallic particles. It is shown that the heating effect becomes significant when such microparticles have a needlelike shape, and therefore, the electric field amplification at the ends is high. In this case, the field-emitted current may cause the heating of the microparticles. The heating in single shots and in repetition-rate regimes is studied for the case of the combined effect of both the RF magnetic and electric fields.
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Publication: ACS Nano 7, 2717 (2013)Doi: 10.1021/nn4001512
Interfacial instability is a fundamental issue in heterostructures ranging from biomaterials to joint replacement and electronic packaging. This challenge is particularly intriguing for lithium ion battery anodes comprising silicon as the ion storage material, where ultrahigh capacity is accompanied by vast mechanical stress that threatens delamination of silicon from the current collectors at the other side of the interface. Here, we describe Si-beaded carbon nanotube (CNT) strings whose interface is controlled by chemical functionalization, producing separated amorphous Si beads threaded along mechanically robust and electrically conductive CNT. In situ transmission electron microscopy combined with atomic and continuum modeling reveal that the chemically tailored Si–C interface plays important roles in constraining the Si beads, such that they exhibit a symmetric “radial breathing” around the CNT string, remaining crack-free and electrically connected throughout lithiation–delithiation cycling. These findings provide fundamental insights in controlling nanostructured interfaces to effectively respond to demanding environments such as lithium batteries.
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Publication: Geophys. Res. Lett. 40, 22 (2013)Doi: 10.1029/2012GL054701
We present full-particle simulations of 2-D magnetotail current sheet equilibria with open boundaries and zero driving. The simulations show that spontaneous formation of dipolarization fronts and subsequent formation of magnetic islands are possible in equilibria with an accumulation of magnetic flux at the tailward end of a sufficiently thin current sheet. These results confirm recent findings in the linear stability of the ion tearing mode, including the predicted dependence of the tail current sheet stability on the amount of accumulated magnetic flux expressed in terms of the specific destabilization parameter. The initial phase of reconnection onset associated with the front formation represents a process of slippage of magnetic field lines with frozen-in electrons relative to the ion plasma species. This non-MHD process characterized by different motions of ion and electron species generates a substantial charge separation electric field normal to the front.
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Publication: Optics Lett. 38, 1034 (2013)Doi: 10.1364/OL.38.001034
We investigate the mechanism of elliptically polarized terahertz (THz) pulse generation in femtosecond two-color laser-produced plasma. In the case of in-line laser focusing, we observe the THz polarization evolves from linear to elliptical with increasing plasma length. This ellipticity arises from two combined effects—successive polarization rotation of local THz plasma sources, caused by laser phase and polarization modulations, and the velocity mismatch between laser and THz, which produces an elliptical THz pulse from a series of time-delayed, polarization-rotating local THz fields.
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Publication: Phys. Rev. E 88, 062910 (2013)Doi: 10.1103/PhysRevE.88.062910
We consider time reversal of electromagnetic waves in a closed, wave-chaotic system containing a discrete, passive, harmonic-generating nonlinearity. An experimental system is constructed as a time-reversal mirror, in which excitations generated by the nonlinearity are gathered, time-reversed, transmitted, and directed exclusively to the location of the nonlinearity. Here we show that such nonlinear objects can be purely passive (as opposed to the active nonlinearities used in previous work), and we develop a higher data rate exclusive communication system based on nonlinear time reversal. A model of the experimental system is developed, using a star-graph network of transmission lines, with one of the lines terminated by a model diode. The model simulates time reversal of linear and nonlinear signals, demonstrates features seen in the experimental system, and supports our interpretation of the experimental results.
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Publication: Geophys. Res. Lett. 40, 4475 (2013)Doi: 10.1002/grl.50917
We surveyed 79 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount and anisotropy of electron bulk heating produced by reconnection depend on the inflow boundary conditions. We find that the amount of heating, ΔTe, is correlated with the asymmetric Alfvén speed, VAL,asym, based on the reconnecting magnetic field and the plasma density measured in both the high-density magnetosheath and low-density magnetospheric inflow regions. Best fit to the data produces the empirical relation ΔTe = 0.017 miVAL,asym2, indicating that the amount of heating is proportional to the inflowing magnetic energy per proton-electron pair, with ~1.7% of the energy being converted into electron heating. This finding, generalized to symmetric reconnection, could account for the lack of electron heating in typical solar wind exhausts at 1 AU, as well as strong heating to keV energies common in magnetotail exhausts. We also find that the guide field suppresses perpendicular heating.
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Publication: Appl. Phys. Lett. 102, 253105 (2013)Doi: 10.1063/1.4812750
We describe controlled, self-limited etching of a polystyrene polymer using a composite etching cycle consisting of sequential deposition of a thin reactive layer from precursors produced from a polymer-coated electrode within the etching chamber, modification using O2 exposure, and subsequent low-pressure Ar plasma etching, which removes the oxygen-modified deposited reactive layer along with ≈0.1 nm unmodified polymer. Deposition prevents net etching of the unmodified polymer during the etching step and enables self-limited etch rates of 0.1 nm/cycle.
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Publication: Lab on a Chip 13, 1854 (2013)Doi: 10.1039/c3lc50079a
We describe an innovation in the immobilization, culture, and imaging of cells in calcium alginate within microfluidic devices. This technique allows unprecedented optical access to the entirety of the calcium alginate hydrogel, enabling observation of growth and behavior in a chemical and mechanical environment favored by many kinds of cells.
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Publication: Optics Express 21, 29612 (2013)Doi: 10.1364/OE.21.029612
We demonstrate spontaneous emission rate enhancement and saturable absorption of cadmium selenide colloidal quantum dots coupled to a nanobeam photonic crystal cavity. We perform time-resolved lifetime measurements and observe an average enhancement of 4.6 for the spontaneous emission rate of quantum dots located at the cavity as compared to those located on an unpatterned surface. We also demonstrate that the cavity linewidth narrows with increasing pump intensity due to quantum dot saturable absorption.
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Publication: Fire Safety J. 58, 204 (2013)Doi: 10.1016/j.firesaf.2013.02.001
Sodium has unusual fire hazards, including autoignition when heated in air or exposed to liquid water. Owing to limitations of existing suppression agents for sodium pool fires, suppression using liquid nitrogen (LN2) is examined here. Sodium pools of 5–80 g were heated in stainless steel beakers. At about 290 °C, pool surface autoignition occurred and caused a rapid pool temperature increase. Vapor phase combustion occurred when the pools reached 320–450 °C, ultimately leading to pool temperatures up to 700 °C. For suppression tests, LN2 delivery (at 2.7 g/s) began when the fires became fully-developed, near a pool temperature of 600 °C. Liquid nitrogen was found to be an effective suppression agent. The minimum amount of LN2 required to suppress a fully-developed sodium pool fire was found to be about three times the initial sodium pool mass.
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Publication: Phys. Rev. Lett. 111, 198701 (2013)Doi: 10.1103/PhysRevLett.111.198701
In many complex systems, large events are believed to follow power-law, scale-free probability distributions so that the extreme, catastrophic events are unpredictable. Here, we study coupled chaotic oscillators that display extreme events. The mechanism responsible for the rare, largest events makes them distinct, and their distribution deviates from a power law. On the basis of this mechanism identification, we show that it is possible to forecast in real time an impending extreme event. Once forecasted, we also show that extreme events can be suppressed by applying tiny perturbations to the system.
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Publication: Astrophys. J. Lett. 774, L8 (2013)Doi: 10.1088/2041-8205/774/1/L8
The picture of the heliopause (HP)—the boundary between the domains of the Sun and the local interstellar medium (LISM)—as a pristine interface with a large rotation in the magnetic field fails to describe recent Voyager 1 (V1) data. Magnetohydrodynamic (MHD) simulations of the global heliosphere reveal that the rotation angle of the magnetic field across the HP at V1 is small. Particle-in-cell simulations, based on cuts through the MHD model at V1's location, suggest that the sectored region of the heliosheath (HS) produces large-scale magnetic islands that reconnect with the interstellar magnetic field while mixing LISM and HS plasma. Cuts across the simulation reveal multiple, anti-correlated jumps in the number densities of LISM and HS particles, similar to those observed, at the magnetic separatrices. A model is presented, based on both the observations and simulations, of the HP as a porous, multi-layered structure threaded by magnetic fields. This model further suggests that contrary to the conclusions of recent papers, V1 has already crossed the HP.
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Publication: Plasma Proc. Polymers 10, 167 (2013)Doi: 10.1002/ppap.201200087
It is widely accepted that plasma-generated energetic and reactive species are responsible for plasma-induced sterilization; however, how these species act alone or synergistically to deactivate endotoxic biomolecules is not completely understood. Using a vacuum beam system, we study the effects of vacuum ultraviolet (VUV) radiation, oxygen and deuterium radicals on lipid A, the immune-stimulating region of lipopolysaccharide. VUV-induced photolysis causes bulk modification of exposed lipid A film up to the penetration depth of VUV photons, ≈200 nm. Although radical-induced etch yield of lipid A is lower than VUV-induced photolysis, secondary ion mass spectrometry and human whole blood-based assay suggest that radicals render a higher degree of modification at the film surface. This study contributes to the fundamental understanding of plasma effects on biomolecules for a better deactivation scheme and applications.
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Publication: J. Mater. Chem. A 1, 8201 (2013)Doi: 10.1039/c3ta10972k
Paper, a low-cost and flexible substrate made from cellulose fiber, is explored in this study as a platform for fast electrochemical energy storage devices. Conductivity and Li-storage capabilities are introduced to the paper by functionalization with carbon nanotubes (CNTs) and V2O5, respectively. The Li-storage paper cathodes present a remarkably high rate performance due to the high conductivity of CNTs, short Li+ diffusion length in V2O5 nanocrystals, and more importantly the hierarchical porosity in paper for Li+ transport. The specific capacity of V2O5 is as high as 410 mA h g−1 at 1 C rate, and retains 116 mA h g−1 at a high rate of 100 C in the voltage range of 4.0–2.1 V. To understand the role of mesoporosity in individual cellulose fibers, we created a control structure by intentionally blocking the mesopores in paper with a 20 nm Al2O3 coating applied via atomic layer deposition (ALD). We found that the V2O5 capacity decreases by about 30% at high rates of 5–100 C after blocking, which serves to be the first confirmative evidence of the critical role of mesoporosity in paper fibers for high-rate electrochemical devices.
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Publication: Appl. Phys. Lett. 103, 151102 (2013)Doi: 10.1063/1.4824712
We demonstrate reversible strain-tuning of a quantum dot strongly coupled to a photonic crystal cavity. We observe an average redshift of 0.45 nm for quantum dots located inside the cavity membrane, achieved with an electric field of 15 kV/cm applied to a piezo-electric actuator. Using this technique, we demonstrate the ability to tune a quantum dot into resonance with a photonic crystal cavity in the strong coupling regime, resulting in a clear anti-crossing. The bare cavity resonance is less sensitive to strain than the quantum dot and shifts by only 0.078 nm at the maximum applied electric field.
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Publication: J. Comput. Phys. 250, 165 (2013)Doi: 10.1016/j.jcp.2013.05.020
We present improvements to the three-dimensional (3D) quasi-static particle-in-cell (PIC) algorithm, which is used to efficiently model short-pulse laser and particle beam–plasma interactions. In this algorithm the fields including the index of refraction created by a static particle/laser beam are calculated. These fields are then used to advance the particle/laser beam forward in time (distance). For a 3D quasi-static code, calculating the wake fields is done using a two-dimensional (2D) PIC code where the time variable is ξ = ct - z and z is the propagation direction of the particle/laser beam. When calculating the wake, the fields, particle positions and momenta are not naturally time centered so an iterative predictor corrector loop is required. In the previous iterative loop in QuickPIC (currently the only 3D quasi-static PIC code), the field equations are derived using the Lorentz gauge. Here we describe a new algorithm which uses gauge independent field equations. It is found that with this new algorithm, the results converge to the results from fully explicitly PIC codes with far fewer iterations (typically 1 iteration as compared to 2–8) for a wide range of problems. In addition, we describe a new deposition scheme for directly depositing the time derivative of the current that is needed in one of the field equations. The new deposition scheme does not require message passing for the particles inside the iteration loop, which greatly improves the speed for parallelized calculations. Comparisons of results from the new and old algorithms and to fully explicit PIC codes are also presented.
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Publication: PLOS One 8, e77944 (2013)Doi: 10.1371/journal.ppone.0077944
Epigenetic modifications to histones may promote either activation or repression of the transcription of nearby genes. Recent experimental studies show that the promoters of many lineage-control genes in stem cells have “bivalent domains” in which the nucleosomes contain both active (H3K4me3) and repressive (H3K27me3) marks. It is generally agreed that bivalent domains play an important role in stem cell differentiation, but the underlying mechanisms remain unclear. Here we formulate a mathematical model to investigate the dynamic properties of histone modification patterns. We then illustrate that our modeling framework can be used to capture key features of experimentally observed combinatorial chromatin states.
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Publication: Nanotechnol. 24, 045305 (2013)Doi: 10.1088/0957-4484/24/4/045305
We present a novel and simple method for forming hexagonal gold nanoparticle arrays that uses Coulombic interactions between negatively charged gold nanoparticles on positively charged vertically oriented poly(4-vinylpyridine) cylinders formed in a spin cast polystyrene-b-poly(4-vinylpyridine) block copolymer film. Exposure of the block copolymer film to dibromobutane vapor quaternizes and crosslinks the poly(4-vinylpyridine) domains which allows for the templated deposition of gold nanoparticles into a self-assembled hexagonal array through electrostatic interactions. These systems can form the basis for sensors or next generation nanoparticle based electronics.
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Publication: Phys. Plasmas 20, 061207 (2013)Doi: 10.1063/14811120
Recently, “phase diagrams” of magnetic reconnection were developed to graphically organize the present knowledge of what type, or phase, of reconnection is dominant in systems with given characteristic plasma parameters. Here, a number of considerations that require caution in using the diagrams are pointed out. First, two known properties of reconnection are omitted from the diagrams: the history dependence of reconnection and the absence of reconnection for small Lundquist number. Second, the phase diagrams mask a number of features. For one, the predicted transition to Hall reconnection should be thought of as an upper bound on the Lundquist number, and it may happen for considerably smaller values. Second, reconnection is never “slow,” it is always “fast” in the sense that the normalized reconnection rate is always at least 0.01. This has important implications for reconnection onset models. Finally, the definition of the relevant Lundquist number is nuanced and may differ greatly from the value based on characteristic scales. These considerations are important for applications of the phase diagrams. This is demonstrated by example for solar flares, where it is argued that it is unlikely that collisional reconnection can occur in the corona.
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Publication: Expeprimental Thermal Fluid Sci. 51, 183 (2013)Doi: 10.1016/j.expthermflusci.2013.07.013
This study is interested in determining the experimental relation between the suction pressure and evaporation rate from the upper surface of a flat, thin porous membrane, which naturally draws water from a reservoir, and its microchannel feeding system. The effects of three main design parameters of a water delivery system on the evaporation rate of the membrane are considered: (i) the diameter of the microchannels irrigating the membrane, (ii) the length of the irrigating microchannels, and (iii) the surface area of the membrane. Additionally, we also evaluated the effect of the pumping height (i.e., the vertical distance between the membrane and the main reservoir) on the evaporation rates for the three design parameters. While the maximum evaporation rate from the membrane is a function of the membrane’s properties (e.g., permeability and porosity), as well as the ambient conditions (e.g., temperature, pressure and humidity), this study focused on determining the geometric parameters of closed water-feeding microchannels that properly hydrate a porous membrane while not impeding evaporation. Results indicated that the evaporation rate was mostly unaffected by the channel dimensions considered. Moreover, evaporation rates increased with increasing surface area (between 20.3 cm2 and 176.7 cm2) but at a decreasing rate of return. Finally, the suction pressures achieved were inversely related to hydrodynamic pressure drop and were unaffected by the membrane diameter.
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Publication: Nano Lett. 13, 3936 (2013)Doi: 10.1021/nl402059u
Synthetic nanostructures, such as nanoparticles and nanowires, can serve as modular building blocks for integrated nanoscale systems. We demonstrate a microfluidic approach for positioning, orienting, and assembling such nanostructures into nanoassemblies. We use flow control combined with a cross-linking photoresist to position and immobilize nanostructures in desired positions and orientations. Immobilized nanostructures can serve as pivots, barriers, and guides for precise placement of subsequent nanostructures.
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Publication: Chaos 23, 033116 (2013)Doi: 10.1063/1.4816361
This paper addresses the behavior of large systems of heterogeneous, globally coupled oscillators each of which is described by the generic Landau-Stuart equation, which incorporates both phase and amplitude dynamics of individual oscillators. One goal of our paper is to investigate the effect of a spread in the amplitude growth parameter of the oscillators and of the effect of a homogeneous nonlinear frequency shift. Both of these effects are of potential relevance to recently reported experiments. Our second goal is to gain further understanding of the macroscopic system dynamics at large coupling strength, and its dependence on the nonlinear frequency shift parameter. It is proven that at large coupling strength, if the nonlinear frequency shift parameter is below a certain value, then there is a unique attractor for which the oscillators all clump at a single amplitude and uniformly rotating phase (we call this a single-cluster "locked state"). Using a combination of analytical and numerical methods, we show that at higher values of the nonlinear frequency shift parameter, the single-cluster locked state attractor continues to exist, but other types of coexisting attractors emerge. These include two-cluster locked states, periodic orbits, chaotic orbits, and quasiperiodic orbits.
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Publication: PLOS Comput. Biol. 9, e1003041 (2013)Doi: 10.1371/journal.pcbi.1003041
We develop a coarse-grained stochastic model for the influence of signal relay on the collective behavior of migrating Dictyostelium discoideum cells. In the experiment, cells display a range of collective migration patterns, including uncorrelated motion, formation of partially localized streams, and clumping, depending on the type of cell and the strength of the external, linear concentration gradient of the signaling molecule cyclic adenosine monophosphate (cAMP). From our model, we find that the pattern of migration can be quantitatively described by the competition of two processes, the secretion rate of cAMP by the cells and the degradation rate of cAMP in the gradient chamber. Model simulations are compared to experiments for a wide range of strengths of an external linear-gradient signal. With degradation, the model secreting cells form streams and efficiently transverse the gradient, but without degradation, we find that model secreting cells form clumps without streaming. This indicates that the observed effective collective migration in streams requires not only signal relay but also degradation of the signal. In addition, our model allows us to detect and quantify precursors of correlated motion, even when cells do not exhibit obvious streaming.
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Publication: Chaos 23, 043117 (2013)Doi: 10.1063/1.4829626
We experimentally study the complex dynamics of a unidirectionally coupled ring of four identical optoelectronic oscillators. The coupling between these systems is time-delayed in the experiment and can be varied over a wide range of delays. We observe that as the coupling delay is varied, the system may show different synchronization states, including complete isochronal synchrony, cluster synchrony, and two splay-phase states. We analyze the stability of these solutions through a master stability function approach, which we show can be effectively applied to all the different states observed in the experiment. Our analysis supports the experimentally observed multistability in the system.
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Publication: Phys. Plasmas 20, 061205 (2013)Doi: 10.1063/1.4811137
The nonlinear development of the strong Buneman instability and the associated fast electron heating in thin current layers with Ωe/ωpe<1 are explored. Phase mixing of the electrons in wave potential troughs and a rapid increase in temperature are observed during the saturation of the instability. We show that the motion of trapped electrons can be described using a Hamiltonian formalism in the adiabatic approximation. The process of separatrix crossing as electrons are trapped and de-trapped is irreversible and guarantees that the resulting electron energy gain is a true heating process.
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Publication: New J. Phys. 15, 085031 (2013)Doi: 10.1088/1367-2630/15/8/085031
We experimentally demonstrate a three-dimensional plasmonic terahertz waveguide by lithographically patterning an array of sub-wavelength pillars on a silicon substrate. Doped silicon can exhibit conductive properties at terahertz frequencies, making it a convenient substitute for conventional metals in plasmonic devices. However, the surface wave solution at a doped silicon surface is usually poorly confined and lossy. Here we demonstrate that by patterning the silicon surface with an array of sub-wavelength pillars, the resulting structure can support a terahertz surface mode that is tightly confined in both transverse directions. Further, we observe that the resonant behavior associated with the surface modes depends on the dimensions of the pillars, and can be tailored through control of the structural parameters. We experimentally fabricated devices with different geometries, and characterized the performance using terahertz time-domain spectroscopy. The resulting waveguide characteristics are confirmed using finite element numerical simulations, and we further show that a simple one-dimensional analytical theory adequately predicts the observed dispersion relation.
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Publication: Nature Photon. 7, 373 (2013)Doi: 10.1038/NPHOTON.2013.48
Integrated quantum photonics provides a promising route towards scalable solid-state implementations of quantum networks, quantum computers, and ultra-low power opto-electronic devices. A key component for many of these applications is the photonic quantum logic gate, where the quantum state of a solid-state quantum bit (qubit) conditionally controls the state of a photonic qubit. These gates are crucial for development of robust quantum networks, non-destructive quantum measurements, and strong photon-photon interactions. Here we experimentally realize a quantum logic gate between an optical photon and a solid-state qubit. The qubit is composed of a quantum dot (QD) strongly coupled to a nano-cavity, which acts as a coherently controllable qubit system that conditionally flips the polarization of a photon on picosecond timescales, implementing a controlled-NOT (cNOT) gate. Our results represent an important step towards solid-state quantum networks and provide a versatile approach for probing QD-photon interactions on ultra-fast timescales.
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Publication: Phys. Rev. E 87, 062906 (2013)Doi: 10.1103/PhysRevE.87.062906
We study the statistical properties of the impedance matrix (related to the scattering matrix) describing the input-output properties of waves in cavities in which ray trajectories that are regular and chaotic coexist (i.e., “mixed” systems). The impedance can be written as a summation over eigenmodes where the eigenmodes can typically be classified as either regular or chaotic. By appropriate characterizations of regular and chaotic contributions, we obtain statistical predictions for the impedance. We then test these predictions by comparison with numerical calculations for a specific cavity shape, obtaining good agreement.
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Publication: New J. Phys. 15, 023015 (2013)Doi: 10.1088/1367-2630/15/2/023025
A sensor was developed to quantitatively measure perturbations which change the volume of a wave chaotic cavity while leaving its shape intact. The sensors work in the time domain by using either scattering fidelity of the transmitted signals or time-reversal mirrors. The sensors were tested experimentally by inducing volume changing perturbations to a 1 m3 mixed chaotic and regular billiard system. Perturbations that caused a volume change that is as small as 54 parts in a million were quantitatively measured. These results were obtained by using electromagnetic waves with a wavelength of about 5 cm; therefore, the sensor is sensitive to extreme sub-wavelength changes of the boundaries of a cavity. The experimental results were compared with finite difference time-domain simulation results, and good agreement was found. Furthermore, the sensor was tested using a frequency-domain approach on a numerical model of the star graph, which is a representative wave chaotic system. These results open up interesting applications such as: monitoring the spatial uniformity of the temperature of a homogeneous cavity during heating up/cooling down procedures, verifying the uniform displacement of a fluid inside a wave chaotic cavity by another fluid, etc.
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Publication: Nano Energy 2, 1197 (2013)Doi: 10.1016/j.nanoen.2013.05.003
This work demonstrates the effect of atomic layer deposited (ALD) Al2O3 on the reactivation of dissolved polysulfides in Li–S batteries. A 0.5 nm thick layer of Al2O3 is conformally coated onto highly porous carbon cloth by ALD, and then assembled in a Li–S battery between the sulfur cathode and the anode side (separator and Li anode) to function as a reactivation component. Compared to half cells with no ALD treatment, the ultrathin Al2O3 coating increases the specific discharge capacity by 25% from 907 to 1136 mA h/g at the 1st cycle, and by 114% from 358 to 766 mA h/g at the 40th cycle. Thus the ALD-Al2O3 improves the initial specific capacity and stabilizes the cycle life remarkably. Scanning electron microscopy and energy-dispersive X-ray spectroscopy results indicate that the ALD-Al2O3 coated carbon cloth sorbs (adsorbs/absorbs) more dissolved sulfur species from the electrolyte. Potential mechanisms for the improved sorption properties are proposed. The combination of an ultrathin ALD-oxide coating with highly porous carbons presents a new strategy to improve the performance of Li–S batteries.
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Publication: Phys. Rev. Lett. 110, 225001 (2013)Doi: 10.1103/PhysRevLett.110.225001
The partition of energy flux in magnetic reconnection is examined experimentally using Cluster satellite observations of collisionless reconnection in Earth’s magnetotail. In this plasma regime, the dominant component of the energy flux is ion enthalpy flux, with smaller contributions from the electron enthalpy and heat flux and the ion kinetic energy flux. However, the Poynting flux is not negligible, and in certain parts of the ion diffusion region the Poynting flux in fact dominates. Evidence for earthward-tailward asymmetry is ascribed to the presence of Earth’s dipole fields.
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Publication: Nano Res. 6, 461 (2013)Doi: 10.1007/s12274-013-0323-9
Contemporary nanostructured transparent electrodes for use in solar cells require high transmittance and high conductivity, dictating nanostructures with high aspect ratios. Optical haze is an equally important yet unstudied parameter in transparent electrodes for solar cells that is also determined by the geometry of the nanostructures that compose the electrode. In this work, the effect of the silver nanowire diameter on the optical haze values in the visible spectrum was investigated using films composed of wires with either small diameters (∼60 nm) or large diameters (∼150 nm). Finite difference time domain (FDTD) simulations and experimental transmittance data confirm that smaller diameter nanowires form higher performing transparent conducting electrode (TCE) films according to the current figure of merit. While maintaining near constant transmittance and conductivity for each film, however, it was observed experimentally that films composed of silver nanowires with larger diameters have a higher haze factor than films with smaller diameters. This confirms the FDTD simulations of the haze factor for single nanowires with similarly large and small diameters. This is the first record of haze properties for Ag NWs that have been simulated or experimentally measured, and also the first evidence that the current figure of merit for TCEs is insufficient to evaluate their performance in solar cell devices.
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Publication: Optics Express 21, 28980 (2013)Doi: 10.1364/OE.21.028980
We investigate beam pointing dynamics in filamentation in gases driven by high repetition rate femtosecond laser pulses. Upon sudden exposure of a gas to a kilohertz train of filamenting pulses, successive filaments are steered from their original direction to a new stable direction whose equilibrium is determined by a balance among buoyant, viscous, and diffusive processes in the gas. The beam mode is preserved. Results are shown for Xe and air, but are broadly applicable to all configurations employing intense, high repetition rate femtosecond laser pulses in gases.
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Publication: Appl. Phys. Lett. 102, 141118 (2013)Doi: 10.1063/1.4802238
We present a method to control the resonant coupling interaction in a coupled-cavity photonic crystal molecule by using a local and reversible photochromic tuning technique. We demonstrate the ability to tune both a two-cavity and a three-cavity photonic crystal molecule through the resonance condition by selectively tuning the individual cavities. Using this technique, we can quantitatively determine important parameters of the coupled-cavity system such as the photon tunneling rate. This method can be scaled to photonic crystal molecules with larger numbers of cavities, which provides a versatile method for studying strong interactions in coupled resonator arrays.
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Publication: IEEE J. Electron Devices 5, 117 (2013)Doi: 10.1109/JEDS.2013.2273794
Random fabrication errors may have detrimental effects on the performance of traveling wave tubes (TWTs) of all types, especially in the sub-millimeter wavelength regime and beyond. Previous studies calculated the standard deviation of the small signal gain and the output phase of a TWT in the presence of small random, axially varying perturbations in the circuit phase velocity, assuming synchronous interaction and zero AC space charge effects. This paper relaxes the latter assumptions. In addition, we calculate the ensemble-average gain and the ensemble-average phase that result from random axial variations in the circuit phase velocity, using two analytic approaches. One is a perturbative approach including all three modes of the coupled beam-circuit equations. The other treats the evolution of only the dominant (exponentially growing) mode. The analytical results on the expected gain and phase compare favorably with results from numerical integrations of the governing equation in the absence of space charge, but are found to deviate from the numerical integrations with the inclusion of space charge effects. The effects of small pitch errors in a 210 GHz folded waveguide TWT are evaluated in an example.
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Publication: Optics Express 21, 7041 (2013)Doi: 10.1364/OE.21.007041
We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with V-shaped grooves. A one-dimensional array of V-grooves is fabricated on a highly-doped silicon surface, using anisotropic wet-etching of crystalline silicon, thereby forming a plasmonic waveguide. Terahertz time domain spectroscopy is used to characterize the propagation of waves near the corrugated surface. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of the V-grooves. The surface modes are confirmed through both numerical simulations and experimental measurements. Not only does the V-groove geometry represent a new and largely unexplored structure for supporting surface waves, but it also enables the practical fabrication of terahertz waveguides directly on semiconductor surfaces, without relying on reactive-ion etching or electroplating of sub-millimeter metallic surfaces.
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Publication: Energy and Environmental Sci. 6, 2578 (2013)Doi: 10.1039/c3ee40898a
Time-varying energy profiles of renewable sources, electric vehicles, end user demands, portable devices, novel military applications and more, require high power as well as high energy density in storage systems. Electrochemical capacitors (EECs), with higher power than batteries, benefit from nanostructured geometries that further increase their power capability. Nanostructured electrostatic capacitors (ESCs) are known to have much higher power capability than EECs, though lower energy density. The physical and chemical mechanisms by which EECs and ESCs function in charge/discharge are completely different, as are their electrical specifications and constraints. We consider for the first time how the contrasting characters of electrochemical and electrostatic nanostructured capacitors might be combined in a heterogeneous hybrid circuit to achieve better power and energy performance than either device alone. While the benefits of hybrid circuits have been previously considered for electrochemical devices – i.e., battery and electrochemical capacitor – this perspective article demonstrates for the first time that hybrid storage circuits of heterogeneous devices can exploit the very high power of electrostatic devices, in concert with electrochemical devices. Using response surface models from our own experimental results with nanostructured ESC and ECC devices, we develop a hybrid simulation model combining the two types of devices, recognizing the intrinsic nonlinearities and constraints of each. We demonstrate that charge capture by the ESC and subsequent rapid transfer to the ECC compensates for the lower power capability of the ECC while avoiding energy loss by ESC leakage currents. Although more sophisticated models and simulations are warranted in the future, these initial results underscore the opportunity that ESC devices and hybrid circuits offer for storage applications which require ultrahigh power performance.
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Publication: Astrophys. J. Lett. 763, L5 (2013)Doi: 10.1088/2041-8205/763/1/L5
Power-law distributions are a near-universal feature of energetic particle spectra in the heliosphere. Anomalous cosmic rays (ACRs), super-Alfvénic ions in the solar wind, and the hardest energetic electron spectra in flares all have energy fluxes with power laws that depend on energy E approximately as E−1.5. We present a new model of particle acceleration in systems with a bath of merging magnetic islands that self-consistently describes the development of velocity-space anisotropy parallel and perpendicular to the local magnetic field and includes the self-consistent feedback of pressure anisotropy on the merging dynamics. By including pitch-angle scattering we obtain an equation for the omnidirectional particle distribution f (v, t) that is solved in closed form to reveal v−5 (corresponding to an energy flux varying as E−1.5) as a near-universal solution as long as the characteristic acceleration time is short compared with the characteristic loss time. In such a state, the total energy in the energetic particles reaches parity with the remaining magnetic free energy. More generally, the resulting transport equation can serve as the basis for calculating the distribution of energetic particles resulting from reconnection in large-scale inhomogeneous systems.
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Publication: Appl. Phys. Lett. 102, 033901 (2013)Doi: 10.1063/1.4758300
An approach for an all lattice-matched multijunction solar cell optimized design is presented with 5.807 Å lattice constant, together with a detailed analysis of its performance by means of full device modeling. The simulations show that a (1.93 eV)In0.37Al0.63As/(1.39 eV)In0.38Ga0.62As0.57P0.43/(0.94 eV)In0.38Ga0.62As 3-junction solar cell can achieve efficiencies >51% under 100-suns illumination (with Voc = 3.34 V). As a key proof of concept, an equivalent 3-junction solar cell lattice-matched to InP was fabricated and tested. The independently connected single junction solar cells were also tested in a spectrum splitting configuration, showing similar performance to a monolithic tandem device, with Voc = 1.8 V.
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Publication: New J. Phys. 15, 075002 (2013)Doi: 10.1088/1367-2630/15/7/075002
We investigate beam pointing dynamics in filamentation in gases driven by high repetition rate femtosecond laser pulses. Upon sudden exposure of a gas to a kilohertz train of filamenting pulses, successive filaments are steered from their original direction to a new stable direction whose equilibrium is determined by a balance among buoyant, viscous, and diffusive processes in the gas. The beam mode is preserved. Results are shown for Xe and air, but are broadly applicable to all configurations employing intense, high repetition rate femtosecond laser pulses in gases.
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Publication: Nature Commun. 4, 1447 (2013)Doi: 10.1038/ncomms2477
Plasmonic nanostructures confine light on the nanoscale, enabling ultra-compact optical devices that exhibit strong light–matter interactions. Quantum dots are ideal for probing plasmonic devices because of their nanoscopic size and desirable emission properties. However, probing with single quantum dots has remained challenging because their small size also makes them difficult to manipulate. Here we demonstrate the use of quantum dots as on-demand probes for imaging plasmonic nanostructures, as well as for realizing spontaneous emission control at the single emitter level with nanoscale spatial accuracy. A single quantum dot is positioned with microfluidic flow control to probe the local density of optical states of a silver nanowire, achieving 12 nm imaging accuracy. The high spatial accuracy of this scanning technique enables a new method for spontaneous emission control where interference of counter-propagating surface plasmon polaritons results in spatial oscillations of the quantum dot lifetime as it is positioned along the wire axis.
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Publication: Plasma Phys. Control. Fusion 55, 124001 (2013)Doi: 10.1088/0741-3335/55/12/124001
Collisionless magnetic reconnection is considered to be one of the most important plasma phenomena because it governs the transport of energy, momentum and plasma in a wide variety of situations. In particular, understanding the central diffusion region is crucial to gaining a full understanding of the physics of reconnection. Although most diffusion region studies have historically focussed on simple reconnection geometries (antiparallel fields and symmetric reconnecting plasmas), in recent years significant progress has been made in understanding the impact of plasma asymmetries, guide fields and flow shear on collisionless diffusion region physics. Here we present a review of this recent progress, which is based both on supercomputer simulations and increasingly detailed multi-point satellite measurements of collisionless magnetic reconnection in space plasmas.
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Publication: Chaos 23, 013101 (2013)Doi: 10.1063/1.4772970
We present a chaotic frequency-modulated microwave source that is governed by a simple, first-order nonlinear delay differential equation. When a sinusoidal nonlinearity is incorporated, the dynamical behaviors range from fixed-point to periodic to chaotic, depending on the feedback strength. When the sinusoidal nonlinearity is replaced by a binary nonlinearity, the system exhibits a complex periodic attractor with no fixed-point solution.
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Publication: J. Vacuum Sci. Technol. A 31, 058503 (2013)Doi: 10.1116/1.4816262
Electrical energy storage is a challenging and pivotal piece of the global energy challenge—the “currency” of the energy economy. The opportunity that nanostructures present for advances in storage, recognized two decades ago, has been substantially bolstered by profound advances in nanoscale science and technology, so that a next generation energy storage technology is in sight. The authors present a perspective on the science issues and technology challenges accompanying this vision, focused primarily on the issues as exemplified by lithium ion batteries and made amenable to science through precision heterogeneous nanostructures. The authors address the synthesis and characterization of heterogeneous nanostructures, architectural designs, and recent results, as well as the scientific and technological challenges of integrating dense arrays of nanostructures for a viable technology.
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Publication: Geophys. Res. Lett. 40, 11 (2013)Doi: 10.1029/2012GL054528
We have performed a statistical study of THEMIS spacecraft crossings of the asymmetric dayside magnetopause to test the prediction that the diamagnetic drift of the X-line due to a plasma pressure gradient across the magnetopause can suppress magnetic reconnection. The study includes crossings both when reconnection exhausts were present and when they were absent in the current sheet. When we restrict the survey to the subsolar region (10 < MLT < 14), we find that for low Δβ (the difference of plasma β on the two sides of the current sheet) the majority of reconnection events occurred over a large range of magnetic shears, whereas when Δβ was high reconnection events occurred only for high shears. Furthermore, nonreconnection events occurred primarily in the Δβ-shear regime in which reconnection is predicted to be suppressed, in good agreement with theory. The Δβ-shear condition should have general consequences for the occurrence of reconnection in space and laboratory plasmas.
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Publication: Phys. Rev. Lett. 111, 121105 (2013)Doi: 10.1103/PhysRevLett.111.121105
Decay in time of undriven weakly collisional kinetic plasma turbulence in systems large compared to the ion kinetic scales is investigated using fully electromagnetic particle-in-cell simulations initiated with transverse flow and magnetic disturbances, constant density, and a strong guide field. The observed energy decay is consistent with the von Kármán hypothesis of similarity decay, in a formulation adapted to magnetohydrodyamics. Kinetic dissipation occurs at small scales, but the overall rate is apparently controlled by large scale dynamics. At small turbulence amplitudes the electrons are preferentially heated. At larger amplitudes proton heating is the dominant effect. In the solar wind and corona the protons are typically hotter, suggesting that these natural systems are in the large amplitude turbulence regime.
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Publication: Optics Express 21, 15878 (2013)Doi: 10.1364/OE.21.015878
We examine the generation of axially modulated plasmas produced from cluster jets whose supersonic flow is intersected by thin wires. Such plasmas have application to modulated plasma waveguides. By appropriately limiting shock waves from the wires, plasma axial modulation periods can be as small as 70 μm, with plasma structures as narrow as 45 µm. The effect of shocks is eliminated with increased cluster size accompanied by a reduced monomer component of the flow.
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Publication: RSC Advances 3, 4294 (2013)Doi: 10.1039/c3ra23031g
Atomic layer deposition (ALD) is attractive for next-generation electrical energy storage in forming passivation layers and more recently active storage material. Here we report a detailed study of ALD V2O5 as a high capacity cathode material, using vanadium tri-isopropoxide (VTOP) precursor with both O3 and H2O as oxidant. The O3-based process produces polycrystalline films with generally higher storage capacity than the amorphous films resulting from the H2O-based process over extended cycling (100 cycles). High capacities are achieved in V2O5 because of the ability to incorporate up to three Li per V2O5 formula unit. To address the central need for both high power and high energy, we identified the crucial tradeoff between higher gravimetric capacity with thinner films and higher material mass with thicker films. For the thickness regime 10–120 nm, we chose areal energy and power density as a useful metric for this tradeoff and found that it is optimized at 60 nm for the O3-VTOP ALD V2O5 films. We believe the control of material quality, thickness, and conformality achievable with ALD processes is valuable as new nanoarchitectures for electrochemical energy storage come into sight.
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Publication: Astrophys. J. 778, 144 (2013)Doi: 10.1088/0004-637X/778/2/144
In observations of flare-heated electrons in the solar corona, a longstanding problem is the unexplained prolonged lifetime of the electrons compared to their transit time across the source. This suggests confinement. Recent particle-in-cell (PIC) simulations, which explored the transport of pre-accelerated hot electrons through ambient cold plasma, showed that the formation of a highly localized electrostatic potential drop, in the form of a double layer (DL), significantly inhibited the transport of hot electrons. The effectiveness of confinement by a DL is linked to the strength of the DL as defined by its potential drop. In this work, we investigate the scaling of the DL strength with the hot electron temperature by PIC simulations and find a linear scaling. We demonstrate that the strength is limited by the formation of parallel shocks. Based on this, we analytically determine the maximum DL strength, and also find a linear scaling with the hot electron temperature. The DL strength obtained from the analytic calculation is comparable to that from the simulations. At the maximum strength, the DL is capable of confining a significant fraction of hot electrons in the source.
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Publication: Phys. Rev. Lett. 110, 064104 (2013)Doi: 10.1103/PhysRevLett.110.064104
We experimentally demonstrate group synchrony in a network of four nonlinear optoelectronic oscillators with time-delayed coupling. We divide the nodes into two groups of two each, by giving each group different parameters and by enabling only intergroup coupling. When coupled in this fashion, the two groups display different dynamics, with no isochronal synchrony between them, but the nodes in a single group are isochronally synchronized, even though there is no intragroup coupling. We compare experimental behavior with theoretical and numerical results.
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Publication: ACS Nano 7, 6354 (2013)Doi: 10.1021/nn402451s
Conversion-type electrodes represent a broad class of materials with a new Li+ reactivity concept. Of these materials, RuO2 can be considered a model material due to its metallic-like conductivity and its high theoretical capacity of 806 mAh/g. In this paper, we use in situ transmission electron microscopy to study the reaction between single-crystal RuO2 nanowires and Li+. We show that a large volume expansion of 95% occurs after lithiation, 26% of which is irreversible after delithiation. Significant surface roughening and lithium embrittlement are also present. Furthermore, we show that the initial reaction from crystalline RuO2 to the fully lithiated mixed phase of Ru/Li2O is not fully reversible, passing through an intermediate phase of LixRuO2. In subsequent cycles, the phase transitions are between amorphous RuO2 in the delithiated state and a nanostructured network of Ru/Li2O in the fully lithiated phase.
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Publication: Asterophys. J. Lett. 762, L14 (2013)Doi: 10.1088/2041-8205/762/1/L14
Voyager 1(V1) and Voyager 2(V2) have observed heliosheath plasma since 2005 December and 2007 August, respectively. The observed speed profiles are very different at the two spacecrafts. Speeds at V1 decreased to zero in 2010 while the average speed at V2 is a constant 150 km s−1 with the direction rotating tailward. The magnetic flux is expected to be constant in these heliosheath flows. We show that the flux is constant at V2 but decreases by an order of magnitude at V1, even after accounting for divergence of the flows and changes in the solar field. If reconnection were responsible for this decrease, the magnetic field would lose 70% of its free energy to reconnection and the energy density released would be 0.6 eV cm−3.
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Publication: Phys. Plasmas 20, 062110 (2013)Doi: 10.1063/1.4812459
A procedure for generating random variates from a relativistic Maxwellian distribution with arbitrary temperature and drift velocity is presented. The algorithm is based on the rejection method and can be used to initialize particle velocities in kinetic simulations of plasmas and gases.
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Scaling and Saturation of High-Power Terahertz Radiation Generation in Two-Color Laser Filamentation
Publication: Appl. Phys. Lett. 102, 201113 (2013)Doi: 10.1063/1.4807790Broadband terahertz generation via two-color femtosecond laser filamentation is studied with laser input energies up to 60 mJ. In the small f-number focusing regime, the output THz energy strongly saturates, which is attributed to ionization-induced laser defocusing in filamentation. This saturation effect can be minimized by elongating the plasma filament with weak focusing. A conversion efficiency of >10−4 is achieved in elongated filamentation.
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Publication: Soft Matter 9, 2703 (2013)Doi: 10.1039/c3sm27581g
The electrodeposition of weak polyelectrolyte hydrogels involves an array of subtle interactions. We report that salt dramatically affects the kinetics of chitosan electrodeposition, and the structure and properties of deposited hydrogel films. The kinetics of film growth was measured using a microfluidic device which demonstrated that salt increases both the rate and extent of deposition. The structure of the deposited film was measured by atomic force microscopy which showed that salt addition to the deposition solution leads to films with greater surface roughness (consistent with the tendency of chitosan to aggregate at high salt concentrations). The properties of the deposited films were measured by quartz crystal microbalance with dissipation (QCM-D) which showed that salt addition to the deposition solution leads to films with substantially reduced moduli (over 3-orders-of-magnitude). These results illustrate the potential to tailor electrodeposition to meet specific requirements for the diverse applications in the life and medical sciences.
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Publication: Astrophys. J. Lett. 778, L26 (2013)Doi: 10.1088/2041-8205/778/2/L26
Based on the difference between the orientation of the interstellar and the solar magnetic fields, there was an expectation by the community that the magnetic field direction will rotate dramatically across the heliopause (HP). Recently, the Voyager team concluded that Voyager 1 (V1) crossed into interstellar space last year. The question is then why there was no significant rotation in the direction of the magnetic field across the HP. Here we present simulations that reveal that strong rotations in the direction of the magnetic field at the HP at the location of V1 (and Voyager 2) are not expected. The solar magnetic field strongly affects the drapping of the interstellar magnetic field (BISM) around the HP. BISM twists as it approaches the HP and acquires a strong T component (East–West). The strong increase in the T component occurs where the interstellar flow stagnates in front of the HP. At this same location the N component BN is significantly reduced. Above and below, the neighboring BISM lines also twist into the T direction. This behavior occurs for a wide range of orientations of BISM. The angle δ = asin (BN/B) is small (around 10°–20°), as seen in the observations. Only after some significant distance outside the HP is the direction of the interstellar field distinguishably different from that of the Parker spiral.
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Publication: Opt. Mater. Exp. 3, 114 (2013)Doi: 10.1364/OME.3.000114
Mid to far-infrared (terahertz) spectroscopy is a valuable tool for probing and characterizing macromolecular structures and motions of complex molecules, including low frequency vibrational and phonon modes in condensed phases. We describe here an improved and readily implemented method for performing terahertz spectroscopic measurements by using a nanoporous silicon substrate to capture and concentrate the substance to be analyzed. We compare the results to conventional sampling methods, including dissolution and crystallization on a flat silicon surface and dispersing crystallites in compressed polyethylene pellets, and show that the use of a transparent, nanoporous substrate provides both increased sensitivity and yields sharper spectral features than conventional solid-state sampling approaches. FTIR measurements are reported over the spectral range from 50–2000 cm−1 (1.5–60 THz), for salicylic acid, dicyanobenzene, glycine, and aspartame.
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Publication: ACS Nano 7, 7037 (2013)Doi: 10.1021/nn401818t
Cellulose fibers with porous structure and electrolyte absorption properties are considered to be a good potential substrate for the deposition of energy material for energy storage devices. Unlike traditional substrates, such as gold or stainless steel, paper prepared from cellulose fibers in this study not only functions as a substrate with large surface area but also acts as an interior electrolyte reservoir, where electrolyte can be absorbed much in the cellulose fibers and is ready to diffuse into an energy storage material. We demonstrated the value of this internal electrolyte reservoir by comparing a series of hierarchical hybrid supercapacitor electrodes based on homemade cellulose paper or polyester textile integrated with carbon nanotubes (CNTs) by simple solution dip and electrodeposited with MnO2. Atomic layer deposition of Al2O3 onto the fiber surface was used to limit electrolyte absorption into the fibers for comparison. Configurations designed with different numbers of ion diffusion pathways were compared to show that cellulose fibers in paper can act as a good interior electrolyte reservoir and provide an effective pathway for ion transport facilitation. Further optimization using an additional CNT coating resulted in an electrode of paper/CNTs/MnO2/CNTs, which has dual ion diffusion and electron transfer pathways and demonstrated superior supercapacitive performance. This paper highlights the merits of the mesoporous cellulose fibers as substrates for supercapacitor electrodes, in which the water-swelling effect of the cellulose fibers can absorb electrolyte, and the mesoporous internal structure of the fibers can provide channels for ions to diffuse to the electrochemical energy storage materials.
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Publication: J. Geophys. Res.-Space Phys. 118, 2073 (2013)Doi: 10.1002/jgra.50211
We re-examine the basic premises of a single-spacecraft data analysis method, developed by Sonnerup and Hasegawa (2005), for determining the axis orientation and proper frame velocity of quasi two-dimensional, quasi-steady structures of magnetic field and plasma. The method, which is based on Faraday's law, makes use of magnetic and electric field data measured by a single spacecraft traversing the structure, although in many circumstances the convection electric field, − v × B, can serve as a proxy for E. It has been used with success for flux ropes observed at the magnetopause but has usually failed to provide acceptable results when applied to real space data from reconnection events as well as to virtual data from numerical MHD simulations of such events. In the present paper, the reasons for these shortcomings are identified, analyzed, and discussed in detail. Certain basic properties of the method are presented in the form of five theorems, the last of which makes use of singular value decomposition to treat the special case where the magnetic variance matrix is non-invertible. These theorems are illustrated using data from analytical models of flux ropes and also from MHD simulations as well as a 2-D kinetic simulation of reconnection. The results make clear that the method requires the presence of a significant, non-removable electric field distribution in the plane transverse to the invariant direction and that it is sensitive to deviations from strict two-dimensionality and strict time stationarity.
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Publication: Optics Express 21, 4740 (2013)Doi: 10.1364/OE.21.004740
Femtosecond laser pulses filamenting in various gases are shown to generate long- lived quasi-stationary cylindrical depressions or ‘holes’ in the gas density. For our experimental conditions, these holes range up to several hundred microns in diameter with gas density depressions up to ~20%. The holes decay by thermal diffusion on millisecond timescales. We show that high repetition rate filamentation and supercontinuum generation can be strongly affected by these holes, which should also affect all other experiments employing intense high repetition rate laser pulses interacting with gases.
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Publication: Soft Matter 9, 6019 (2013)Doi: 10.1039/c3sm50527h
Biology is a master of mesoscale science, possessing unprecedented capabilities for fabricating components with nano-scale precision and then assembling them over a hierarchy of length scales. Biology's fabrication prowess is well-recognized and there has been considerable effort to mimic these capabilities to create materials with diverse and multiple functions. In this review, we pose the question – why mimic, why not directly use the materials and mechanisms that biology provides to biofabricate functional materials? This question seems especially relevant when considering that many of the envisioned applications – from regenerative medicine to bioelectronics – involve biology. Here, we provide a sampling to illustrate how self-assembly, enzymatic-assembly and the emerging tools of modern biology can be enlisted to create functional soft matter. We envision that biofabrication will provide a biocompatible approach to mesoscale science and yield products that are safe, sustainable and potentially even edible.
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Publication: Astrophys. J. Lett. 776, L32 (2013)Doi: 10.1088/2041-8205/776/2/L32
We are able to show by comparing modeled energetic neutral atoms (ENAs) spectra to those measured by Interstellar Boundary Explorer (IBEX) that the models along the Voyager 1 (V1) trajectory that best agree with the low energy IBEX data include extra heating due to ram and magnetic energy in the quasi-stagnation region or a kappa ion distribution (with κ = 2.0) in the outer heliosheath. The model explored is the multi-ion, multi-fluid (MI–MF) which treats the pick-up ions and the thermal ion fluids with separate Maxwellian distributions. These effects are included ad hoc in the modeled ENA since they are not present in the model. These results indicate that the low energy spectra of ENAs as measured by IBEX is sensitive to the physical nature of the heliosheath and to effects not traditionally present in current global models. Therefore, by comparing the low energy ENA spectra to models, we can potentially probe the heliosheath in locations beyond those probed by V1 and Voyager 2 (V2).
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Publication: IEEE Trans. Plasma Sci. 41, 862 (2013)Doi: 10.1109/TPS.2013.2247635
We present experimental results of the modes excited during the voltage rise of a 1.5-MW, 110-GHz gyrotron operating in the TE22,6,1 forward-wave mode. Results were obtained by two different experimental techniques: measurements with a time-gated heterodyne receiver and measurements during the flat-top portion of the voltage pulse with a sequence of increasing voltages. Two operating points were selected: a high-efficiency 1.2-MW power-level point at 4.38 T and a highly stable 600-kW point at 4.45 T. In the former case, the TE21,6,3 and TE21,6,4 backward-wave modes far from cutoff were excited during the voltage rise of the pulse before the desired TE22,6,1 operating mode was excited; in the latter case, the excitation of a TE22,6,2 backward-wave mode dominated the voltage rise before eventually exciting the desired operating mode. Analysis of the microwave output beam spatial pattern and the frequency and power levels recorded indicate that these modes are indeed excited within the cavity. Single-mode MAGY simulations provide further evidence that such modes can exist in the gyrotron during the voltage rise. Knowledge of the modes excited during start-up is important for achieving high efficiency and avoiding power at unwanted frequencies.
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Publication: Appl. Phys. Lett. 102, 173501 (2013)Doi: 10.1063/1.4801979
Defects in electronic devices can lead to poor performance and device failure. We used deuterium doping to investigate the source of hydrogen defects in Atomic Layer Deposited (ALD) Al2O3 films and in situ fabrication techniques to produce ultraclean metal-insulator-metal trilayers. We compare leakage current and defect density of ALD Al2O3 dielectrics deposited using different oxidation conditions. The plasma O2 ALD process has lowest number of entrained defects and exhibits a leakage current 104 times lower than the thermal ALD process. Deuterium doping during the ALD process shows that the majority of the hydrogen defects contained in the ALD films are due to entrained water.
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Publication: Astrophys. J. 764, 126 (2013)Doi: 10.1088/0004-637x/764/2/126
Voyager spacecraft observations have revealed that contrary to expectations, the source of anomalous cosmic rays (ACRs) is not at the local termination shock. A possible mechanism of ACR acceleration is magnetic reconnection in the heliosheath. Using a particle-in-cell code, we investigate the effects of β on reconnection-driven particle acceleration by studying island growth in multiple interacting Harris current sheets. Many islands are generated, and particles are dominantly heated through Fermi reflection in contracting islands during island growth and merging. There is a striking difference between the heating of electrons versus the heating of ions. There is a strong dependence of β on electron heating, while the ion heating is insensitive to β. Anisotropies develop with T∥ ≠ T⊥ for both electrons and ions. The electron anisotropies support the development of a Weibel instability that suppresses the Fermi acceleration of the electrons. Since the Weibel instability develops at a larger T∥/T⊥ in lower β systems, electrons are able to accelerate more efficiently by the Fermi mechanism at low β. The variance in anisotropy implies less electron acceleration in higher β systems, and thus less heating. This study sheds light on particle acceleration mechanisms within the sectored magnetic field regions of the heliosheath and the dissipation of turbulence such as that produced by the magnetorotational instability in accreting systems.