Publications Database

2024

  1. Author(s): Alexander Wikner, Joseph Harvey, Michelle Girvan, Brian R. Hunt, Andrew Pomerance, Thomas Antonsen, Edward Ott
    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.

  2. Author(s): Svilen Sabchevski, Gregory S. Nusinovich, Mikhail Glyavin
    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.

2023

  1. Author(s): Ying Uy, Shunfa Liu, Chang-Min Lee, Peter Michler, Stephan Reitzenstein, Kartik Srinivasan, Edo Waks, Jin Liu
    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.

  2. Author(s): Jeong Woo Han, Pavlo Sai, Dmytro B. But, Ece Uykur, Stephan Winnerl, Kagan Kumar, Matthew L. Chin, Rachael Myers-Ward, Matthew T. Dejarld, Kevin M. Daniels, Thomas E. Murphy, Wojciech Knap, Martin Mittendorff
    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.

  3. Author(s): Mohammad Rezaei-Pandari, Mohammad Mirzaie, Calin Ioan Hojbota, Tae Gyu Pak, Sang Beom Kim, Geon Woo Lee, Reza Massudi, Ali Reza Niknam, Seong Ku Lee, Ki-Yong Kim, Chang Hee Nam
    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.

  4. Author(s): Mohammad Habibur Rahaman, Chang-Min Lee, Mustafa Atabey Buyukkaya, Yuqi Zhao, Edo Waks
    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

  5. Author(s): Nour E. Raouafi, G. Stenborg, D.B. Seaton, H. Wang, J. Wang, C.E. DeForest, S.D. Bale, J.F. Drake, V.M. Uritsky, J.T. Karpen, et al.
    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.

  6. Author(s): Alan A. Kaptanoglu, Gabriel P. Langlois, Matt Landreman
    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.

  7. Author(s): Chang-Min Lee, Fariba Islam, Samuel Harper, Mustafa Atabey Buyukkaya, Daniel Higginbottom, Stephanie Simmons, Edo Waks
    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.

  8. Author(s): Rahul Gaur, Stefan Buller, Maximilian E. Ruth, Matt Landreman, Ian G. Abel, William D. Dorland
    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.

  9. Author(s): David Bindel, Matt Landreman, Misha Padidar
    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.

  10. Author(s): Jiashen Tang, Zechuan Yin, Connor A. Hart, John W. Blanchard, Jner Tzern Oon, Smriti Bhalerao, Jennifer M. Schloss, Matthew J. Turner, Ronald L. Walsworth
    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.

  11. Author(s): Rahul Gaur, Ian G. Abel, David Dickinson, William D. Dorland
    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):.”

     

  12. Author(s): P.S. Pyakurel, T.D. Phan, J.F. Drake, M.A. Shay, M. Oieroset, C.C. Haggerty, J. Stawarz, J.L. Burch, R.E. Ergun, D.J. Gershman, B.L. Giles, R.B. Tolbert, R.J. Strangeway, C.T. Russell
    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.

  13. Author(s): Nithya Arunkumar, Kevin S. Olsson, Jner Tzern Oon, Connor A. Hart, Dominik B. Bucher, David R. Glenn, Mikhail D. Lukin, Hongkun Park, Donhee Ham, Ronald L. Walsworth
    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

  14. Author(s): Noah Hoppis, Kathryn M. Sturge, Jonathan E. Barney, Brian L. Beaudoin, Ariana M. Bussio, Ashley E. Hammell, Samuel L. Henderson, James E. Krutzler, Joseph P. Lichthardt, Alexander H. Mueller, Karl Smith, Bryce C. Tappan, Timothy W. Koeth
    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.

  15. Author(s): Mitsuo Oka, Joachim Birn, Jan Egedal, Fan Guo, Robert E. Ergun, Drew L. Turner, Yuri Khotyaintsev, Kyoung-Joo Hwang, Ian J. Cohen, James F. Drake
    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.

  16. Author(s): Alisa Galishnikova, Alexander Philippov, Eliot Quataert
    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.

  17. Author(s): Yanne K. Chembo, Elham Heidari, Curtis R. Menyuk
    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.

  18. Author(s): Elias R. Most, Alexander A. Philippov
    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)erg s−1 as an upper bound on the luminosity, where B is the magnetic field strength on the surface of the neutron star.

  19. Author(s): O.V. Agapitov, J.F. Drake, M. Swisdak, K.-E. Choi, N. Raouafi
    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 ΔV= 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.

  20. Author(s): Philipp Kempski, Drummond B. Fielding, Eliot Quataert, Alisa K. Galishnikova, Matthew W. Kunz, Alexander A. Philippov, Bart Ripperda
    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.

  21. Author(s): Sang-Jin Chung, Pingshan Luan, Minjoon Park, Andrew Metz, Gottlieb S. Oehrlein
    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.

  22. Author(s): M. Swisdak, J. Giacalone, J.F. Drake, M. Opher, G.P. Zank, B. Zieger
    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.

  23. Author(s): Kenneth McAfee, Peter B. Sunderland, Oded Rabin
    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.

  24. Author(s): Michael Hinshelwood, Gottlieb S. Oehrlein
    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.

  25. Author(s): M. Oieroset, T.D. Phan, J.F. Drake, S.A. Fuselier, D.J. Gershman, K. Maheshwari, B.L. Giles, Q. Zhang, F. Guo, J.L. Burch, R.B. Tolbert, R.J. Strangeway
    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.

  26. Author(s): Haotian Wang, Yueming Song, Victoria Castagna Ferrari, Nam Soo Kim, Sang Bok Lee, Paul Albertus, Gary Rubloff, David Murdock Stewart
    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.

  27. Author(s): Alexander Chernoglazov, Hayk Hakobyan, Alexander Philippov
    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.

  28. Author(s): J. Qiu, M. Alaoui, S.K. Antiochos, J.T. Dahlin, M. Swisdak, J.F. Drake, A. Robison, C.R. DeVore, V.M. Uritsky
    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.

  29. Author(s): Fariba Islam, Chang-Min Lee, Samuel Harper, Mohammad Habibur Rahaman, Yuqi Zhao, Neelesh Kumar Vij, Edo Waks
    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.

  30. Author(s): J.S. Halekas, S.D. Bale, M. Berthomier, B.D.G. Chandran, J.F. Drake, J.C. Kasper, K.G. Klein, D.E. Larson, R. Livi, M.P. Pulupa, M.L. Stevens, J.L. Verniero, P. Whittlesey
    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(☉).

  31. Author(s): David Bindel, Matt Landreman, Misha Padidar
    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.

  32. Author(s): Junghyun Lee, Mamiko Tatsuta, Andrew Xu, Erik Bauch, Mark J.H. Ku, Ronald L. Walsworth
    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.

  33. Author(s): Shukai Ma, Thomas M. Antonsen, Jr., Steven M. Anlage
    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.

  34. Author(s): Zach Levy, Victoria Castagna Ferrari, Pablo Rosas, Mitchell J. Walker, Kalpak Duddella, Micah Haseman, David Stewart, Gary Rubloff, Leonard J. Brillson
    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.

  35. Author(s): Uday Saha, James D. Siverns, John Hannegan, Qudsia Quraishi, Edo Waks
    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.

  36. Author(s): Richard A. Brewster, Gerald Baumgartner, Yanne K. Chembo
    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.

  37. Author(s): Junghyun Lee, Keigo Arai, Huiliang Zhang, Mark J.H. Ku, Ronald L. Walsworth
    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.

  38. Author(s): R. Jorge, A. Goodman, M. Landreman, J. Rodrigues, F. Wechsung
    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.

  39. Author(s): Guoping Lin, Fengyu Liu, Damia Gomila, Curtis Menyuk, Yanne K. Chembo
    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.

  40. Author(s): Yueming Song, Bhuvsmita Bhargava, David M. Stewart, A. Alec Talin, Gary W. Rubloff, Paul Albertus
    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.

  41. Author(s): Richard A. Brewster, Gerald Baumgartner, Yanne K. Chembo
    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.

  42. Author(s): Alfred Leitenstorfer, Andrey S. Moskalenko, Tobias Kampfrath, Junichiro Kono, Enrique Castro-Camus, Lun Peng, Naser Qureshi, Dmitry Turchinovich, Koichiro Tanaka, Andrea G. Markelz, Martina Havenith, Cameron Hough, Hannah J. Joyce, Willie J. Padilla, Binbin Zhou, Ki-Yong Kim, et al.
    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.

  43. Author(s): Taegyu Pak, Mohammad Rezaei-Pandari, Sang Beom Kim, Geonwoo Lee, Dae Hee Wi, Calin Ioan Hojbota, Mohammad Mirzaie, Hyeongmun Kim, Jae Hee Sung, Chul Kang, Ki-Yong Kim
    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.

  44. Author(s): Yang Wang, Jeonghyun Ko, Myungsuk Lee, Sam Klueter, Elias Kallon, John Hoerauf, Daniela Fontecha, Cholho Lee, Gary W. Rubloff, Sang Bok Lee, Alexander C. Kozen
    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.

  45. Author(s): Kunal Ahuja, Valentin Sallaz, Ramsay Blake Nuwayhid, Frederic Voiron, Patrick McCluskey, Gary W. Rubloff, Keith E. Gregorczyk
    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.

  46. Author(s): Evan Dowling, Mark Morris, Gerald Baumgartner, Rajarshi Roy, Thomas E. Murphy
    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.

  47. Author(s): Guoping Lin, Jingyi Tian, Tang Sun, Qinghai Song, Yanne K. Chembo
    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.

  48. Author(s): Tae Gyu Pak, Yong Joo Rhee, Mohammad Mirzaie, Calin Ioan Hoybota, Jong Ho Jeon, Sung Ha Jo, Chang Hee Nam, Mohammad Rezaei-Pandari, Jae Hee Sung, Seong Ku Lee, Ki-Yong Kim
    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.

  49. Author(s): Dhruvit Patel, Edward Ott
    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.

  50. Author(s): Nathan Youngblood, Carlos A. Rios Ocampo, Wolfram H.P. Pernice, Harish Bhaskaran
    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.

  51. Author(s): Troy Arcomano, Istvan Szunyogh, Alexander Wikner, Brian R. Hunt, Edward Ott
    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.

  52. Author(s): Subhojit Dutta, Yuqi Zhao, Uday Saha, Demitry Farfurnik, Elizabeth A. Goldschmidt, Edo Waks
    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.

  53. Author(s): Fengyu Liu, Curtis R. Menyuk, Yanne K. Chembo
    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.

  54. Author(s): Charles J. Turner, Andrew I. Harris, Thomas E. Murphy, Mark Stephen
    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.

  55. Author(s): Silvia Trinczek, Felix I. Parra, Peter J. Catto, Ivan Calvo, Matt Landreman
    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.

  56. Author(s): Konstantin Y. Bliokh, Ebrahim Karimi, Miles J. Padgett, Miguel A. Alonson, Mark R. Dennis, Angela Dudley, Andrew Forbes, Sina Zahedpour, Scott W. Hancock, Howard M. Milchberg
    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.

  57. Author(s): Gavin Blair, Phillip Sprangle
    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.

  58. Author(s): Henry F. Elder, Phillip Sprangle
    Publication: Optics Lett. 48, 606 (2023)
    Doi: 10.1364/OL.481303

    We 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.

  59. Author(s): Hanqing Ma, J.F. Drake, M. Swisdak
    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.

  60. Author(s): Brandon F. Lee, Elizabeth J. Paul, Georg Stadler, Matt Landreman
    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.

  61. Author(s): Charles J. Turner, Thomas Stevenson, Robin Cantor, Lawrence Hillard, Thomas E. Murphy, Berhanu Bulcha
    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.

  62. Author(s): Tornike Ghutishvili, Lei Chen, Steven M. Anlage, Thomas M. Antonsen
    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.

  63. Author(s): Demitry Farfurnik, Harjot Singh, Zhouchen Luo, Allan S. Bracker, Samuel G. Carter, Robert M. Pettit, Edo Waks
    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.

  64. Author(s): Henry P. Freund, Michael V. Fazio, Patrick G. O'Shea, Richard B. True
    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.

  65. Author(s): Bo Miao, Jaron Shrock, Howard Milchberg
    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.

  66. Author(s): Haotian Wang, Nam Soo Kim, Yueming Song, Paul Albertus, Sang Bok Lee, Gary Rubloff, David Stewart
    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.

  67. Author(s): Calin Ioan Hojbota, Mohammad Mirzaie, Do Yeon Kim, Tae Gyu Pak, Mohammad Rezaei-Pandari, Bandhu Vishwa, Jong Ho Jeon, Jin Woo Yoon, Jae Hee Sung, Seong Ku Lee, Chul Min Kim, Ki-Yong Kim, Chang Hee Nam
    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.

  68. Author(s): Kang-Yi Lin, Christian Preischl, Christian Felix Hermanns, Daniel Rhinow, Hans-Michael Solowan, Michael Budach, Hubertus Marbach, Klaus Edinger, Gottlieb S. Oehrlein
    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.

  69. Author(s): Drummond B. Fielding, Bart Ripperda, Alexander A. Philippov
    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.

  70. Author(s): Rahul Gaur, Ian G. Abel, David Dickinson, William D. Dorland
    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.

  71. Author(s): Elias R. Most, Alexander A. Philippov
    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.

  72. Author(s): Nam Kim, Marco Casareto, Miles Mowbray, Robert Henry, John Hayden, Gary Rubloff, Sang Bok Lee, Keith E. Gregorczyk
    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. 

  73. Author(s): Farasatul Adnan, Steven M. Anlage, Thomas M. Antonsen, Jr., Edward Ott
    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.

  74. Author(s): Alisa Galishnikova, Alexander Philippov, Eliot Quataert, Fabio Bacchini, Kyle Parfrey, Bart Ripperda
    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.

  75. Author(s): Andrew Giuliani, Florian Wechsung, Antoine Cerfon, Matt Landreman, Georg Stadler
    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.

  76. Author(s): R. Blake Nuwayhid, Alexander C. Kozen, Daniel M. Long, Kunal Ahuja, Gary W. Rubloff, Keith E. Gregorczyk
    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.

  77. Author(s): Shen Lin, Sangrui Luo, Shukai Ma, Junda Feng, Yang Shao, Zachary B. Drikas, Bisrat D. Addissie, Steven M. Anlage, Thomas M. Antonsen, Zhen Peng
    Publication: IEEE Trans. Electromagnetic Compatiblity 65, 436 (2023)

    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.

  78. Author(s): Alan A. Kaptanoglu, Rory Conlin, Matt Landreman
    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.

  79. Author(s): Nabeel Aslam, Hengyun Zhou, Elana K. Urbach, Matthew J. Turner, Ronald L. Walsworth, Mikhail D. Lukin, Hongkun Park
    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.

  80. Author(s): Hayk Hakobyan, Alexander Philippov, Anatoly Spitkovsky
    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.

  81. Author(s): Alan A. Kaptanoglu, Christopher Hansen, Jeremy D. Lore, Matt Landreman, Steven L. Brunton
    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.

  82. Author(s): Merav Opher, John Richardson, Gary Zank, Vladimir Florinski, Joe Giacalone, Justyna M. Sokol, Gabor Toth, Sanlyn Buxner, Marc Kornbleuth, Matina Gkioulidou, Romina Nikoukar, Bart Van der Holst, Drew Turner, Nicholas Gross, James Drake, Marc Swisdak, et al.
    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.

  83. Author(s): Artur Perevalov, Ruben E. Rojas, Daniel P. Lathrop
    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.

  84. Author(s): S.D. Bale, J.F. Drake, M.D. McManus, M.I. Desai, S.T. Badman, D.E. Larsen, M. Swisdak, T.S. Horbury, N.E. Raouafi, T. Phan, M. Velli, D.J. McComas, C.M.S. Cohen, D. Mitchell, O. Panasenco, J.C. Kasper
    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.

  85. Author(s): B. Lavraud, M. Opher, K. Dialynas, D.L. Turner, S. Eriksson, E. Provornikova, M.Z. Kornbleuth, P. Mostafavi, A. Fedorov, J.D. Richardson, S.A. Fuselier, J. Drake, M. Swisdak, et al.
    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.

  86. Author(s): M.R. Hardman, F.I. Parra, C.M. Roach, J. Ruiz Ruiz, M. Barnes, D. Dickinson, William Dorland, et al.
    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.

  87. Author(s): Sadra Rahimi Kari, Carlos A. Rios Ocampo, Lei Jiang, Jiawei Meng, Nicola Peserico, Volker J. Sorger, Juejun Hu, Nathan Youngblood
    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.

  88. Author(s): Uday Saha, James D. Siverns, John Hannegan, Mihika Prabhu, Qudsia Quraishi, Dirk Englund, Edo Waks
    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.

2012

  1. Author(s): Michel Rieutord, Santiago Andres Triana, Daniel S. Zimmerman, Daniel P. Lathrop
    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.

  2. Author(s): Young-Noh Yoon, Brian R. Hunt, Edward Ott, Istvan Szunyogh
    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.

  3. Author(s): Jeremiah J. Wathen, Vincent R. Pagan, Thomas E. Murphy
    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.

  4. Author(s): Enrico Fonda, Katepalli R. Sreenivasan, Daniel P. Lathrop
    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.

  5. Author(s): Sanjeev Chauhan, Michelle Girvan, Edward Ott
    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.

  6. Author(s): Aaron M. Hagerstrom, Thomas E. Murphy, Rajarshi Roy, Philipp Hoevel, Iryna Omelchenko, Eckehard Schoell
    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.

  7. Author(s): Ki-Yong Kim, James H. Glownia, Antoinette J. Taylor, George Rodriguez
    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.

  8. Author(s): Shane Squires, Edward Ott, Michelle Girvan
    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.

  9. Author(s): Thomas E. Murphy, Halil Berberoglu
    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.

  10. Author(s): Dustin Anderson, Ari Tenzer, Gilad Barlev, Michelle Girvan, Thomas M. Antonsen, Edward Ott
    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.

  11. Author(s): J.F. Drake, M. Swisdak
    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.

  12. Author(s): Kimberly Glass, Edward Ott, Wolfgang Losert, Michelle Girvan
    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.

  13. Author(s): Jinan Xia, Andrea M. Rossi, Thomas E. M Murphy
    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.

  14. Author(s): Ming-Jer Lee, Thomas M. Antonsen, Edward Ott, Louis M. Pecora
    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.

  15. Author(s): T.C. Li, J.F. Drake, M. Swisdak
    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.

  16. Author(s): Daniel B. Larremore, Marshall Y. Carpenter, Edward Ott, Juan G. Restrepo
    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.

  17. Author(s): Justin K. Markunas, Peter J. Smith, John Melngailis
    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.

  18. Author(s): Gabriele Gradoni, Thomas M. Antonsen, Jr., Edward Ott
    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.

  19. Author(s): R.E. Denton, B.U.Oe. Sonnerup, M. Swisdak, J. Birn, J.F. Drake, M. Hesse
    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%.

  20. Author(s): Jordi Zamora-Munt, Edward Ott
    Publication: EPL 98, 40007 (2012)
    Doi: 10.1209/0295-5075/98/40007

    We study a system of ≫ 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.

  21. Author(s): Marko J. Tadjer, Travis J. Anderson, Karl D. Hobart, Tatyana I. Feygelson, Joshua D. Caldwell, Charles R. Eddy, Jr., Fritz J. Kub, James E. Butler, Bradford Pate, John Melngailis
    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 .

  22. Author(s): Sameer Hemmady, Thomas M. Antonsen, Jr., Edward Ott, Steven M. Anlage
    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.

  23. Author(s): R.L. Fermo, J.F. Drake, M. Swisdak
    Publication: Phys. Rev. Lett. 108, 255005
    Doi: 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.

  24. Author(s): Nicholas A. Mecholsky, Edward Ott, Thomas M. Antonsen, Jr., Parvez Guzdar
    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.

  25. Author(s): Jared K. Wahlstrand, Yu-Hsin Chen, Yu-Hsiang Cheng, Sanjay R. Varma, Howard M. Milchberg
    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.

  26. Author(s): Alexander N. Vlasov, Thomas M. Antonsen, Jr., Igor A. Chernyavskiy, David P. Chernin, Baruch Levush
    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.

  27. Author(s): K.M. Schoeffler, J.F. Drake, M. Swisdak
    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.

  28. Author(s): Wai Lim Ku, Geet Duggal, Yuan Li, Michelle Girvan, Edward Ott
    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.

  29. Author(s): J.H. Odhner, D.A. Romanov, E.T. McCole, J.K. Wahlstrand, H.M. Milchberg, R.J. Levis
    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.

  30. Author(s): Oded Rabin
    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.

  31. Author(s): Urmila Bhanja, Sudipta Mahapatra, Rajarshi Roy
    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.

  32. Author(s): Florian Weilnboeck, Elliot Bartis, Sivan Shachar, Gottlieb S. Oehrlein, David Farber, Tom Lii, Chet Lenox
    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.

  33. Author(s): J.D. Rowley, J.K. Wahlstrand, K.T. Zawilski, P.G. Schunemann, N.C. Giles, A.D. Bristow
    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.

  34. Author(s): Seung Yong Lee, Oded Rabin
    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.

  35. Author(s): Oded Rabin, Seung Yong Lee
    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.

  36. Author(s): Saara A. Khan, Eric J. Montgomery, Blake C. Riddick, Patrick O'Shea
    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.

  37. Author(s): J.K. Wahlstrand, Y.-H. Cheng, H.M. Milchberg
    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.

  38. Author(s): Jane E. Cornett, Oded Rabin
    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.

  39. Author(s): Jen-Hao Yeh, Thomas M. Antonsen, Edward Ott, Steven M. Anlage
    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.

  40. Author(s): Yong Han, Gregory S. Nusinovich, Thomas M. Antonsen, Jr., John Rodgers, Oleksandr V. Sinitsyn
    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.

  41. Author(s): Yi-Hsin Liu, J.F. Drake, M. Swisdak
    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.

  42. Author(s): Ranojoy Bose, Tao Cai, Glenn S. Solomon, Edo Waks
    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.

  43. Author(s): Ranojoy Bose, Deepak Sridharan, Hyochul Kim, Deepak Sridharan, Hyochul Kim, Glenn S. Solomon, Edo Waks
    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.

  44. Author(s): Roland Probst, Zachary Cummins, Chad Ropp, Edo Waks, Benjamin Shapiro
    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.

  45. Author(s): Li-Bo Chen, L.J. Sham, Edo Waks
    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.

  46. Author(s): Mike Armani, Zach Cummins, Jian Gong, Pramod Mathai, Roland Probst, Chad Ropp, Edo Waks, Shawn Walker, Benjamin Shapiro
    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. 

  47. Author(s): Michael A. Holloway, Zeynep Dilli, Nuttiiya Seekhao, John C. Rodgers
    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.

  48. Author(s): H.D. Zhang, R.B. Fiorito, A.G. Shkvarunets, R.A. Kishek, C.P. Welsch
    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.

  49. Author(s): A. Masters, J.P. Eastwood, M. Swisdak, M.F. Thomsen, C.T. Russell, N. Sergis, F.J. Crary, M.K. Dougherty, A.J. Coates, S.M. Krimigis
    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.

  50. Author(s): M.I. Sitnov, M. Swisdak
    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.

2022

  1. Author(s): Alexander N. Vlasov, Thomas M. Antonsen, Jr., David Chernin, Igor A. Chernyavskiy
    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.

  2. Author(s): Prajit Dhara, Norbert M. Linke, Edo Waks, Saikat Guha, Kaushik P. Seshadreesan
    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.

  3. Author(s): Yudong Li, Jingkai Jiang, Michael Hinshelwood, Shiqiang Zhang, Peter J. Bruggeman, Gottlieb S. Oehrlein
    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.

  4. Author(s): Andrew Giuliani, Florian Wechsung, George Stadler, Antoine Cerfon, Matt Landreman
    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.

  5. Author(s): Stephen J. Naus, Jiong Qiu, C. Richard DeVore, Spiro K. Antiochos, Joel T. Dahlin, J..F. Drake, M. Swisdak, Vadim M. Uritsky
    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.

  6. Author(s): Carlos Rios, et al.
    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.

  7. Author(s): H. Arnold, J.F. Drake, M. Swisdak, G. Guo, J. Dahlin, Q. Zhang
    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.

  8. Author(s): Alan A. Kaptanoglu, Tian Qian, Florian Wechsung, Matt Landreman
    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.

  9. Author(s): Haoying Dai, Yanne Kouomou Chembo
    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%.

  10. Author(s): Yang Wang, Sam Klueter, Myungsuk Lee, Junnyeong Yun, Binh Hoang, Elias Kallon, Cholho Lee, Chuan-Fu Lin, Gary W. Rubloff, Sang Bok Lee, Alexander C. Kozen
    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.

  11. Author(s): Souleymane Diallo, Curtis R. Menyuk, Yanne K. Chembo
    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.

  12. Author(s): Yudong Li, Michael Hinshelwood, Gottlieb S. Oehrlein
    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.

  13. Author(s): Nicholas A. Nobile, John R. Erickson, Carlos Rios, Yifei Zhang, Juejun Hu, Steven A. Vitale, Feng Xiong, Nathan Youngblood
    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.

  14. Author(s): Andrew Giuliani, Florian Wechsung, Antoine Cerfon, Georg Stadler, Matt Landreman
    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.

  15. Author(s): S.P. Sabchevski, M. Yu Glyavin, Gregory S. Nusinovich
    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.

  16. Author(s): Maitreyi Ashok, Matthew J. Turner, Ronald L. Walsworth, Edlyn V. Levine, Anatha P. Chandrakasan
    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.

  17. Author(s): Aziz Karasahin, Robert M. Pettit, Nils von den Driesch, Marvin Marco Jansen, Alexander Pawlis, Edo Waks
    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.

  18. Author(s): Meenwook Ha, Yanne Chembo
    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.

  19. Author(s): Florian Wechsun, Andrew Giliani, Matt Landreman, Antoine Cerfon, Georg Stadler
    Publication: Nucl. Fusion 62, 076034 (2022)
    Doi: 10.1088/1741-4326/ac45f3

    It 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.

  20. Author(s): Ekaterina A. Dolgopolova, Dongfang Li, Steven T. Hartman, John Watt, Carlos Rios, et al.
    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.

  21. Author(s): Kang-Yi Lin, Christian Preischl, Christian Felix Hermanns, Daniel Rhinow, Hans-Michael Solowan, Michael Budach, Klaus Edinger, Gottlieb S. Oehrlein
    Publication: J. Vac. Sci. Technol. A: Vacuum, Surfaces, and Films 40, 063004 (2022)
    Doi: 10.1116/6.0002038

    Electron-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.

  22. Author(s): R. Blake Nuwayhid, Daniela Fontecha, Alexander C. Kozen, Angelique Jarry, Sang Bok Lee, Gary W. Rubloff, Keith E. Gregorczyk
    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.

  23. Author(s): Hairui Liu, Zuhong Zhang, Weiwei Zuo, Rajarshi Roy, Meng Li, Malekshahi Mahdi Byranvand, Michael Saliba
    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.

  24. Author(s): Andrew Goffin, J. Griff-McMahon, Ilia Larkin, Howard M. Milchberg
    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.

  25. Author(s): Uday Saha, Edo Waks
    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.

  26. Author(s): A. Baillod, J. Loizu, J.P. Graves, M. Landreman
    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.

  27. Author(s): Vi Kaushik, S. Rajput, S. Srivastav, L. Singh, P. Babu, E. Heidari, et al.
    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).

  28. Author(s): Daniel Woodbury, Robert Schwartz, Howard Milchberg
    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.

  29. Author(s): Levon Dovlatyan, Brian L. Beaudoin, Santiago Bernal, Irving Haber, David Sutter, Thomas M. Antonsen, Jr.
    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.

  30. Author(s): Shukai Ma, Thomas M. Antonsen, Jr., Steven M. Anlage, Edward Ott
    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.

  31. Author(s): M. Oka, T.D. Phan, M. Øieroset, D.L. Turner, J.F. Drake, et al.
    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.

  32. Author(s): Jonathan M. Goodwill, Nitin Prassad, Brian D. Hospins, Matthew W. Daniels, Advait Madhavan, et al.
    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.

  33. Author(s): Nicole Menkart, Joseph D. Hart, Thomas E. Murphy, Rajarshi Roy
    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.

  34. Author(s): Peiyun Shi, Prabhakar Srivastav, M. Hasan Barbhuiya, Paul A. Cassak, Earl E. Scime, M. Swisdak, et al.
    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.

  35. Author(s): M. Hasan Barbuiya, P.A. Cassak, M.A. Shay, Vadim Roytershteyn, M. Swisdak, Amir Caspi, Andrei Runov, Haoming Liang
    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.

  36. Author(s): Soaram Kim, Yoo Byongseok, Matthew Miller, David Bowen, Darryll J. Pines, Kevin M. Daniels
    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.

  37. Author(s): Yajie Yuan, Andrei M. Beloborodov, Alexander Y. Chen, Yuri Levin, Elias R. Most, Alexander A. Philippov
    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.

  38. Author(s): Reza Ebadi, et al. , Ronald L. Walsworth
    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.

  39. Author(s): J.F. Mahlmann, A.A. Philippov, A. Levinson, A. Spitkovsky, H. Hakobyan
    Publication: Astrophys. J. Lett. 932, L20 (2022)
    Doi: 10.3847/2041-8213/ac7156

    One 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 .

  40. Author(s): Jner Tzern Oka, Jiashen Tang, Connor A. Hart, Kevin S. Olsson, Matthew J. Turner, Jennifer M. Schloss, Ronald L. Walsworth
    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.

  41. Author(s): D.B. Graham, Yu. V. Khotyaintsev, M. Andre, A. Valvads, A. Divin, J.F. Drake, et al.
    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.

  42. Author(s): Yu Shi, Edo Waks
    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.

  43. Author(s): Jeong Hyun Shim, Seong-Joo Lee, Santosh Ghimire, Ju Il Hwang, Kwang-Geol Lee, Kiwoong Kim, Matthew J. Turner, Connor A. Hart, Ronald L. Walsworth, Sangwon Oh
    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.

  44. Author(s): Mustafa Atabey Buyukkaya, Chang-Min Lee, Ahmad Mansoori, Ganesh Baumgartner, Yanne K. Chembo
    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.

  45. Author(s): Soaram Kim, Heeju Ryu, Sheldon Tai, Michael Pedowitz, John Robertson Rzasa, Daniels J. Pennachio, Jenifer R. Hajzus, Donald K. Milton, Rachel Myers-Ward, Kevin M. Daniels
    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.

  46. Author(s): Xin Zhang, Kunyi Zhang, Hasson von Bredow, Christopher Metting, George Atanasoff, Robert M. Briber, Oded Rabin
    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.

  47. Author(s): Henry F. Elder, Phillip Sprangle
    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.

  48. Author(s): Mason C. Marshall, Reza Ebadi, Connor Hart, Matthew J. Turner, Mark J.H. Ku, David F. Phillips, Ronald L. Walsworth
    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.

  49. Author(s): Kevin J. Palm, Micah E. Karahadian, Marina S. Leite, Jeremy N. Munday
    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.

  50. Author(s): C.C. Hegna, M. Landreman, E. Paul, et al.
    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.

  51. Author(s): Santiago Bernal
    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.

  52. Author(s): Jaron Shrock, Bo Miao, Linus Feder, Howard M. Milchberg
    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.

  53. Author(s): Xianfei Chen, Gregory S. Nusinovich, Olgierd Dumbrajs, Houxiu Xiao, Xiaotao Han, Donghui Xia, Tao Peng
    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.

  54. Author(s): Matt Landreman
    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.

  55. Author(s): Chuanyu Lian, Christos Vagionas, Theonitsa Alexoudi, Nikos Pleros, Nathan Youngblood, Carlos Rios
    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.

  56. Author(s): Henry Elder, Phillip Sprangle
    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.

  57. Author(s): A. Mazzoni, R. Burke, M. Chin, S. Najmaei, M. Dubey, N. Goldsman, K. Daniels
    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.

  58. Author(s): Kevin J. Palm, Lisa J. Krayer, Jeremy N. Munday
    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.

  59. Author(s): E. Heidari, H. Dalir, F. Mokhtari Koushyar, B. Movahhed Nouri, C. Patil, M. Miscuglio, D. Akinwande, V.J. Sorger
    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.

  60. Author(s): Harjot Singh, Demitry Farfurnik, Zhouchen Luo, Allan S. Bracker, Samuel G. Carter, Edo Waks
    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.

  61. Author(s): Rui Chen, Zhuoran Fang, Jiajiu Zheng, Abhi Saxena, Johannes E. Froech, Arka Majumdar, Asir Intisar Khan, Kathryn M. Neilson, Michelle E. Chen, Eric Pop, Carlos Rios, Peipeng Xu, Juejun Hu
    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.

  62. Author(s): Matt Landreman, Elizabeth Paul
    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.

  63. Author(s): Stefan Eriksson, M. Swisdak, et.al.
    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.

  64. Author(s): B. Miao, J.E. Shrock, L. Feder, R.C. Hollinger, J. Morrison, R. Nedbailo, A. Picksley, H. Song, S. Wang, J.J. Rocca, H. Milchberg
    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.

  65. Author(s): E.J. Paul, A. Bahattacharjee, M. Landreman, D. Alex, J.L. Velasco, R. Nies
    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.

  66. Author(s): I. Adamovich, G.S. Oehrlein, et al.
    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.

  67. Author(s): John M. Howard, Qiong Wang, Meghna Srivastava, Tao Gong, Erica Lee, Antonio Abate, Marina S. Leite
    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.

  68. Author(s): Jeong Woo Han, Matthew L. Chin, Sebastian Matschy, Jayaprakash Poojali, Angelika Seidl, Stephan Winnerl, Hassan A. Hafez, Dmitry Turchinovich, Gagan Kumar, Rachael L. Myers-Ward, Matthew T. Dejarld, Kevin M. Daniels, Howard Dennis Drew, Thomas E. Murphy, Martin Mittendorff
    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.

  69. Author(s): Keshav Srinivasan, Nolan Coble, Joy Hamlin, Thomas M. Antonsen, Jr., Edward Ott, Michelle Girvan
    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.

  70. Author(s): Benjamin Crinquand, Benoit Cerutti, Guillaume Dubus, Kyle Parfrey, Alexander Philippov
    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.

  71. Author(s): Matt Landreman, Stefan Buller, M. Drevlak
    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.

  72. Author(s): Victoria Ferrari, Nam S. Kim, Sang Bok Lee, Gary W. Rubloff, David M. Stewart
    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.

  73. Author(s): B. Miao, Linus Feder, Jaron E. Shrock, Howard M. Milchberg
    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.

  74. Author(s): Robert M. Pettit, Aziz Karasahin, Nils von den Driesch, Marvin Marco Jansen, Alexander Pawlis, Edo Waks
    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.

  75. Author(s): O. Agapitov, J.F. Drake, M. Swisdak, S.D. Bale, T.S. Horbury, J.C. Kasper , R.J. MacDowall, F.S. Mozer, T.D. Phan, M. Pulupa, N.E. Raouafi, M. Velli
    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.

  76. Author(s): Piao Hu, Gary W. Rubloff, et al.
    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.

  77. Author(s): Elizabeth A. Tolman, A.A. Philippov, A.N. Timokhin
    Publication: Astrophys. J. Lett. 933, L37
    Doi: 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.

  78. Author(s): Sai Kanth Dacha, Wenqi Zhu, Amit Agrawal, Kenneth J. Ritter, Thomas E. Murphy
    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.

  79. Author(s): Michael Le Bars, Ankit Barik, Fabian Burmann, Daniel P. Lathrop, Jerome Noir, Nathanael Schaefer, Santiago A. Triana
    Publication: Surveys Geophys. 43, 229
    Doi: 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.

  80. Author(s): Daniela Fontecha, R. Blake Nuwayhid, Alexander C. Kozen, David M. Stewart, Gary W. Rubloff, Keith E. Gregorczyk
    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.

  81. Author(s): Elias R. Most, Jorge Noronha, Alexander A. Philippov
    Publication: Mon. Not. R. Astron. Soc. 514, 4989
    Doi: 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.

  82. Author(s): A. Philippov, M. Kramer
    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.

  83. Author(s): Troy Arcomano, Istvan Szunyogh, Alexander Wikner, Jaideep Pathak, Brian R. Hunt, Edward Ott
    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.

  84. Author(s): Woong Bae Jeon, Jong Sung Moon, Kyu-Young Kim, Young-Ho Ko, Christopher J.K. Richardson, Edo Waks, Je-Hyung Kim
    Publication: Adv. Quantum Technol. 5, 2200022
    Doi: 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.

  85. Author(s): Steven A. Vitale, Carlos Rios, et al.
    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.

  86. Author(s): M.I. Desai, D.G. Mitchell, D.J. McComas, J.F. Drake, et al.
    Publication: Astrophys. J. 927, 62
    Doi: 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/E∝ (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.

  87. Author(s): Daniel Lewis, Brendan Jordan, Michael Pedowitz, Daniel J. Pennachio, Jenifer R. Hajzus, Rachel Myers-Ward, Kevin M. Daniels
    Publication: Nanotechnol. 33, 375202 (2022)
    Doi: 10.1088/1361-6528/ac7653

    Electron 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.

  88. Author(s): Benjamin W. Frazier, Thomas M. Antonsen, Jr., Steven M. Anlage, Edward Ott
    Publication: Phys. Rev. Appl. 17, 024027
    Doi: 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.

  89. Author(s): Elias R. Most, Alexander A. Philippov
    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.

  90. Author(s): Carlos Rios, Linjie Zhou, Ann-Katrin U. Michel, Arka Majumdar, Juejun Hu
    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.

  91. Author(s): Paul Argyle, Phillip Sprangle
    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.

  92. Author(s): R. Jorge, G.G. Plunk, M. Drevlak, M. Landreman, J.-F. Lobsien, K. Camacho Mata, P. Helander
    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.

  93. Author(s): Richard A. Brewster, Julius Goldhar, Mark Morris, Gerald Baumgartner, Yanne K. Chembo
    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.

2021

  1. Author(s): Matt Landreman, Bharat Medasani, Caoxiang Zhu
    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.

  2. Author(s): Amitava Banerjee, Joseph D. Hart, Rajarshi Roy, Edward Ott
    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.

  3. Author(s): Yu Shi, Edo Waks
    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.

  4. Author(s): Meenwook Ha, Yanne K. Chembo
    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.

  5. Author(s): Yuqi Zhao, Hamidreza Chalabi, Edo Waks
    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.

  6. Author(s): Advait Madhavan, Matthew W. Daniels, Mark D. Stiles
    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.

  7. Author(s): Arthur Carlton-Jones, Elizabeth J. Paul, William Dorland
    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.

  8. Author(s): Mikhail Sitnov, Grant Stephens, Tetsuo Motoba, Marc Swisdak
    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.

  9. Author(s): Scott W. Hancock, Sina Zahedpour, Howard M. Milchberg
    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.

  10. Author(s): Sunil Mittal, Gregory Moille, Kartik Srinivasan, Yanne K. Chembo, Mohammad Hafezi
    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.

  11. Author(s): Xianfei Chen, Gregory S. Nusinovich, et al.
    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.

  12. Author(s): M.I. Sitnov, T. Motoba, M. Swisdak
    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.

  13. Author(s): Scott W. Hancock, Sina Zahedpour, Howard M. Milchberg
    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.

  14. Author(s): Richard A. Brewster, Gerald Baumgartner, Yanne K. Chembo
    Publication: Phys. Rev. A 104, 022411 (2021)
    Doi: 10.1103/PhysRevA.104.022411

    Polarization-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.

  15. Author(s): Gregoy Moille, Edgar F. Perez, Jordan R. Stone, Ashutosh Rao, Xiyuan Lu, Tahmid Sami Rahman, Yanne K. Chembo, Kartik Srinivasan
    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.

  16. Author(s): Kevin M. Daniels, Thomas E. Murphy, Martin Mittendorf, et al.
    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.

  17. Author(s): Meghna Srivastava, John M. Howard, Tao Gong, Mariama Rebello Sousa, Marina S. Leite
    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.

  18. Author(s): Scott W. Hancock, Sina Zahedpour, Howard M. Milchberg
    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.

  19. Author(s): Helene Nguewou-Hyousse, Yanne K. Chembo
    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.

  20. Author(s): Haoying Dai, Yanne K. Chembo
    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.

  21. Author(s): Dustin Swanson, Phillip Sprangle
    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.

  22. Author(s): A. Higginson, Y. Wang, H. Chi, A. Goffin, I. Larkin, H.M. Milchberg, J.J. Rocca
    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.

  23. Author(s): Elizabeth J. Paul, Matt Landreman, Thomas M. Antonsen, Jr.
    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.

  24. Author(s): Thomas Daugey, Cyril Billet, John Dudley, Jean-Marc Merolla, Yanne K. Chembo
    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.

  25. Author(s): Philippe Talatchian, Matthew W. Daniels, Advait Madhavan, et al.
    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.

  26. Author(s): Emily M. Hitz, Hua Xie, Yi Lin, John W. Connell, Gary W. Rubloff, Chuan-Fu Lin, Liangbing Hu
    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.

  27. Author(s): Michael D. Pedowitz, Soaram Kim, Daniel I. Lewis, Balaadithya Uppalapati, Digangana Khan, Ferhat Bayram, Goutam Koley, Kevin M. Daniels
    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.

  28. Author(s): J.F. Drake, I. Agapitov, M. Swisdak, et al.
    Publication: Astron. Astrophys. 650, A2 (2021)
    Doi: 10.1051/0004-6361/202039432

    The 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.

  29. Author(s): Advait Madhavan, et al.
    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.

  30. Author(s): John M. Howard, Kevin J. Palm, Qiong Wang, Erica Lee, Antonio Abate, Jeremy N. Munday, Marina S. Leite
    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.

  31. Author(s): Cristian Albert, Matio Brillinger, Paulina Guerrero, Sarah Gottwald, Jennifer Henze, Stefam Schmidt, Edward Ott, Barbara Schroeter
    Publication: AMBIO 50, 1446
    Doi: 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.

  32. Author(s): Gabriel G. Plunk, Matt Landreman, Per Helander
    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.

  33. Author(s): J.F. Drake, C. Pfrommer, C.S. Reynolds, M. Ruszkowski, M. Swisdak, A. Einarsson, T. Thomas, A.B. Hassam, G.T. Roberg-Clark
    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 ~ vtee « 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 Vand 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.

  34. Author(s): Yang Wang, Binh Hoang, John Hoerauf, Cholho Lee, Chuan-Fu Lin, Gary W. Rubloff, Sang Bok Lee, Alexander Kozen
    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.

  35. Author(s): Kyu-Young Kim, Christopher J.K. Richardson, Edo Waks
    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.

  36. Author(s): H. Arnold, J.F. Drake, M. Swisdak, et al.
    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.

  37. Author(s): Adam Pranda, Chen Li, Yongsik Seo, Gottlieb Oehrlein
    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.

  38. Author(s): Alexander Englesbe, Jennifer Elle, Robert Schwartz, Travis Garrett, Daniel Woodbury, Dogeun Jang, Ki-Yong Kim, Howard Milchberg, Remington Reid, et al.
    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.

  39. Author(s): Alexander Wikner, Jaideep Pathak, Brian R. Hunt, Istvan Szunyogh, Michelle Girvan, Edward Ott
    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.

  40. Author(s): Kevin M. Daniels, et al.
    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.

  41. Author(s): Carlos Rios, et al.
    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.

  42. Author(s): Ruben E. Rojas, Artur Perevalov, Till Zürner, Daniel P. Lathrop
    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.

  43. Author(s): Thomas G. Farinha, Tao Gong, Peifen Lyu Ece Deniz, John M. Hoerauf, Marina S. Leite
    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.

  44. Author(s): M. Xuan, M. Swisdak, J.F. Drake
    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.

  45. Author(s): Matt Landreman
    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.

  46. Author(s): Shuo Sun, Edo Waks, Ronald Walsworth, et al.
    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.

  47. Author(s): Fatholah Salehi, Le Manh, Lucas Railing, M. Kolesik, Howard M. Milchberg
    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.

  48. Author(s): Kevin M. Daniels, et al.
    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.

  49. Author(s): Guodong Yu, Zhichen Feng, Peiyou Jiang, Neil Pomphrey, Matt Landreman, Guo Yong Fu
    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.

2020

  1. Author(s): Matthew W. Daniels, Advait Madhavan, Philippe Talatchian, Alice Mizrahi, Mark D. Stiles
    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.

  2. Author(s): Sarvenaz Memarzadeh, Kevin J. Palm, Thomas E. Murphy, Marina S. Leite, Jeremy N. Munday
    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.

  3. Author(s): Dogeun Jang, Ki-Yong Kim
    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.

  4. Author(s): Marina A. Pranda, Brittney J. Murugesan, Andrew J. Knoll, Gottlieb S. Oehrlein, Kimberly M. Stroka
    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.

  5. Author(s): Seung-Hwan Lee, Ki-Yong Kim, Jung-Rag Yoon
    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.

  6. Author(s): Alexander Wikner, Jaideep Pathak, Brian Hunt, Michelle Girvan, Troy Arcomano, Istvan Szunyogh, Andrew Pomerance, Edward Ott
    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).

  7. Author(s): Tristan Deppe, Jeremy N. Munday
    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.

  8. Author(s): Sunil Mittal, Edo Waks
    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.

  9. Author(s): Dogeun Jang, Jae Hee Sung, Seong Ku Lee, Chul Kang, Ki-Yong Kim
    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.

  10. Author(s): Mikhail Proyavin, Olgierd Dumbrajs, Gregory S. Nusinovich, Mikhail Glyavin
    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.

  11. Author(s): Li Chen, Valery Godyak, Thorsten Hofmann, Klaus Edinger, Gottlieb S. Oehrlein
    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.

  12. Author(s): Kunyi Zhang, Andrew P. Lawson, Chase T. Ellis, Matthew S. Davis, Thomas E. Murphy, Hans A. Bechtel, Joseph G. Tischler, Oded Rabin
    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.

  13. Author(s): Sabyasachi Barik, Aziz Karasahin, Sunil Mittal, Edo Waks, Mohammad Hafezi
    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.

  14. Author(s): Subhojit Dutta, Elizabeth A. Goldschmmidt, Sabyasachi Barik, Uday Saha, Edo Waks
    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.

  15. Author(s): David Muller-Bender, Andreas Otto, Guenter Radons, Joseph D. Hart, Rajarshi Roy
    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.

  16. Author(s): Angelique Jarry, Mitchell Walker, Stefan Theodoru, Leonard J. Brillson, Gary W. Rubloff
    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.

  17. Author(s): Malte C. Schroeder, Ilia Larkin, Thomas Produit, Eric W. Rosenthal, Howard M. Milchberg, Jean-Pierre Wolf
    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.

  18. Author(s): Chen Li, Thorsten Hofmann, Klaus Edinger, Valery Godyak, Gottlieb S. Oehrlein
    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.

  19. Author(s): Yuanzhao Zhang, Zachary G. Nicolaou, Joseph Hart, Rajarshi Roy, Adilson E. Motter
    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.

  20. Author(s): Yaqin Cao, Ping Liu, Cuifang Hou, Yanne K. Chembo, Zehuang Lu, Guoping Lin
    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.

  21. Author(s): Zhichen Feng, David A. Gates, Samuel A. Lazerson, Matt Landreman, Neil Pomphrey, Guo Yong Fu
    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.

  22. Author(s): Helene Nguewou-Hyousse, Yanne K. Chembo
    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.

  23. Author(s): Je-Hyung Kim, Shahriar Aghaeimeibodi, Jacques Carolan, Dirk Englund, Edo Waks
    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.

  24. Author(s): Istvan Szunyogh, Troy Arcomano, Jaideep Pathak, Alexander Wikner, Brian Hunt, Edward Ott
    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.

  25. Author(s): Kang-Yi Lin, Chen Li, Sebastian Engelmann, Robert L. Bruce, Eric A. Joseph, Dominik Metzler, Gottlieb S. Oehrlein
    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.

  26. Author(s): Hamidreza Chalabi, Sabyasachi Barik, Sunil Mittal, Thomas E. Murphy, Mohammad Hafezi, Edo Waks
    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.

  27. Author(s): Benjamin W. Frazier, Thomas M. Antonsen, Jr., Steven M. Anlage, Edward Ott
    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.

  28. Author(s): Gregory S. Nusinovich, Xianfei Chen, Olgierd Dumbrajs, Houxiu Xiao, Xiaotao Han
    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.

  29. Author(s): V.S. Santosh K. Kondeti, Yashuang Zheng, Pingshan Luan, Gottlieb S. Oehrlein, Peter J. Bruggeman
    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.

  30. Author(s): Toshihiro Taguchi, Thomas M. Antonsen, Jr., Kunioki Mima
    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.

  31. Author(s): Shen Lin, Zhen Peng, Thomas M. Antonsen, Jr.
    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.

  32. Author(s): Sarthak Chandra, Edward Ott, Michelle Girvan
    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.

  33. Author(s): Joseph L. Garrett, Jongbum Kim, Jeremy N. Munday
    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.

  34. Author(s): R. Blake Nuwayhid, Angelique Jarry, Gary W. Rubloff, Keith E. Gregorczyk
    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.

  35. Author(s): Jimmy C. Kotsakidis, Antonija Grubisi-Cabo, Yuefeng Yin, Anton Tadich, Rachael L. Myers-Ward, Matthew DeJarld, Shojan P. Puvunny, Marc Currie, Kevin M. Daniels, et al.
    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.

  36. Author(s): Yang Tang, Juergen Kurths, Wei Lin, Edward Ott, Ljupco Kocarev
    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.

  37. Author(s): Shukai Ma, Bo Siao, Zachary Drikas, Bisrat Addissie, Ronald Hong, Thomas M. Antonsen, Jr., Edward Ott, Steven M. Anlage
    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.

  38. Author(s): Yanne K. Chembo
    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.

  39. Author(s): Tao Gong, Peifen Lyu, Kevin J. Palm, Sarvenaz Memarzadeh, Jeremy N. Munday, Marina S. Leite
    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.

  40. Author(s): Jason F. Parisi, Felix I. Parra, Colin M. Roach, Carine Giroud, William Dorland, et al.
    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 

    $k_y \rho_i \gtrsim 0.1$

     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 

    $k_y \rho_i \gtrsim 1$

    , the fastest growing mode is often a novel type of toroidal ETG instability. This toroidal ETG mode is driven at scales as large as 

    $k_y \rho_i \sim (\rho_i/\rho_e) L_{Te} / R_0 \sim 1$

     and at a sufficiently large radial wavenumber that electron finite Larmor radius effects become important; that is, 

    $K_x \rho_e \sim 1$

    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 

    ${\bf E} \times {\bf B} $

     shear. ETG modes are very resilient to 

    ${\bf E} \times {\bf B}$

     shear. Heuristic quasilinear arguments suggest that the novel toroidal ETG instability is important for transport.

  41. Author(s): Lisa J. Krayer, Kevin J. Palm, Chen Gong, Alberto Torres, Cesar E. Villegas, Alexandre R. Rocha, Marina S. Leite, Jeremy N. Munday
    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.

  42. Author(s): Elizabeth M. Tennyson, Mojtaba Abdi-Jalebi, Kangyu Jiang, Joseph L. Garrett, Chen Gong, Alison A. Pawlicki, Olga S. Ovchinnikova, Jeremy N. Munday, Samuel D. Stranks, Marina S. Leite
    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.

  43. Author(s): Shukai Ma, Sendy Phang, Zachary Drikas, Bisrat Addissie, Ronald Hong, Valon Blakaj, Gabriele Gradoni, Gregor Tanner, Thomas M. Antonsen, Jr., Edward Ott
    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.

  44. Author(s): Yanne K. Chembo, Aurelien Coillet, Guoping Lin, Pere Colet, Damia Gomila
    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.

  45. Author(s): Chang-Mu Han, Edo Waks, Benjamin Shapiro
    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.

  46. Author(s): Dhruvit Patel, Dogeun Jang, Scott W. Hancock, Howard M. Milchberg, Ki-Yong Kim
    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.

  47. Author(s): Sanjukta Krishnagopal, Michelle Girvan, Edward Ott, Brian R. Hunt
    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.

  48. Author(s): Yogesh S. Virkar, Juan G. Restrepo, Woodrow L. Shew, Edward Ott
    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.

  49. Author(s): Mike F. Martin, Matt Landreman
    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.

  50. Author(s): Geraud R. Goune Chengui, Kengne Jacques, Paul Woafo, Yanne K. Chembo
    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.

  51. Author(s): John M. Howard, Richa Lahoti, Marina S. Leite
    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.

  52. Author(s): Elizabeth J. Paul, Thomas M. Antonsen, Jr., Matt Landreman, W. Anthony Cooper
    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.

  53. Author(s): Ramesh Kudalippalliyalil, Thomas E. Murphy, Karen E. Grutter
    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.

  54. Author(s): Mitchell J. Walker, Angelique Jarry, Nick Pronin, Jake Ballard, Gary W. Rubloff, Leonard J. Brillson
    Publication: J. Mater. Chem. A 8, 11800 (2020)
    Doi: 10.1039/d0ta03204b

    Vanadium 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.

  55. Author(s): Jimmi Hervé Talla Mbé, Yanne K. Chembo
    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.

  56. Author(s): J. Mehdi Jadidi, Mehdi Kargarian, Martin Mittendorff, Yigit Aytac, Bing Shen, Jacob C. Koenig-Otto, Stephan Winnerl, Alexander L. Gaeta, Thomas E. Murphy, H. Dennis Drew
    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.

  57. Author(s): Michael D. Pedowitz, Soaram Kim, Daniel I. Lewis, Balaadithya Uppalapati, Digangana Khan, Ferhat Bayram, Goutam Koley, Kevin M. Daniels
    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.

  58. Author(s): Matt Landreman, Rogerio Jorge
    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.

  59. Author(s): Fei Wang, Chuan-Fu Lin, Xiao Ji, Gary W. Rubloff, Chunsheng Wang
    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.

  60. Author(s): Sai Kanth Dacha, Thomas E. Murphy
    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.

  61. Author(s): Sarvenaz Memarzadeh, Jongbum Kim, Yigit Aytac, Thomas E. Murphy, Jeremy N. Munday
    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.

  62. Author(s): Dogeun Jang, Hanran Jin, Ki-Yong Kim
    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.

  63. Author(s): Adam Pranda, Kang-Yi Lin, Sebastian Engelmann, Robert L. Bruce, Eric A. Joseph, Dominik Metzler, Gottlieb S. Oehrlein
    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.

     

  64. Author(s): Rachel M. Lee, Leonard Campanello, Matt J. Hourwitz, Phillip Alvarez, Ava Omidvar, John T. Fourkas, Wolfgang Losert
    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.

  65. Author(s): Haotian Wang, Keith E. Gregorczyk, Sang Bok Lee, Gary W. Rubloff, Chuan-Fu Lin
    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.

2019

  1. Author(s): Pingshan Luan, V.S. Santosh K. Konteti, Andrew J. Knoll, Peter J. Bruggeman, Gottlieb S. Oehrlein
    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.

  2. Author(s): Keith Burghardt, William Rand, Michelle Girvan
    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.

  3. Author(s): Shojan P. Pavunny, Rachael L. Myers-Ward, Kevin M. Daniels, et al.
    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.

  4. Author(s): Joseph L. Garrett, David A.T. Somers, Kyle Sendgikoski, Jeremy N. Munday
    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.

  5. Author(s): Timothy Koeth, Miriam Hiebert
    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.”

  6. Author(s): Yang Wang, Emily Sahadeo, Gary W. Rubloff, Chuan-Fu Lin, Sang Bok Lee
    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.

  7. Author(s): Pingshan Luan, Gottlieb S. Oehrlein
    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.

  8. Author(s): Hamidreza Chalabi, Sabyasachi Barik, Sunil Mittal, Thomas E. Murphy, Mohammad Hafezi, Edo Waks
    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.

  9. Author(s): Daniel J. Pennachio, Chance C. Ornelas-Skarin, Nathaniel S. Wilson, Samantha G. Rosenberg, Kevin M. Daniels, et al.
    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.

  10. Author(s): Joseph D. Hart, Yuanzhao Zhang, Rajarshi Roy, Adilson E. Motter
    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.

  11. Author(s): Lisa J. Krayer, Jongbum Kim, Joseph L. Garrett, Jeremy N. Munday
    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.

  12. Author(s): Daniel Woodbury, Robert M. Schwartz, Howard M. Milchberg
    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.

  13. Author(s): Zhouchen Luo, Shuo Sun, Aziz Karasahin, Alan S. Backer, Samuel Carter, Michael K. Yakes, Daniel Gammon, Edo Waks
    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.

  14. Author(s): Sarthak Chandra, Edward Ott
    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 are D-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(+)> 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 = 2⁠, for even > 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 KK(+)⁠, there are also incoherent equilibria with a range of possible Kc values below K(+)c⁠, (K(+)c/2) ≤ K< K(+)c⁠. How can the possible instability of incoherent states arising at K = K< K(+)⁠ be reconciled with the previous finding that, at large time (t→∞)⁠, the state is always incoherent unless K(+)? We find, for a given incoherent equilibrium, that, if K is rapidly increased from K < Kc to K< K < K(+)⁠, 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 Kvalue 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 K≈ K(+)⁠. Thus, instability-mediated resetting leads to a unique critical point of the t→∞ time asymptotic state (⁠K(+)⁠) in spite of the existence of an infinity of possible pretransition incoherent states.

  15. Author(s): Anton Peshkov, Michelle Girvan, Derek C. Richardson, Wolfgang Losert
    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.

  16. Author(s): Joseph D. Hart, Rajarshi Roy, David Mueller-Bender, Andreas Otto, Gunter Radons
    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.

  17. Author(s): Matt Landreman, Wrick Sengupta
    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.

  18. Author(s): Zhili Yang, Shahriar Aghaeimeibodi, Edo Waks
    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.

  19. Author(s): Jimmi H. Talla Mbe, Paul Woafo, Yanne K. Chembo
    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.

  20. Author(s): Sarthak Chandra, Michelle Girvan, Edward Ott
    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.

  21. Author(s): Adam Pranda, Sandra A. Gutierrez Razo, John T. Fourkas, Gottlieb S. Oehrlein
    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.

  22. Author(s): John M. Howard, Elizabeth M. Tennyson, Bernardo R.A. Neves, Marina S. Leite
    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.

  23. Author(s): Chang-Min Lee, Mustafa Atabey Buyukkaya, Shahriar Aghaeimeibodi, Aziz Karasahin, Christopher J.K. Richardson, Edo Waks
    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.

  24. Author(s): Min Zhou, Edward Ott, Thomas M. Antonsen, Jr., Steven M. Anlage
    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.

  25. Author(s): Shahriar Aghaeimeibodi, Chang-Min Lee, Mustafa Atabey Buyukkaya, Christopher J.K. Richardson, Edo Waks
    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.

  26. Author(s): Jimmi H. Talla Mbe, Juliette S.D. Kamaha, Yanne K. Chembo, Paul Woalo
    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).

  27. Author(s): Sarthak Chandra, Michelle Girvan, Edward Ott
    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.

  28. Author(s): Marina A. Pranda, Brittney J. Murugesan, Andrew J. Knoll, Gottlieb S. Oehrlein, Kimberly M. Stroka
    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.

  29. Author(s): Joshua Isaac, Phillip Sprangle
    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.

  30. Author(s): Sllvia Xin Li, Nam S. Kim, Kim McKelvey, Chanyuan Liu, Henry S. White, Gary W. Rubloff, Sang Bok Lee, Mark A. Reed
    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.

  31. Author(s): John M. Howard, Richa Lahoti, Marina S. Leite
    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.

  32. Author(s): Shahriar Aghaeimeibodi, Ju-Hyung Kim, Chang-Min Lee, Mustafa Atabey Buyukkaya, Christopher Richardson, Edo Waks
    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.

  33. Author(s): Joshua P. McClure, Jonathan Boltersdorf, David R. Baker, Thomas G. Farinha, Nicholas Dzuricky, Cesar E.P. Villegas, Alexandre R. Rocha, Marina S. Leite
    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.

  34. Author(s): Yanne K. Chembo, Daniel Brunner, Maxime Jacquot, Laurent Larger
    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.

  35. Author(s): Christopher J.K. Richardson, Richard P. Leavitt, Je-Hyung Kim, Edo Waks, Ilke Arslan, Bruce Arey
    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.

  36. Author(s): Vadim Jabotinski, David Ehermin, Thomas M. Antonsen, Jr., Alexander N. Vlasov, Igor A. Chernyavskiy
    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.

  37. Author(s): Haoming Liang, Paul A. Cassak, Sergio Servidio, Michael A. Shay, James F. Drake, Marc Swisdak, et al.
    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.

  38. Author(s): Jose Miguel Algarin, Bharath Ramaswamy, Lucy Venuti, Matthews E. Swierzbinski, James Baker-McKee, Irving N. Weinberg, Yu-Jin Chen, Ilya N. Krivorotov, Jordan A. Katine, Jens Herberholz, Ricardo C. Araneda, Benjamin Shapiro, Edo Waks
    Publication: IEEE Magnetics Lett. 10, 3101405 (2019)
    Doi: 10.1109/LMAG.2019.2896307

    Action 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.

  39. Author(s): Howard M. Milchberg
    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 TodayDecember 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.

  40. Author(s): Aurelien Coillet, Zhen Qi, Irina V. Balakireva, Guoping Lin, Curtis R. Menyuk, Yanne K. Chembo
    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.

  41. Author(s): M. Mehdi Jadidi, Kevin M. Daniels, Rachael L. Myers-Ward, D. Kurt Gaskill, Jacob C. Koenig-Otto, Stephan Winnerl, Andrei B. Sushkov, H. Dennis Drew, Thomas E. Murphy, Martin Mittendorff
    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.

  42. Author(s): Robert M. Schwartz, Daniel Woodbury, Joshua Isaacs, Phillip Sprangle, Howard M. Milchberg
    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.

  43. Author(s): Guoping Lin, Yaqin Cao, Zehuang Lu, Yanne K. Chembo
    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.

  44. Author(s): Thomas M. Antonsen, Jr., David Chernin, John J. Petillo
    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.

  45. Author(s): Jeremy N. Munday
    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.

  46. Author(s): Carles Corbella, Adam Pranda, Sabine Portal, Teresa de los Arcos, Guido Grundmeier, Gottlieb S. Oehrlein, Achim von Keudell
    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.

  47. Author(s): Dogeun Jang, Robert M. Schwartz, Daniel Woodbury, Jesse Griff-McMahon, Abdurrahman H. Younis, Howard M. Milchberg, Ki-Yong Kim
    Publication: Optica 6, 1338 (2019)
    Doi: 10.1364/OPTICA.6.001338

    Nonlinear 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.

  48. Author(s): Itamar Shani, Liam Shaughnessy, John Rzasa, Alessandro Restelli, Brian R. Hunt, Heidi Komkov, Daniel P. Lathrop
    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.

  49. Author(s): Steven T. Lipkowitz, Timothy U. Horton, Thomas E. Murphy
    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.

  50. Author(s): Amitava Banerjee, Jaideep Pathak, Rajarshi Roy, Juan G. Restrepo, Edward Ott
    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.

  51. Author(s): Jeremy N. Munday
    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.

  52. Author(s): Zachary Epstein, Robert H. Lehmberg, Phillip Sprangle
    Publication: Phys. Rev. A 100, 023831 (2019)
    Doi: 10.1103/PhysRevA.100.023831

    In 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.

  53. Author(s): Jayakrishnan Appanam Karakkad, Gregory S. Nusinovich, Brian L. Beaudoin, Antonio C. Ting, Amith H. Narayan, Thomas M. Antonsen, Jr.
    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.

  54. Author(s): Ryan J. Suess, Joseph D. Hart, Edward Leong, Martin Mittendorff, Thomas E. Murphy
    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.

  55. Author(s): Pingshan Luan, Gottlieb Oehrlein
    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.

  56. Author(s): Zachery A. Benson, Chen Gong, Marina S. Leite
    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.

  57. Author(s): Kevin J. Paul, Joseph B. Murray, Joshua P. McClure, Marina S. Leite, Jeremy N. Munday
    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.

  58. Author(s): Thomas G. Farinha, Chen Gong, Zachery A. Benson, Marina S. Leite
    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.

  59. Author(s): Jongbum Kim, Lisa J. Krayer, Joseph L. Garrett, Jeremy N. Munday
    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.

  60. Author(s): Elizabeth M. Tennyson, Bart Roose, Joseph L. Garrett, Chen Gong, Jeremy N. Munday, Antonio Abate, Marina S. Leite
    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.

  61. Author(s): Pingshan Luan, Luis J. Bastarrachea, Andrea R. Gilbert, Rohan Tikekar, Gottlieb S. Oehrlein
    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.

  62. Author(s): Curtis P. Berlinguette, Yet-Ming Chiang, Jeremy N. Munday, Thomas Schenkel, David K. Fork, Ross Koningstein, Matthew D. Trevithick
    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.

  63. Author(s): Elizabeth J. Paul, Ian G. Abel, Matt Landreman, William Dorland
    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.

  64. Author(s): Batiste Galmes, Phan-Huy Kien, Luca Furfaro, Yanne K. Chembo, Jean-Marc Merolla
    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.

  65. Author(s): Andrew J. Knoll, Adam Pranda, Hoki Lee, Gottlieb S. Oehrlein
    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.

  66. Author(s): Elizabeth M. Tennyson, John M. Howard, Bart Roose, Joseph L. Garrett, Jeremy N. Munday, Antonio Abate, Marina S. Leite
    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 reduce