Publications Database


  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. Author(s): Henry F. Elder, Phillip Sprangle
    Publication: Opt. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. 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.

  32. 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.

  33. 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.

  34. 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.

  35. 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.

  36. 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.

  37. 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.

  38. 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.

  39. 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.

  40. 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.

  41. 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.

  42. 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.

  43. 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.

  44. 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.

  45. 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.

  46. 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.

  47. 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.

  48. 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.

  49. 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.

  50. 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.

  51. 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.

  52. 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.

  53. 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.


  1. 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).

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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, 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.

  13. 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 .

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. 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.

  32. 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.

  33. 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.

  34. 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.

  35. 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.

  36. 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.

  37. 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.

  38. 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)

    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.

  39. 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.

  40. Author(s): Stefan Eriksson, M. Swisdak,
    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.

  41. 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.

  42. 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.

  43. 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.

  44. 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.

  45. 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.

  46. 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.

  47. 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.

  48. 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.

  49. 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.

  50. 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.

  51. 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.

  52. 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.

  53. 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.

  54. 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.

  55. 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.

  56. 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.

  57. 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.

  58. 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.

  59. 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.

  60. 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), 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.

  61. 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.

  62. 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.

  63. 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.

  64. 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.

  65. 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.

  66. 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.

  67. 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.

  68. 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.

  69. 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.

  70. 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.

  71. 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.

  72. 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.

  73. 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.

  74. 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.

  75. 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.

  76. 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%.

  77. 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.

  78. 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.

  79. 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.

  80. 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.

  81. 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.

  82. 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.

  83. Author(s): Andrew Giuliani, Florian Wechsung, Antoine Cerfon, Georg Stadler, Matt Landreman
    Publication: J. Comp. Phys. 459, 111147 (2022)
    Doi: 10.1016/

    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.

  84. 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.

  85. 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.

  86. 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.

  87. 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.

  88. 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.

  89. 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.

  90. 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.

  91. 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.

  92. 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.

  93. 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.


  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. 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.

  32. 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.

  33. 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.

  34. 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.

  35. 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.

  36. 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.

  37. 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.

  38. 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.

  39. 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.

  40. 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.

  41. 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.

  42. 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.

  43. 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.

  44. 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.

  45. 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.

  46. 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.

  47. 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.

  48. 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.

  49. 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.


  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. 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.

  32. 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.

  33. 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.

  34. 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.

  35. 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.

  36. 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.

  37. 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.

  38. 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.

  39. 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.

  40. 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.

  41. 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.

  42. 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.

  43. 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.

  44. 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.

  45. 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.

  46. 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.


  47. 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.

  48. 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.

  49. 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.

  50. 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.

  51. 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.

  52. 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.

  53. 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).

  54. 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.

  55. 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.

  56. 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.

  57. 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.

  58. 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.

  59. 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.

  60. 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.

  61. 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.

  62. 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.

  63. 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.

  64. 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.

  65. 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.


  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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).

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. 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.

  32. 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.

  33. 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.

  34. 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.

  35. 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.

  36. 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.

  37. 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.

  38. 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.

  39. 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.

  40. 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.

  41. 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.

  42. 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.

  43. 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.

  44. 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.

  45. 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.

  46. 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.

  47. 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.

  48. 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.

  49. 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.

  50. 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 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.

  51. 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.

  52. 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.

  53. 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.

  54. 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.

  55. 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.”

  56. 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.

  57. 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.

  58. 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.

  59. 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.

  60. 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.

  61. 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.

  62. 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.

  63. 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.

  64. 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.

  65. 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.

  66. 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.


  1. Author(s): Matt Landreman, Wrick Sengupta
    Publication: J. Plasma Phys. 84, 905840616 (2018)
    Doi: 10.1017/S0022377818001289

    The confinement of the guiding-centre trajectories in a stellarator is determined by the variation of the magnetic field strength B in Boozer coordinates (r,θ,φ), but B(r,θ,φ) depends on the flux surface shape in a complicated way. Here we derive equations relating B(r,θ,φ) in Boozer coordinates and the rotational transform to the shape of flux surfaces in cylindrical coordinates, using an expansion in distance from the magnetic axis. A related expansion was done by Garren and Boozer (Phys. Fluids B, vol. 3, 1991a, 2805) based on the Frenet-Serret frame, which can be discontinuous anywhere the magnetic axis is straight, a situation that occurs in the interesting case of omnigenity with poloidally closed B contours. Our calculation in contrast does not use the Frenet-Serret frame. The transformation between the Garren-Boozer approach and cylindrical coordinates is derived, and the two approaches are shown to be equivalent if the axis curvature does not vanish. The expressions derived here help enable optimized plasma shapes to be constructed that can be provided as input to VMEC and other stellarator codes, or to generate initial configurations for conventional stellarator optimization.

  2. Author(s): Jaideep Pathak, Alexander Wikner, Rebeckah Fussell, Sarthak Chandra, Brian R. Hunt, Michelle Girvan, Edward Ott
    Publication: Chaos 28, 041101 (2018)
    Doi: 10.1063/1.5028373

    A model-based approach to forecasting chaotic dynamical systems utilizes knowledge of the mechanistic processes governing the dynamics to build an approximate mathematical model of the system. In contrast, machine learning techniques have demonstrated promising results for forecasting chaotic systems purely from past time series measurements of system state variables (training data), without prior knowledge of the system dynamics. The motivation for this paper is the potential of machine learning for filling in the gaps in our underlying mechanistic knowledge that cause widely-used knowledge-based models to be inaccurate. Thus, we here propose a general method that leverages the advantages of these two approaches by combining a knowledge-based model and a machine learning technique to build a hybrid forecasting scheme. Potential applications for such an approach are numerous (e.g., improving weather forecasting). We demonstrate and test the utility of this approach using a particular illustrative version of a machine learning known as reservoir computing, and we apply the resulting hybrid forecaster to a low-dimensional chaotic system, as well as to a high-dimensional spatiotemporal chaotic system. These tests yield extremely promising results in that our hybrid technique is able to accurately predict for a much longer period of time than either its machine-learning component or its model-based component alone.

  3. Author(s): Hamidreza Chalabi, Edo Waks
    Publication: Phys. Rev. A 98, 063832 (2018)
    Doi: 10.1103/PhysRevA.98.063832

    In this paper, we have developed a method for describing the dynamics of an arbitrary quantum system under a bidirectional time-delayed feedback loop. For this purpose, we have described the evolution in terms of the time propagation of the quantum system of interest without feedback together with several identical systems, which represent the history of the quantum system under study. This technique provides a numerically efficient solution for describing a system's dynamics in the case of significant time delays in which direct investigation of the state of the reservoirs becomes numerically intractable. Using this method, we have studied two scenarios of multiple scatterings of photons incident on a cavity with a two-level atom positioned inside it, coupled to two waveguides that are connected at their ends. In the first scenario, two photons impinge on the cavity through separate waveguides with a delay between them. We have demonstrated that the maximum difference between the two output photon numbers occurs when the delay between the incident photons becomes close to the inverse of their linewidth. In the second scenario, multiple photons impinge on the cavity through the same waveguide and go through multiple interactions. We have shown that, for a fixed atom-cavity coupling rate, the transmission rate enhances as the number of photons increases and have quantified this enhancement. The developed method enables us to study a broad range of nonlinear dynamics in complex quantum networks.

  4. Author(s): Joseph L. Garrett, Marina S. Leite, Jeremy N. Munday
    Publication: ACS Appl. Mater. Interfaces 10, 38850 (2018)
    Doi: 10.1021/acsami.8b08097

    The spatial resolution of atomic force microscopy (AFM) needed to resolve material interfaces is limited by the tip–sample separation (d) dependence of the force used to record an image. Here, we present a new multiscale functional imaging technique that allows for in situ tunable spatial resolution, which can be applied to a wide range of inhomogeneous materials, devices, and interfaces. Our approach uses a multifrequency method to generate a signal whose d-dependence is controlled by mixing harmonics of the cantilever’s oscillation with a modulated force. The spatial resolution of the resulting image is determined by the signal’s d-dependence. Our measurements using harmonic mixing (HM) show that we can change the d-dependence of a force signal to improve spatial resolution by up to a factor of two compared to conventional methods. We demonstrate the technique with both Kelvin probe force microscopy (KPFM) and bimodal AFM to show its generality. Bimodal AFM with harmonic mixing actuation separates conservative from dissipative forces and is used to identify the regions of adhesive residue on exfoliated graphene. Our electrostatic measurements with open-loop KPFM demonstrate that multiple force modulations may be applied at once. Further, this method can be applied to any tip–sample force that can be modulated, for example, electrostatic, magnetic, and photoinduced forces, showing its universality.  Because HM enables in situ switching between high sensitivity and high spatial resolution with any periodic driving force, we foresee this technique as a critical advancement for multiscale functional imaging.

  5. Author(s): Shuo Sun, Hyochul Kim, Zhouchen Luo, Glenn S. Solomon, Edo Waks
    Publication: Science 361, 57 (2018)
    Doi: 10.1126/science.aat3581

    Single-photon switches and transistors generate strong photon-photon interactions that are essential for quantum circuits and networks. However, the deterministic control of an optical signal with a single photon requires strong interactions with a quantum memory, which has been challenging to achieve in a solid-state platform. We demonstrate a single-photon switch and transistor enabled by a solid-state quantum memory. Our device consists of a semiconductor spin qubit strongly coupled to a nanophotonic cavity. The spin qubit enables a single 63-picosecond gate photon to switch a signal field containing up to an average of 27.7 photons before the internal state of the device resets. Our results show that semiconductor nanophotonic devices can produce strong and controlled photon-photon interactions that could enable high-bandwidth photonic quantum information processing.

  6. Author(s): P.A. Cassak, R.N. Baylor, R.L. Fermo, M.T. Beidler, M.A. Shay, M. Swisdak, J.F. Drake, H. Karimabadi
    Publication: Phys. Plasmas 22, 20705 (2015)
    Doi: 10.1063/1.4908545

    A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks.

  7. Author(s): Kevin Leung, Alexander J. Pearse, A. Alec Talin, Elliott J. Fuller, Gary W. Rubloff, Normand A. Modine
    Publication: Chemsuschem 11, 1956 (2018)
    Doi: 10.1002/cssc.201800027

    Detailed understanding of solid–solid interface structure–function relationships is critical for the improvement and wide deployment of all-solid-state batteries. The interfaces between lithium phosphorous oxynitride (LiPON) solid electrolyte material and lithium metal anode, and between LiPON and LixCoO2 cathode, have been reported to generate solid–electrolyte interphase (SEI)-like products and/or disordered regions. Using electronic structure calculations and crystalline LiPON models, we predict that LiPON models with purely P−N−P backbones are kinetically inert towards lithium at room temperature. In contrast, transfer of oxygen atoms from low-energy LixCoO2(104) surfaces to LiPON is much faster under ambient conditions. The mechanisms of the primary reaction steps, LiPON structural motifs that readily reacts with lithium metal, experimental results on amorphous LiPON to partially corroborate these predictions, and possible mitigation strategies to reduce degradations are discussed. LiPON interfaces are found to be useful case studies for highlighting the importance of kinetics-controlled processes during battery assembly at moderate processing temperatures.

  8. Author(s): Alexander Pearse, Thomas Schmitt, Emily Sahadeo, David M. Stewart, Alexander Kozen, Kostantinos Gerasopoulos, A. Alec Talin, Sang Bok Lee, Gary W. Rubloff, Keith E. Gregorczyk
    Publication: ACS Nano 12, 4286 (2018)
    Doi: 10.1021/acsnano.7b08751

    Three-dimensional thin-film solid-state batteries (3D TSSB) were proposed by Long et al. in 2004 as a structure-based approach to simultaneously increase energy and power densities. Here, we report experimental realization of fully conformal 3D TSSBs, demonstrating the simultaneous power-and-energy benefits of 3D structuring. All active battery components—electrodes, solid electrolyte, and current collectors—were deposited by atomic layer deposition (ALD) onto standard CMOS processable silicon wafers microfabricated to form arrays of deep pores with aspect ratios up to approximately 10. The cells utilize an electrochemically prelithiated LiV2O5 cathode, a very thin (40–100 nm) Li2PO2N solid electrolyte, and a SnNx anode. The fabrication process occurs entirely at or below 250 °C, promising compatibility with a variety of substrates as well as integrated circuits. The multilayer battery structure enabled all-ALD solid-state cells to deliver 37 μAh/cm2·μm (normalized to cathode thickness) with only 0.02% per-cycle capacity loss. Conformal fabrication of full cells over 3D substrates increased the areal discharge capacity by an order of magnitude while simulteneously improving power performance, a trend consistent with a finite element model. This work shows that the exceptional conformality of ALD, combined with conventional semiconductor fabrication methods, provides an avenue for the successful realization of long-sought 3D TSSBs which provide power performance scaling in regimes inaccessible to planar form factor cells.

  9. Author(s): Geraud R. Goune Chengui, Jimmi H. Talla Mbe, Alain Francis Talla, Paul Woafo, Yanne K. Chembo
    Publication: IEEE J. Quantum Electron. 54, 5000207 (2018)
    Doi: 10.1109/JQE.2017.2782319

    We present a theoretical and experimental study of a low-frequency optoelectronic oscillator featuring both laser-diode and Van der Pol-like nonlinearities. In this architecture, the device performing the electrical-to-optical conversion is the laser-diode itself instead of an external electro-optical modulator, while the electric branch of the oscillator is characterized by a Van der Pol nonlinear transfer function. We show that the system displays a complex autonomous dynamics, induced by the competition between these two nonlinearities. In the case of small delay, the system displays harmonic and relaxation oscillations. When the delay is large, the interplay between the two nonlinearities leads to a period-doubling route of bifurcations as the feedback gain is increased, and ultimately to fully developed chaos. Our experimental measurements are in good agreement with the theoretical analysis.

  10. Author(s): Chen Gong, Alan Kaplan, Zachery A. Benson, David R. Baker, Joshua P. McClure, Alexandre R. Rocha, Marina S. Leite
    Publication: Adv. Opt. Mater. 6, 1800218 (2018)
    Doi: 10.1002/adom.201800218

    Surface plasmon polaritons (SPPs) enable the deep subwavelength confinement of an electromagnetic field, which can be used in optical devices ranging from sensors to nanoscale lasers. However, the limited number of metals that satisfy the required boundary conditions for SPP propagation in a metal/dielectric interface severely limits its occurrence in the visible range of the electromagnetic spectrum. We introduce the strategy of engineering the band structure of metallic materials by alloying. We experimentally and theoretically establish the control of the dispersion relation in Ag–Au alloys by varying the film chemical composition. Through X-ray photoelectron spectroscopy (XPS) measurements and partial density-of-states calculations we deconvolute the d band contribution of the density-of-states from the valence band spectrum, showing that the shift in energy of the d band follows the surface plasmon resonance change of the alloy. Our density functional theory calculations of the alloys band structure predict the same variation of the threshold of the interband transition, which is in very good agreement with our optical and XPS experiments. By elucidating the correlation between the optical behavior and band structure of alloys, we anticipate the fine control of the optical properties of metallic materials beyond pure metals.

  11. Author(s): Dinhduy Than Vu, Dogeun Sang, Ki-Yong Kim
    Publication: Opt. Exp. 26, 20572 (2018)
    Doi: 10.1364/OE.26.020572

    Single-shot supercontinuum spectral interferometry (SSSI) is an optical technique that can measure ultrafast transients in the complex index of refraction. This method uses chirped supercontinuum reference/probe pulses that need to be pre-characterized prior to use. Conventionally, the spectral phase (or chirp) of those pulses can be determined from a series of phase or spectral measurements taken at various time delays with respect to a pump-induced modulation. Here we propose a novel method to simplify this process and characterize reference/probe pulses up to the third order dispersion from a minimum of 2 snapshots taken at different pump-probe delays. Alternatively, without any pre-characterization, our method can retrieve both unperturbed and perturbed reference/probe phases, including the pump-induced modulation, from 2 time-delayed snapshots. From numerical simulations, we show that our retrieval algorithm is robust and can achieve high accuracy even with 2 snapshots. Without any apparatus modification, our method can be easily applied to any experiment that uses SSSI.

  12. 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 36, 040601 (2018)
    Doi: 10.1116/1.5035291

    The authors demonstrate that complex hydrofluorocarbon (HFC) precursors offer significant advantages relative to gas mixtures of comparable elemental ratios for plasma-based selective atomic layer etching (ALE). This work compares mixtures of a fluorocarbon precursor and H2 with an HFC precursor, i.e., mixtures of octafluorocyclobutane (C4F8) with H2 and 3,3,3-trifluoropropene (C3H3F3), for SiO2 ALE and etching of SiO2 selective to Si3N4 or Si. For continuous plasma etching, process gas mixtures, e.g., C4F8/H2, have been employed and enable highly selective material removal based on reduction of the fluorine content of deposited steady-state HFC films; however this approach is not successful for ALE since hydrogen-induced etching reduces the thickness of the ultrathin HFC passivation layer which is required for both etching of SiO2 and passivation of the Si3N4 and Si underlayers, leading to lower materials etching selectivity. Conversely, the experimental results show that C3H3F3-based ALE enables ultrahigh ALE selectivity of SiO2 over Si3N4 and Si. The hydrogen in the precursor structure allows to reduce the fluorine content of the deposited HFC film without suppressing the formation of the passivation layer on the surface. Gas pulsing of complex reactive precursors in ALE provides the prospect of utilizing the precursor chemical structure for achieving high materials selectivity in ALE.

  13. Author(s): Adam Pranda, Sandra S. Gutierrez Razo, Zuleykhan Tomova, John T. Fourkas, Gottlieb S. Oehrlein
    Publication: J. Vac. Sci. Technol. A 36, 021304 (2018)
    Doi: 10.1116/1.5009640

    The development of new photoresists for semiconductor manufacturing applications requires an understanding of the material properties that control the material's plasma etching behavior. Ion bombardment at ion energies of the order 100 s of eV is typical of plasma-based pattern-transfer processes and results in the formation of a dense amorphous carbon (DAC) layer on the surface of a photoresist, such as the PR193-type of photoresist that currently dominates the semiconductor industry. Prior studies have examined the physical properties of the DAC layer, but the correlation between these properties and the photoresist etching behavior had not been established. In this work, the authors studied the real-time evolution of a steady-state DAC layer as it is selectively depleted using an admixture of oxygen into an argon plasma. Observations of the depletion behavior for various DAC layer thicknesses motivate a new model of DAC layer depletion. This model also correlates the impact of the DAC layer thickness with the etch rate of the bulk photoresist. The authors find that up to a 40% depletion of the DAC layer thickness does not have a significant impact on the bulk photoresist etch rate. However, further depletion results in an exponential increase in the etch rate, which can be up to ten times greater at full depletion than for the fully formed DAC layer. Thus, with these trends the authors show that the photoresist etch rate is controlled by the thickness of the DAC layer. Furthermore, thickness loss of the DAC layer in an O2-containing plasma coincides with a chemical modification of the layer into an oxygen-rich surface overlayer with properties that are intermediate between those of the DAC layer and the bulk photoresist. Support for this interpretation was provided via x-ray photoelectron spectroscopy characterization. Atomic force microscopy was used to gauge the impact on surface roughness as the DAC layer is formed and depleted. The trends established in this work will provide a benchmark in our development of new photoresists, which will be suitable for pattern transfer processes that will ultimately be a part of enabling smaller semiconductor device feature sizes and pitches.

  14. Author(s): Tao Cai, Je-Hyung Kim, Zhili Yang, Subhojit Dutta, Shahriar Aghaeimeibodi, Edo Waks
    Publication: ACS Photon. 5, 3466 (2018)
    Doi: 10.1021/acsphotonics.8b00580

    Plasmonic nanostructures provide an efficient way to control and enhance the radiative properties of quantum emitters. Coupling these structures to single defects in two-dimensional materials provides a particularly promising material platform to study emitter–plasmon interactions because these emitters are not embedded in a surrounding dielectric. They can therefore approach a near-field plasmonic mode to nanoscale distances, potentially enabling strong light–matter interactions. However, this coupling requires precise alignment of the emitters to the plasmonic mode of the structures, which is particularly difficult to achieve in a site-controlled structure. We present a technique to generate quantum emitters in two-dimensional tungsten diselenide coupled to site-controlled plasmonic nanopillars. The plasmonic nanopillar induces strains in the two-dimensional material which generate quantum emitters near the high-field region of the plasmonic mode. The electric field of the nanopillar mode is nearly parallel to the two-dimensional material and is therefore in the correct orientation to couple to the emitters. We demonstrate both an enhanced spontaneous emission rate and increased brightness of emitters coupled to the nanopillars. This approach may enable bright site-controlled nonclassical light sources for applications in quantum communication and optical quantum computing.

  15. Author(s): Colby C. Haggerty, Michael A. Shay, Alexandros Chasapis, Tai D. Phan, James F. Drake, Kittipat Malakit, Paul A. Cassak, Rungployphan Kieokaew
    Publication: Phys. Plasmas 25, 102120 (2018)
    Doi: 10.1063/1.5050530

    The outflow velocity of jets produced by collisionless magnetic reconnection is shown to be reduced by the ion exhaust temperature in fully kinetic particle in cell simulations and in situ satellite observations. We derive a scaling relationship for the outflow velocity based on the upstream Alfvén speed and the parallel ion exhaust temperature, which is verified in kinetic simulations and observations. The outflow speed reduction is shown to be due to the firehose instability criterion, and so, for large enough guide fields, this effect is suppressed and the outflow speed reaches the upstream Alfvén speed based on the reconnecting component of the magnetic field.

  16. Author(s): Roscoe White, Adil Hassam, Alain Brizard
    Publication: Phys. Plasmas 25, 012514 (2018)
    Doi: 10.1063/1.5003359

    The use of supersonic rotation of a plasma in mirror geometry has distinct advantages for thermonuclear fusion. The device is steady state, there are no disruptions, the loss cone is almost closed, sheared rotation stabilizes magnetohydrodynamic instabilities as well as plasma turbulence, there are no runaway electrons, and the coil configuration is simple. In this work, we examine the effect of rotation on mirror confinement using a full cyclotron orbit code. The full cyclotron simulations give a much more complete description of the particle energy distribution and losses than the use of guiding center equations. Both collisionless loss as a function of rotation and the effect of collisions are investigated. Although the cross field diffusion is classical, we find that the local rotating Maxwellian is increased to higher energy, increasing the fusion rate and also enhancing the radial diffusion. We find a loss channel not envisioned with a guiding center treatment, but a design can be chosen that can satisfy the Lawson criterion for ions. Of course, the rotation has a minimal effect on the alpha particle birth distribution, so there is initially loss through the usual loss cone, just as in a mirror with no rotation. However after this loss, the alphas slow down on the electrons with little pitch angle scattering until reaching low energy, so over half of the initial alpha energy is transferred to the electrons. The important problem of energy confinement, with losses primarily through the electron channel, is not addressed in this work. We also discuss the use of rotating mirror geometry to produce an ion thruster.

  17. Author(s): Zhixin Lu, Brian R. Hunt, Edward Ott
    Publication: Chaos 28, 061104 (2018)
    Doi: 10.1063/1.5039508

    A machine-learning approach called “reservoir computing” has been used successfully for short-term prediction and attractor reconstruction of chaotic dynamical systems from time series data. We present a theoretical framework that describes conditions under which reservoir computing can create an empirical model capable of skillful short-term forecasts and accurate long-term ergodic behavior. We illustrate this theory through numerical experiments. We also argue that the theory applies to certain other machine learning methods for time series prediction.

  18. Author(s): Shahriar Aghaeimeibodi, Boris Desiatov, Je-Hyung Kim, Chang-Min Lee, Mustafa Atabey Buyukkaya, Aziz Karasahin, Christopher J. Richardson, Richard P. Leavitt, Marko Loncar, Edo Waks
    Publication: Appl. Phys. Lett. 113, 221102 (2018)
    Doi: 10.1063/1.5054865

    The integration of quantum emitters with integrated photonics enables complex quantum photonic circuits that are necessary for photonic implementation of quantum simulators, computers, and networks. Thin-film lithium niobate is an ideal material substrate for quantum photonics because it can tightly confine light in small waveguides and has a strong electro-optic effect that can switch and modulate single photons at low power and high speed. However, lithium niobate lacks efficient single-photon emitters, which are essential for scalable quantum photonic circuits. We demonstrate deterministic coupling of single-photon emitters with a lithium niobate photonic chip. The emitters are composed of InAs quantum dots embedded in an InP nanobeam, which we transfer to a lithium niobate waveguide with nanoscale accuracy using a pick-and-place approach. An adiabatic taper transfers single photons emitted into the nanobeam to the lithium niobate waveguide with high efficiency. We verify the single photon nature of the emission using photon correlation measurements performed with an on-chip beamsplitter. Our results demonstrate an important step toward fast, reconfigurable quantum photonic circuits for quantum information processing.

  19. Author(s): G.T. Roberg-Clark, J.F. Drake, C.S. Reynolds, M. Swisdak
    Publication: Astrophys. J. 867, 154 (2018)
    Doi: 10.3847/1538-4357/aae393

    Heat flux suppression in collisionless plasmas for a large range of plasma β is explored using two-dimensional particle-in-cell simulations with a strong, sustained thermal gradient. We find that a transition takes place between whistler-dominated (high-β) and double-layer-dominated (low-β) heat flux suppression. Whistlers saturate at small amplitude in the low beta limit and are unable to effectively suppress the heat flux. Electrostatic double layers (DLs) suppress the heat flux to a mostly constant factor of the free-streaming value once this transition happens. The DL physics is an example of ion–electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of β explored. The range of plasma-βs studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including the intracluster medium of galaxy clusters, hot accretion flows, stellar and accretion disk coronae, and the solar wind.

  20. Author(s): David Darmon, William Rand, Michelle Girvan
    Publication: Phys. Rev. E 98, 062306 (2018)
    Doi: 10.1103/PhysRevE.98.062306

    With the increasing abundance of “digital footprints” left by human interactions in online environments, e.g., social media and app use, the ability to model complex human behavior has become increasingly possible. Many approaches have been proposed, however, most previous model frameworks are fairly restrictive. We introduce a new social modeling approach that enables the creation of models directly from data with minimal a priori restrictions on the model class. In particular, we infer the minimally complex, maximally predictive representation of an individual's behavior when viewed in isolation and as driven by a social input. We then apply this framework to a heterogeneous catalog of human behavior collected from 15 000 users on the microblogging platform Twitter. The models allow us to describe how a user processes their past behavior and their social inputs. Despite the diversity of observed user behavior, most models inferred fall into a small subclass of all possible finite-state processes. Thus, our work demonstrates that user behavior, while quite complex, belies simple underlying computational structures.

  21. Author(s): John M. Howard, Elizabeth M. Tennyson, Sabyasachi Barik, Rodrigo Szostak, Edo Waks, Michael Toney, Ana F. Nogueira, Bernardo R.A. Neves, Marina S. Leite
    Publication: J. Phys. Chem. Lett. 9, 3463 (2018)
    Doi: 10.1021/acs.jpclett.8b01357

    Hybrid organic–inorganic perovskites containing Cs are a promising new material for light-absorbing and light-emitting optoelectronics. However, the impact of environmental conditions on their optical properties is not fully understood. Here, we elucidate and quantify the influence of distinct humidity levels on the charge carrier recombination in CsxFA1–xPb(IyBr1–y)3 perovskites. Using in situ environmental photoluminescence (PL), we temporally and spectrally resolve light emission within a loop of critical relative humidity (rH) levels. Our measurements show that exposure up to 35% rH increases the PL emission for all Cs (10–17%) and Br (17–38%) concentrations investigated here. Spectrally, samples with larger Br concentrations exhibit PL redshift at higher humidity levels, revealing water-driven halide segregation. The compositions considered present hysteresis in their PL intensity upon returning to a low-moisture environment due to partially reversible hydration of the perovskites. Our findings demonstrate that the Cs/Br ratio strongly influences both the spectral stability and extent of light emission hysteresis. We expect our method to become standard when testing the stability of emerging perovskites, including lead-free options, and to be combined with other parameters known for affecting material degradation, e.g., oxygen and temperature.

  22. Author(s): D. Woodbury, L. Feder, V. Shumakova, C. Gollner, R. Schwartz, B. Miao, F. Salehi, A. Korolov, A. Pugzlys, A. Baltuska, H.M. Milchberg
    Publication: Opt. Lett. 43, 1131 (2018)
    Doi: 10.1364/OL.43.001131

    ►We report on, to the best of our knowledge, the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared (IR) laser pulses (lambda = 3.9 μm, 100 fs, 0.25 TW), which enable near- and above-critical density interactions with moderate-density gas jets. Relativistic electron acceleration up to  ∼12 MeV ∼ 12 MeV occurs when the jet width exceeds the threshold scale length for relativistic self-focusing. We present scaling trends in the accelerated beam profiles, charge, and spectra, which are supported by particle-in-cell simulations and time-resolved images of the interaction. For similarly scaled conditions, we observe significant increases in the accelerated charge, compared to previous experiments with near-infrared (lambda = 800 nm) pulses.



  23. Author(s): Wei Luo, John Hayden, Soo-Hwan Jang, Yilin Wang, Ying Zhang, Yudi Kuang, Yanbin Wang, Yubing Zhou, Gary W. Rubloff, Chuan-Fu Lin
    Publication: Adv. Energy Mater. 8, 1702615 (2018)
    Doi: 10.1002/aenm.201702615

    Lithium-ion batteries (LIBs) are integral parts of modern technology, but can raise safety concerns because of their flammable organic electrolytes with low flash points. Aqueous electrolytes can be used in LIBs to overcome the safety issues that come with organic electrolytes while avoiding poor kinetics associated with solid state electrolytes. Despite advances in aqueous electrolytes, current collectors for aqueous battery systems have been neglected. Current collectors used in today's aqueous battery systems are usually metal-based materials, which are heavy, expensive, bulky, and prone to corrosion after prolonged use. Here, a carbon nanotube (CNT)–cellulose nanofiber (CNF) all-fiber composite is developed that takes advantage of the high conductivity of CNT while achieving high mechanical strength through the interaction between CNT and CNF. By optimizing the CNT/CNF weight ratio, this all-fiber current collector can be made very thin while maintaining high conductivity (≈700 S cm−1) and strength (>60 MPa), making it an ideal replacement for heavy metal current collectors in aqueous battery systems.

  24. Author(s): H. Arnold, M. Swisdak, J.F. Drake
    Publication: J. Geophys. Res.-Space Phys. 123, 6327 (2018)
    Doi: 10.1029/2018JA025604

    The structure of dipolarization jets with finite width in the dawn-dusk direction relevant to magnetic reconnection in the Earth's magnetotail is explored with particle-in-cell simulations. We carry out Riemann simulations of the evolution of the jet in the dawn-dusk, north-south plane to investigate the dependence of the jet structure on the jet width in the dawn-dusk direction. We find that the magnetic field and Earth-directed ion flow structure depend on the dawn-dusk width. A reversal in the usual Hall magnetic field near the center of the current sheet on the duskside of larger jets is observed. For small widths, the maximum velocity of the earthward flow is significantly reduced below the theoretical limit of the upstream Alfvén speed. However, the ion flow speed approaches this limit once the width exceeds the ion Larmor radius based on the normal magnetic field, Bz.

  25. Author(s): Matthew DeJarld, Paul M. Campbell, Adam L. Friedman, Marc Currie, Rachael L. Myers-Ward, Anthony K. Boyd, Samantha G. Rosenberg, Shojan P. Pavunny, Kevin M. Daniels, D.K. Gaskill
    Publication: Sci. Rep. 8, 16487 (2018)
    Doi: 10.1038/s41598-018-34595-1

    Metal films deposited on graphene are known to influence its electronic properties, but little is known about graphene’s interactions with very low work function rare earth metals. Here we report on the work functions of a wide range of metals deposited on n-type epitaxial graphene (EG) as measured by Kelvin Probe Force Microscopy (KPFM). We compare the behaviors of rare earth metals (Pr, Eu, Er, Yb, and Y) with commonly used noble metals (Cr, Cu, Rh, Ni, Au, and Pt). The rare earth films oxidize rapidly, and exhibit unique behaviors when on graphene. We find that the measured work function of the low work function group is consistently higher than predicted, unlike the noble metals, which is likely due to rapid oxidation during measurement. Some of the low work function metals interact with graphene; for example, Eu exhibits bonding anomalies along the metal-graphene perimeter. We observe no correlation between metal work function and photovoltage, implying the metal-graphene interface properties are a more determinant factor. Yb emerges as the best choice for future applications requiring a low-work function electrical contact on graphene. Yb films have the strongest photovoltage response and maintains a relatively low surface roughness, ~5 nm, despite sensitivity to oxidation.

  26. Author(s): Emily Sahadeo, Jaehee Song, Karen Gaskell, Nam Kim, Gary W. Rubloff, Sang Bok Lee
    Publication: Phys. Chem. Chem. Phys. 20, 2517 (2018)
    Doi: 10.1039/c7cp06312a

    Batteries based on magnesium chemistry are being widely investigated as an alternative energy storage system to replace lithium-ion batteries. Mg batteries have multiple challenges, especially on the cathode side. The divalent Mg ion has slow insertion kinetics in many metal oxide cathodes conventionally used in Li-ion batteries. One solution that has been explored is adding water molecules into an organic electrolyte, which has been shown to aid in Mg insertion and improve performance of manganese oxide (MnO2) cathodes. While there have been studies on Mg insertion mechanisms into MnO2 in solely aqueous or organic electrolytes for some crystalline MnO2 polymorphs, our work is focused on water-containing organic electrolyte, where an H2O to Mg ratio of 6 : 1 is present. In this study, we report results based on ex situ XPS experiments, including both angle resolved and depth profiling studies to assess the surface reactions and determine the mechanism of Mg insertion into an amorphous, electrodeposited MnO2 cathode. We propose that in this mixed electrolyte system, there is a combined insertion/conversion reaction mechanism whereby Mg and H2O molecules co-insert into the MnO2 structure and a reaction between H2O and Mg creates an observable Mg(OH)2 layer at the surface of the MnO2. A more full understanding of the role of the water molecules is important to aid in the future design of cathode materials, especially when determining potential ways to integrate metal oxides in Mg batteries.

  27. Author(s): Je-Hyung Kim, Shahriar Aghaeimeibodi, Christopher J.K. Richardson, Richard P. Leavitt, Edo Waks
    Publication: Nano Lett. 18, 4734 (2018)
    Doi: 10.1021/acs.nanolett.8b01133

    Future scalable photonic quantum information processing relies on the ability of integrating multiple interacting quantum emitters into a single chip. Quantum dots provide ideal on-chip quantum light sources. However, achieving quantum interaction between multiple quantum dots on-a-chip is a challenging task due to the randomness in their frequency and position, requiring local tuning technique and long-range quantum interaction. Here, we demonstrate quantum interactions between separated two quantum dots on a nanophotonic waveguide. We achieve a photon-mediated long-range interaction by integrating the quantum dots to the same optical mode of a nanophotonic waveguide and overcome spectral mismatch by incorporating on-chip thermal tuners. We observe their quantum interactions of the form of super-radiant emission, where the two dots collectively emit faster than each dot individually. Creating super-radiant emission from integrated quantum emitters could enable compact chip-integrated photonic structures that exhibit long-range quantum interactions. Therefore, these results represent a major step toward establishing photonic quantum information processors composed of multiple interacting quantum emitters on a semiconductor chip.

  28. Author(s): Bo Xiao, Thomas M. Antonsen, Jr., Edward Ott, Zachary B. Drikas, Jesus Gil, Steven M. Anlage
    Publication: Phys. Rev. E 97, 062220 (2018)
    Doi: 10.1103/PhysRevE.97.062220

    The Random Coupling Model (RCM) predicts the statistical properties of waves inside a ray-chaotic enclosure in the semiclassical regime by using Random Matrix Theory, combined with system-specific information. Experiments on single cavities are in general agreement with the predictions of the RCM. It is now desired to test the RCM on more complex structures, such as a cascade or network of coupled cavities, that represent realistic situations but that are difficult to test due to the large size of the structures of interest. This paper presents an experimental setup that replaces a cubic-meter-scale microwave cavity with a miniaturized cavity, scaled down by a factor of 20 in each dimension, operated at a frequency scaled up by a factor of 20 and having wall conductivity appropriately scaled up by a factor of 20. We demonstrate experimentally that the miniaturized cavity maintains the statistical wave properties of the larger cavity. This scaled setup opens the opportunity to study wave properties in large structures such as the floor of an office building, a ship, or an aircraft, in a controlled laboratory setting.

  29. Author(s): Dakang Ma, Jeremy N. Munday
    Publication: Sci. Rep. 8, 15930 (2018)
    Doi: 10.1038/s41598-018-34381-z


    Opto-mechanical forces result from the momentum transfer that occurs during light-matter interactions. One of the most common examples of this phenomenon is the radiation pressure that is exerted on a reflective surface upon photon reflection. For an ideal mirror, the radiation pressure is independent of the wavelength of light and depends only on the incident power.  Here we consider a different regime where, for a constant input optical power, wavelength-dependent radiation pressure is observed due to coherent thin film Fabry-Perot interference effects. We perform measurements using a Si microcantilever and utilize an in-situ optical transmission technique to determine the local thickness of the cantilever and the light beam’s angle of incidence. Although Si is absorptive in the visible part of the spectrum, by exploiting the Fabry-Perot modes of the cantilever, we can determine whether momentum is transferred via reflection or absorption by tuning the incident wavelength by only ~20 nm. Finally, we demonstrate that the tunable wavelength excitation measurement can be used to separate photothermal effects and radiation pressure.

  30. Author(s): Justin Ball, Felix I. Parra, Matt Landreman, Michael Barnes
    Publication: Nucl. Fusion 58, 026003 (2018)
    Doi: 10.1088/1741-4326/aa9a50

    Breaking the up–down symmetry of the tokamak poloidal cross-section can significantly increase the spontaneous rotation due to turbulent momentum transport. In this work, we optimize the shape of flux surfaces with both tilted elongation and tilted triangularity in order to maximize this drive of intrinsic rotation. Nonlinear gyrokinetic simulations demonstrate that adding optimally-tilted triangularity can double the momentum transport of a tilted elliptical shape. This work indicates that tilting the elongation and triangularity in an ITER-like device can reduce the energy transport and drive intrinsic rotation with an Alfvén Mach number of roughly  1%.  This rotation is four times larger than the rotation expected in ITER and is approximately what is needed to stabilize MHD instabilities. It is shown that this optimal shape can be created using the shaping coils of several present-day experiments.

  31. Author(s): Mariama Rebello Sousa Dias, Chen Gong, Zachery A. Benson, Marina S. Leite
    Publication: Adv. Opt. Mater. 6, 1700830 (2018)
    Doi: 10.1002/adom.201700830

    Superabsorbers based on metasurfaces have recently enabled the control of light at the nanoscale in unprecedented ways. Nevertheless, the sub-wavelength features needed to modify the absorption band usually require complex fabrication methods, such as electron-beam lithography. To overcome the scalability limitations associated with the fabrication of metallic nanostructures, engineering the optical response of superabsorbers by metal alloying is proposed, instead of tuning the geometry/size of the nanoscale building blocks. The superior performance of thin film AlCu alloys as the metallic component of planar bilayer superabsorbers is numerically demonstrated. This alloy outperforms its pure constituents as well as other metals, such as Ag, Au, and Cr. As a model system, a Si/AlCu structure is analyzed that presents >99% absorption at selected wavelength ranging from the visible to the near-infrared regions of the spectrum, depending on the subwavelength thickness of the semiconductor. The multi-wavelength near-unity absorption behavior of Si/AlCu persists even for oblique angle of incidence, up to 70°. Additionally, the findings are validated by fabricating and testing a-Si/AlCu superabsorbers, where good agreement is found between the numerically and experimentally determined optical response. The system investigated here is relevant for integration in complementary metal-oxide-semiconductor (CMOS) technologies.

  32. Author(s): R.J. Shalloo, C. Aaran, L. Corner, J. Holloway, J. Jonnerby, R. Walczak, H.M. Milchberg, S.M. Hooker
    Publication: Phys. Rev. E 97, 053203 (2018)
    Doi: 10.1103/PhysRevE.97.053203

    We present experiments and numerical simulations which demonstrate that fully ionized, low-density plasma channels could be formed by hydrodynamic expansion of plasma columns produced by optical field ionization. Simulations of the hydrodynamic expansion of plasma columns formed in hydrogen by an axicon lens show the generation of 200 mm long plasma channels with axial densities of order ne(0) = 1 × 1017cm−3 and lowest-order modes of spot size W≈ 40 μm. These simulations show that the laser energy required to generate the channels is modest: of order 1 mJ per centimeter of channel. The simulations are confirmed by experiments with a spherical lens which show the formation of short plasma channels with 1.5 × 1017cm−3 ≲ ne(0) ≲ 1 × 1018cm−3 and 61 μm ≳ W≳ 33 μm. Low-density plasma channels of this type would appear to be well suited as multi-GeV laser-plasma accelerator stages capable of long-term operation at high pulse repetition rates.

  33. Author(s): Nam Kim, Emily Sahadeo, Chanyuan Liu, Olivia Rose, Gary W. Rubloff, Sang Bok Lee
    Publication: Phys. Chem. Chem. Phys. 20, 29708 (2018)
    Doi: 10.1039/c8cp04706e

    Considering the tortuous, random porous nanostructures existing in many battery electrodes, it is essential to understand electronic and ionic behaviors in such a confined nanoscale porous geometry in which electron and ion transports can change dynamically. Here, we have carefully designed three dimensional (3D) interconnected porous electrode structures and performed experiments to probe how the ion and electron transport is impacted within these controlled geometries. By using anodized aluminum oxide as a template, we were able to fabricate both 1D array electrodes and 3D electrodes with varying numbers of interconnections, utilizing vanadium oxide (V2O5) as the active material. We demonstrate that the inherent properties of the electrode material in combination with the structural properties of the electrodes can both positively and negatively impact electrochemical characteristics. Most notably, electrodes with seven interconnecting layers in their structure had 19.7% less capacity at 25C than electrodes with zero interconnecting layers, demonstrating the negative effect of interconnections combined with poor electronic conductivity of V2O5 upon lithiation beyond one Li insertion. These results indicate that a careful consideration of the material and structural properties is needed for the design of high performance battery systems.

  34. Author(s): Yunlu Xu, Elizabeth M. Tennyson, Jehyung Kim, Sabyasachi Barik, Joseph Murray, Edo Waks, Marina S. Leite, Jeremy N. Munday
    Publication: Adv. Opt. Mater. 6, 1701323 (2018)
    Doi: 10.1002/adom.201701323

    Optoelectronic materials are the backbone of today's high-tech industry. To customize their response, one can directly modify the atomic arrangement, chemical composition, lattice strain, or doping of the semiconductor. However, these processes frequently cause undesirable effects resulting from induced defects. Here, a novel concept is demonstrated to actively tune the optoelectronic response of a material through tailored photon recycling. Without altering the material's intrinsic structure, doping, or temperature, the reabsorption of emitted photons within GaAs is modulated to control its carrier density. This approach is used to create a diode that can change its emission wavelength, a solar cell with improved open-circuit voltage, and an actively controlled, gate-free current modulator. These results represent a new platform to enable materials with tailored optoelectronic response based on photonic manipulation rather than semiconductor engineering.

  35. Author(s): B. Miao, L. Feder, J. Elle, A.J. Goers, D. Woodbury, F. Salehi, J.K. Wahlstrand, H.M. Milchberg
    Publication: Phys. Rev. E 98, 943206 (2018)
    Doi: 10.1103/PhysRevE.98.043206

    The injection of electrons into a laser wakefield accelerator (LWFA) is observed to generate an intense coherent ultra-broadband and ultrashort pulse radiation flash, consistent with the acceleration of electrons from rest to nearly the speed of light in a distance <∼1μm. Under certain conditions, the radiation occurs at harmonics of the local plasma frequency. The flash is sufficiently bright to induce large nonlinear refractive index shifts in optical materials; we estimate a source brightness temperature of ∼1018K. We present measurements of the flash spectra, coherence, pulse duration, polarization, and angular distribution, providing a detailed picture of electron injection dynamics in LWFA. These are characteristic of laser-assisted injection of off-axis electrons, which preserves wake coherence.

  36. Author(s): Sabyasachi Barik, Aziz Karasahin, Christopher Flower, Tao Cai, Hirokazu Miyake, Wade DeGottardi, Mohammad Hafezi, Edo Waks
    Publication: Sci. 359, 666 (2018)
    Doi: 10.1126/science.aaq0327

    Exploiting topological properties of a system allows certain properties to be protected against the disorder and scattering caused by defects. Barik et al. demonstrate a strong light-matter interaction in a topological photonic structure (see the Perspective by Amo). They created topological edge states at the interface between two photonic, topologically distinct regions and coupled them to a single quantum emitter. The chiral nature of single-photon emission was used to inject single photons of opposite polarization into counterpropagating topological edge states. Such a topological quantum optics interface may provide a powerful platform for developing robust integrated quantum optical circuits.

  37. Author(s): Subhojit Dutta, Tao Cai, Mustafa Atabey Buyukkaya, Sabyasachi Barik, Shahriar Aghaeimeibodi, Edo Waks
    Publication: Appl. Phys. Lett. 113,191105 (2018)
    Doi: 10.1063/1.5045727

    Coupling single photon emitters to surface plasmons provides a versatile ground for on chip quantum photonics. However, achieving good coupling efficiency requires precise alignment of both the position and dipole orientation of the emitter relative to the plasmonic mode. We demonstrate coupling of single emitters in the 2-D semiconductor, WSe2 self-aligned with propagating surface plasmon polaritons in silver-air-silver, metal-insulator-metal waveguides. The waveguide produces strain induced defects in the monolayer which are close to the surface plasmon mode with favorable dipole orientations for optimal coupling. We measure an average enhancement in the rate of spontaneous emission by a factor of 1.89 for coupling the single defects to the plasmonic waveguide. This architecture provides an efficient way of coupling single photon emitters to propagating plasmons which is an important step towards realizing active plasmonic circuits on chip.

  38. Author(s): M.I. Sitnov, V.G. Merkin, V. Roytershteyn, M. Swisdak
    Publication: Geophys. Res. Lett. 45, 4639 (2018)
    Doi: 10.1029/2018GL077874

    Kinetic aspects of energy conversion and dissipation near a dipolarization front (DF) in the magnetotail are considered using fully kinetic 3-D particle-in-cell simulations. The energy conversion is described in terms of the pressure dilatation, as well as the double contraction of deviatoric pressure tensor and traceless strain rate tensor, also known as the Pi-D parameter in turbulence studies. It is shown that in contrast to the fluid dissipation measure, the Joule heating rate, which cannot distinguish between ion and electron dissipation and reveals deep negative dips at the DF, the Pi-D parameters, as kinetic analogs of the Joule heating rate, are largely positive and drastically different for ions and electrons. Further analysis of these parameters suggests that ions are heated at and ahead of the DF due to their reflection from the front, while electrons are heated at and behind the DF due to the long-wavelength lower-hybrid drift instability.

  39. Author(s): Andrew J. Knoll, Pingshan Luan, Adam Pranda, Robert L. Bruce, Gottlieb S. Oehrlein
    Publication: Plasma Processes Polymers 15, e1700217 (2018)
    Doi: 10.1002/ppap.201700217

    Treatments of polymer films using either a MHz atmospheric pressure plasma jet (APPJ) or an atmospheric pressure surface micro-discharge (SMD) plasma are investigated. While the typical approach to determine relevant reactive species is to correlate surface effects with gas phase species measurement, this does not capture potential synergistic or other complex effects that may be occurring. Activation energy and directionality of the etching process can characterize what is occurring at the surface for these processes in more detail. The APPJ source shows an apparent activation energy of ∼0.18 eV at 8 mm distance and up to ∼0.34 eV at 16 mm distance for a temperature range of 20–80 °C tested with thin polymer films. The APPJ source shows directional etching at 8 mm distance with less anisotropy the more distance is increased. The SMD source has an apparent activation energy of ∼0.8–0.9 eV at a distance of 3 mm. The SMD also only shows isotropic etching behavior. However the SMD surface chemistry changes significantly to less oxidation with increased temperature while the APPJ source induced modifications remain very similar with temperature change. The lower apparent activation energy of the APPJ-induced etching reactions as compared with low pressure work (0.5 eV) and observation of line-of-sight contribution to etching suggests the involvement of a directional species at closer distances facilitating the etching which falls off with increasing distance. The high activation energy of the SMD suggests that species with less capability for etching is responsible compared to the APPJ and low pressure plasma. The high surface oxidation from low temperature SMD treatments shows that the surface is being oxidized but not sufficiently to reach the desorption step of the etching process.

  40. Author(s): Sarang Yeola, Donghoon Kuk, Ki-Yong Kim
    Publication: Opt. Soc. Am. B - Opt. Phys. 35, 2822 (2018)
    Doi: 10.1364/JOSAB.35.002822

    We have developed a single-shot imaging technique that can capture ultrafast events occurring on femtosecond to picosecond time scales. The technique is based on an optical pump-probe method, in which multiple time-delayed femtosecond pulses simultaneously probe a pump-excited sample. Here we use two sets of 2-by-2 mirror arrays for spatial/temporal division and routing of multiple probe pulses. This single-shot scheme is successfully applied to capture femtosecond ionization fronts propagating at the speed of light in air, as well as laser-induced ablation of solid targets.

  41. Author(s): Joseph B. Murray, Kevin J. Palm, Tarun C. Narayan, David K. Fork, Seid Sada, Jeremy N. Munday
    Publication: Rev. Sci. Instrum. 89, 085106 (2018)
    Doi: 10.1063/1.5040503

    We present an apparatus that allows for the simultaneous measurement of mass change, heat evolution, and stress of thin film samples deposited on quartz crystal microbalances (QCMs). We show device operation at 24.85 ± 0.05 °C under 9.31 ± 0.02 bars of H2 as a reactive gas. Using a 335 nm palladium film, we demonstrate that our apparatus quantifies curvature changes of 0.001 m−1. Using the QCM curvature to account for stress induced frequency changes, we demonstrate the measurement of mass changes of 13 ng/cm2 in material systems exhibiting large stress fluctuations. We use a one-state nonlinear lumped element model to describe our system with thermal potentials measured at discrete positions by three resistance temperature devices lithographically printed on the QCM. By inputting known heat amounts through lithographically defined Cr/Al wires, we demonstrate a 150 μW calorimetric accuracy and 20 μW minimum detectable power. The capabilities of this instrument will allow for a more complete characterization of reactions occurring in nanoscale systems, such as the effects of hydrogenation in various metal films and nanostructures, as well as allow for direct stress compensation in QCM measurements.

  42. Author(s): Bobby G. Barker, Jr., Venkata Surya N. Chava, Kevin M. Daniels, M.V.S. Chandrashekhar, Andrew B. Greytak
    Publication: 2D Mater. 5, 011033 (2018)
    Doi: 10.1088/2053-1583/aa90b1

    Graphene layers grown epitaxially on SiC substrates are attractive for a variety of sensing and optoelectronic applications because the graphene acts as a transparent, conductive, and chemically responsive layer that is mated to a wide-bandgap semiconductor with large breakdown voltage. Recent advances in control of epitaxial growth and doping of SiC epilayers have increased the range of electronic device architectures that are accessible with this system. In particular, a recently-introduced Schottky-emitter bipolar phototransistor (SEPT) based on an epitaxial graphene (EG) emitter grown on a p-SiC base epilayer has been found to exhibit a maximum common emitter current gain of 113 and a UV responsivity of 7.1 A W−1. The behavior of this device, formed on an n+-SiC substrate that serves as the collector, was attributed to a very large minority carrier injection efficiency at the EG/p-SiC Schottky contact. This large minority carrier injection efficiency is in turn related to the large built-in potential found at a EG/p-SiC Schottky junction. The high performance of this device makes it critically important to analyze the sub bandgap visible response of the device, which provides information on impurity states and polytype inclusions in the crystal. Here, we employ scanning photocurrent microscopy (SPCM) with sub-bandgap light as well as a variety of other techniques to clearly demonstrate a localized response based on the graphene transparent electrode and an approximately 1000-fold difference in responsivity between 365 nm and 444 nm excitation. A stacking fault propagating from the substrate/epilayer interface, assigned as a single layer of the 8H-SiC polytype within the 4H-SiC matrix, is found to locally increase the photocurrent substantially. The discovery of this polytype heterojunction opens the potential for further development of heteropolytype devices based on the SEPT architecture.

  43. Author(s): Abdel El Fatimy, Anindya Nath, Byoumng Don Kong, Anthony K. Boyd, Rachael L. Myers-Ward, Kevin M. Daniels, M. Mehdi Jadidi, Thomas E. Murphy, D. Kurt Gaskill, Paola Barbara
    Publication: Nanophotonics 7, 735 (2018)
    Doi: 10.1515/nanoph-2017-0100

    Graphene is an ideal material for hot-electron bolometers due to its low heat capacity and weak electron-phonon coupling. Nanostructuring graphene with quantum-dot constrictions yields detectors of electromagnetic radiation with extraordinarily high intrinsic responsivity, higher than 1×109 V W−1 at 3 K. The sensing mechanism is bolometric in nature: the quantum confinement gap causes a strong dependence of the electrical resistance on the electron temperature. Here, we show that this quantum confinement gap does not impose a limitation on the photon energy for light detection and these quantum-dot bolometers work in a very broad spectral range, from terahertz through telecom to ultraviolet radiation, with responsivity independent of wavelength. We also measure the power dependence of the response. Although the responsivity decreases with increasing power, it stays higher than 1×108 V W−1 in a wide range of absorbed power, from 1 pW to 0.4 nW.

  44. Author(s): David A.T. Somers, Joseph L. Garrett, Kevin J. Palm, Jeremy N. Munday
    Publication: Nature 564, 386 (2018)
    Doi: 10.1038/s41586-018-0777-8

    Intermolecular forces are pervasive in nature and give rise to various phenomena including surface wetting1, adhesive forces in biology2,3, and the Casimir effect4, which causes two charge-neutral, metal objects in vacuum to attract each other. These interactions are the result of quantum fluctuations of electromagnetic waves and the boundary conditions imposed by the interacting materials. When the materials are optically anisotropic, different polarizations of light experience different refractive indices and a torque is expected to occur that causes the materials to rotate to a position of minimum energy5,6. Although predicted more than four decades ago, the small magnitude of the Casimir torque has so far prevented direct measurements of it. Here we experimentally measure the Casimir torque between two optically anisotropic materials—a solid birefringent crystal (calcite, lithium niobite, rutile or yttrium vanadate) and a liquid crystal (5CB). We control the sign and strength of the torque, and its dependence on the rotation angle and the separation distance between the materials, through the choice of materials. The values that we measure agree with calculations, verifying the long-standing prediction that a mechanical torque induced by quantum fluctuations can exist between two separated objects. These results open the door to using the Casimir torque as a micro- or nanoscale actuation mechanism, which would be relevant for a range of technologies, including microelectromechanical systems and liquid crystals.

  45. Author(s): Lisa J. Krayer, Elizabeth M. Tennyson, Marina S. Leite, Jeremy N. Munday
    Publication: ACS Photonics 5, 306 (2018)
    Doi: 10.1021/acsphotonics.7b01021

    Silicon is the most widely used material for visible photodetection, with extensive applications in both consumer and industrial products. Further, its excellent optoelectronic properties and natural abundance have made it nearly ideal for microelectronic devices and solar cells. However, its lack of absorption in the infrared precludes its use in infrared detectors and imaging sensors, severely constraining its implementation in telecommunications. Here we show that this limitation can be overcome by exploiting resonant absorption in ultrathin metal films (<20 nm). Through appropriate optical design, a zeroth-order Fabry–Perot resonance is achieved, enabling ∼80% light absorption below the bandgap of the semiconductor. Absorption within the metal film results in excitation and injection of hot carriers through a Schottky junction into the Si. We experimentally demonstrate this phenomenon with four ultrathin planar metal films (Pt, Fe, Cr, and Ti), chosen to satisfy the resonant condition over a wide range of wavelengths (1200–1600 nm), and realize a near-infrared imaging detector. Our approach paves the way to implement a scalable, lithography free, and low-cost route to obtain silicon-based optoelectronics beyond the material bandgap.

  46. Author(s): Alexander Yulaev, Vladimir Oleshko, Paul Janey, Jialin Liu, Yue Qi, A. Alec Talin, Marina S. Leite, Andrei Kolmakov
    Publication: Nano Lett. 18, 1644 (2018)
    Doi: 10.1021/acs.nanolett.7b04518

    Li metal is the preferred anode material for all-solid-state Li batteries. However, a stable plating and stripping of Li metal at the anode–solid electrolyte interface remains a significant challenge particularly at practically feasible current densities. This problem usually relates to high and/or inhomogeneous Li-electrode–electrolyte interfacial impedance and formation and growth of high-aspect-ratio dendritic Li deposits at the electrode–electrolyte interface, which eventually shunt the battery. To better understand details of Li metal plating, we use operando electron microscopy and Auger spectroscopy to probe nucleation, growth, and stripping of Li metal during cycling of a model solid-state Li battery as a function of current density and oxygen pressure. We find a linear correlation between the nucleation density of Li clusters and the charging rate in an ultrahigh vacuum, which agrees with a classical nucleation and growth model. Moreover, the trace amount of oxidizing gas (≈10–6 Pa of O2) promotes the Li growth in a form of nanowires due to a fine balance between the ion current density and a growth rate of a thin lithium-oxide shell on the surface of the metallic Li. Interestingly, increasing the partial pressure of O2 to 10–5 Pa resumes Li plating in a form of 3D particles. Our results demonstrate the importance of trace amounts of preexisting or ambient oxidizing species on lithiation processes in solid-state batteries.

  47. Author(s): Gregory S. Nusinovich, Olgierd Dumbrajs
    Publication: Phys. Plasmas 25, 093108 (2018)
    Doi: 10.1063/1.5045317

    It is known that gyrotrons (as well as other electron beam driven microwave and millimeter-wave oscillators) can operate in the regime of either soft or hard self-excitation. In the regime of soft self-excitation, the beam current exceeds its starting value; thus, the oscillations can start to grow from the noise produced by electrons. In the regime of hard self-excitation, the beam current is less than its starting value. Therefore, for exciting the oscillations, a certain start-up scenario is required, which may include the variation of the mod-anode and/or beam voltage or the guiding magnetic field. It was found recently [O. Dumbrajs and G. S. Nusinovich, Phys. Plasmas 25, 013121 (2018)] that some gyrotrons can also operate in the region of magnetic fields where there is no start current at all. In the present paper, it is shown that this sort of operation can be attributed to the presence of the axial dependence of the phase of the resonator field.

  48. Author(s): Brian L. Beaudoin, Antonio Ting, Steven Gold, Amith H. Narayan, Richard Fischer, Jayakrishnan A. Karakkad, Gregory S. Nusinovich, Thomas M. Antonsen, Jr.
    Publication: Phys. Plasmas 25, 103166 (2018)
    Doi: 10.1063/1.5052183

    The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be affected by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, high frequency source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. For developing a mobile heater with a much smaller antenna array, a new highly efficient megawatt-class Radio Frequency (RF) source is required to reduce the overall power demands on a fully deployable system. The concept of such a source has been described previously [Beaudoin et al., J. Electromagn. Waves Appl. 31(17), 1786–1801 (2017)]. Here, experimental results using an electron beam produced by a gridded thermionic electron gun to drive an external lumped element circuit for a high efficiency RF generation are reported. The gun produces an electron beam bunched at the driving frequency with a narrow phase angle spread that is then collected by an external circuit for resonant impedance matching to the load. The results showed that effects, such as the internal resistance of the inductor and deflection of the beam electrons by the induced RF voltages on the beam collector, are important considerations to be included in the design of a practical device using this configuration for high efficiency RF generation.

  49. Author(s): Zachary Epstein, Bahman Hafizi, Joseph Penano, Phillip Sprangle
    Publication: J. Opt. Soc. Am. B - Opt. Phys. 35, 2718 (2018)
    Doi: 10.1364/JOSAB.35.002718

    High-average-power ultra-broadband mid-IR radiation can be generated by illuminating a nonlinear medium with a multi-line laser radiation. Propagation of a multi-line pulsed CO2CO2 laser beam in a nonlinear medium, e.g., gallium arsenide or chalcogenide, can generate directed broadband IR radiation in the atmospheric window (2–13 μm). A 3D laser code for propagation in a nonlinear medium has been developed to incorporate extreme spectral broadening resulting from the beating of several wavelengths. The code has the capability to treat coupled forward and backward-propagating waves, as well as transverse and full linear dispersion effects. Methods for enhancing the spectral broadening are proposed and analyzed. Grading the refractive index radially or using a cladding will tend to guide the CO2CO2 radiation and extend the interaction distance, allowing for enhanced spectral broadening. Nonlinear coupling of the CO2CO2 laser beam to a backward-propagating reflected beam can increase the rate of spectral broadening in the anomalous dispersion regime of a medium. Laser phase noise associated with the finite CO2CO2 linewidths can significantly enhance the spectral broadening, as well. In a dispersive medium, laser phase noise results in laser intensity fluctuations. These intensity fluctuations result in spectral broadening due to the self-phase modulation mechanism. Finally, we present propagation through a chalcogenide fiber as an alternative for extreme spectral broadening of a frequency-doubled CO2CO2 multi-line laser beam.

  50. Author(s): David M. Stewart, Alexander J. Pearse, Nam S. Kim, Elliot J. Fuller, A. Alec Talin, Keith Gregorczyk, Sang Bok Lee, Gary W. Rubloff
    Publication: Chem. Mater. 30, 2526 (2018)
    Doi: 10.1021/acs.chemmater.7b04666

    Major advances in thin-film solid-state batteries (TFSSBs) may capitalize on 3D structuring using high-aspect-ratio substrates such as nanoscale pits, pores, trenches, flexible polymers, and textiles. This will require conformal processes such as atomic layer deposition (ALD) for every active functional component of the battery. Here we explore the deposition and electrochemical properties of SnO2, SnNy, and SnOxNy thin films as TFSSB anode materials, grown by ALD using tetrakisdimethylamido(tin), H2O, and N2 plasma as precursors. By controlling the dose ratio between H2O and N2, the N–O fraction can be tuned between 0% N and 95% N. The electrochemical properties of these materials were tested across a composition range varying from pure SnO2, to SnON intermediates, and pure SnNy. In TFSSBs, the SnNy anodes are found to be more stable during cycling than the SnO2 or SnOxNy films, with an initial reversible capacity beyond that of Li–Sn alloying, retaining 75% of their capacity over 200 cycles compared to only 50% for SnO2. Furthermore, the performance of the SnOxNy anodes indicates that SnNy anodes should not be negatively impacted by small levels of O contamination.

  51. Author(s): Sanjukta Krishnagopal, Yiannis Alohnonos, Michelle Girvan
    Publication: Complexity 2018, 6953836 (2018)
    Doi: 10.1155/2018/6953836

    We investigate the ways in which a machine learning architecture known as Reservoir Computing learns concepts such as “similar” and “different” and other relationships between image pairs and generalizes these concepts to previously unseen classes of data. We present two Reservoir Computing architectures, which loosely resemble neural dynamics, and show that a Reservoir Computer (RC) trained to identify relationships between image pairs drawn from a subset of training classes generalizes the learned relationships to substantially different classes unseen during training. We demonstrate our results on the simple MNIST handwritten digit database as well as a database of depth maps of visual scenes in videos taken from a moving camera. We consider image pair relationships such as images from the same class; images from the same class with one image superposed with noise, rotated 90°, blurred, or scaled; images from different classes. We observe that the reservoir acts as a nonlinear filter projecting the input into a higher dimensional space in which the relationships are separable; i.e., the reservoir system state trajectories display different dynamical patterns that reflect the corresponding input pair relationships. Thus, as opposed to training in the entire high-dimensional reservoir space, the RC only needs to learns characteristic features of these dynamical patterns, allowing it to perform well with very few training examples compared with conventional machine learning feed-forward techniques such as deep learning. In generalization tasks, we observe that RCs perform significantly better than state-of-the-art, feed-forward, pair-based architectures such as convolutional and deep Siamese Neural Networks (SNNs). We also show that RCs can not only generalize relationships, but also generalize combinations of relationships, providing robust and effective image pair classification. Our work helps bridge the gap between explainable machine learning with small datasets and biologically inspired analogy-based learning, pointing to new directions in the investigation of learning processes.

  52. Author(s): J.K. Wahlstrand, S. Zahedpour, A. Bahl, M. Kolesik, H.M. Milchberg
    Publication: Phys. Rev. Lett. 120, 183901 (2018)
    Doi: 10.1103/PhysRevLett.120.183901

    We present absolute space- and time-resolved measurements of the ultrafast laser-driven nonlinear polarizability in argon, krypton, xenon, nitrogen, and oxygen up to ionization fractions of a few percent. These measurements enable determination of the strongly nonperturbative bound-electron nonlinear polarizability well beyond the ionization threshold, where it is found to remain approximately quadratic in the laser field, a result normally expected at much lower intensities where perturbation theory applies.

  53. Author(s): Kevin J. Palm, Joseph B. Murray, Tarun C. Narayan, Jeremy N. Munday
    Publication: ACS Photonics 5, 4677 (2018)
    Doi: 10.1021/acsphotonics.8b01243

    Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices, such as optical hydrogen sensors and switchable mirrors. While some of these metals, such as palladium, have been well studied, many other promising materials have only been characterized over a limited optical range and lack direct in situ measurements of hydrogen loading, limiting their potential applications. Further, there have been no systematic studies that allow for a clear comparison between these metals. In this work, we present such a systematic study of the dynamically tunable optical properties of Pd, Mg, Zr, Ti, and V throughout hydrogenation with a wavelength range of 250–1690 nm. These measurements were performed in an environmental chamber, which combines mass measurements via a quartz crystal microbalance with ellipsometric measurements in up to 7 bar of hydrogen gas, allowing us to determine the optical properties during hydrogen loading. In addition, we demonstrate a further tunability of the optical properties of titanium and its hydride by altering annealing conditions, and we investigate the optical and gravimetric hysteresis that occurs during hydrogenation cycling of palladium. Finally, we demonstrate several nanoscale optical and plasmonic structures based on these dynamic properties. We show structures that, upon hydrogenation, demonstrate 5 orders of magnitude change in reflectivity, resonance shifts of >200 nm, and relative transmission switching of >3000%, suggesting a wide range of applications.

  54. Author(s): Jacob Bringewatt, William Dorland, Stephen P. Jordan, Alan Mink
    Publication: Phys. Rev. A 97, 022323 (2018)
    Doi: 10.1103/PhysRevA.97.022323

    Most research regarding quantum adiabatic optimization has focused on stoquastic Hamiltonians, whose ground states can be expressed with only real non-negative amplitudes and thus for whom destructive interference is not manifest. This raises the question of whether classical Monte Carlo algorithms can efficiently simulate quantum adiabatic optimization with stoquastic Hamiltonians. Recent results have given counterexamples in which path-integral and diffusion Monte Carlo fail to do so. However, most adiabatic optimization algorithms, such as for solving MAX-k-SAT problems, use k-local Hamiltonians, whereas our previous counterexample for diffusion Monte Carlo involved n-body interactions. Here we present a 6-local counterexample which demonstrates that even for these local Hamiltonians there are cases where diffusion Monte Carlo cannot efficiently simulate quantum adiabatic optimization. Furthermore, we perform empirical testing of diffusion Monte Carlo on a standard well-studied class of permutation-symmetric tunneling problems and similarly find large advantages for quantum optimization over diffusion Monte Carlo.

  55. Author(s): Masafumi Fukunari, Gregory S. Nusinovich, Yoshinori Tatematsu, Teruo Saito, Yuusuke Yamaguchi
    Publication: IEEE Trans. Plasma Sci. 46, 2848 (2018)
    Doi: 10.1109/TPS.2018.2849379

    Spectroscopic applications have stimulated strong interest in frequency tunable gyrotrons. One of the possibilities to tune gyrotron frequency is known as frequency pulling, which is based on the effect of the electron beam on the oscillation frequency. Recently, the linear theory of this effect was developed, and the results were obtained for a sequence of modes with different axial indices. This paper is intended to analyze saturation effects in such gyrotrons. The nonlinear theory describing the frequency pulling and the efficiency in gyrotrons is developed in the cold-cavity approximation. The results are obtained for the modes with different axial indices (from 1 to 4). The frequency shift and the efficiency are calculated in a wide range of the dimensionless parameters characterizing the beam current and the external magnetic field. These results related to the frequency pulling are compared with those of the linear theory. Also, comparison is made with the results of the self-consistent theory and experiments.

  56. Author(s): Shou Sun, Hyochul Kim, Glenn S. Solomon, Edo Waks
    Publication: Phys. Rev. Appl. 9, 054013 (2018)
    Doi: 10.1103/PhysRevApplied.9.054013

    We demonstrate optical readout of a single spin using cavity quantum electrodynamics. The spin is based on a single trapped electron in a quantum dot that has a poor branching ratio of 0.43. Selectively coupling one of the optical transitions of the quantum dot to the cavity mode results in a spin-dependent cavity reflectivity that enables spin readout by monitoring the reflected intensity of an incident optical field. Using this approach, we demonstrate spin-readout fidelity of 0.61. Achieving this fidelity using resonance fluorescence from a bare dot would require 43 times improvement in photon collection efficiency.

  57. Author(s): Matt Landreman, Elizabeth Paul
    Publication: Nucl. Fusion 58, 076023 (2018)
    Doi: 10.1088/1741-4326/aac197

    Tight tolerances have been a leading driver of cost in recent stellarator experiments, so improved definition and control of tolerances can have significant impact on progress in the field. Here we relate tolerances to the shape gradient representation that has been useful for shape optimization in industry, used for example to determine which regions of a car or aerofoil most affect drag, and we demonstrate how the shape gradient can be computed for physics properties of toroidal plasmas. The shape gradient gives the local differential contribution to some scalar figure of merit (shape functional) caused by normal displacement of the shape. In contrast to derivatives with respect to quantities parameterizing a shape (e.g. Fourier amplitudes), which have been used previously for optimizing plasma and coil shapes, the shape gradient gives spatially local information and so is more easily related to engineering constraints. We present a method to determine the shape gradient for any figure of merit using the parameter derivatives that are already routinely computed for stellarator optimization, by solving a small linear system relating shape parameter changes to normal displacement. Examples of shape gradients for plasma and electromagnetic coil shapes are given. We also derive and present examples of an analogous representation of the local sensitivity to magnetic field errors; this magnetic sensitivity can be rapidly computed from the shape gradient. The shape gradient and magnetic sensitivity can both be converted into local tolerances, which inform how accurately the coils should be built and positioned, where trim coils and structural supports for coils should be placed, and where magnetic material and current leads can best be located. Both sensitivity measures provide insight into shape optimization, enable systematic calculation of tolerances, and connect physics optimization to engineering criteria that are more easily specified in real space than in Fourier space.

  58. Author(s): Jaideep Pathak, Brian Hunt, Michelle Girvan, Zhixin Lu, Edward Ott
    Publication: Phys. Rev. Lett. 120, 024102 (2018)
    Doi: 10.1103/PhysRevLett.120.024102

    We demonstrate the effectiveness of using machine learning for model-free prediction of spatiotemporally chaotic systems of arbitrarily large spatial extent and attractor dimension purely from observations of the system’s past evolution. We present a parallel scheme with an example implementation based on the reservoir computing paradigm and demonstrate the scalability of our scheme using the Kuramoto-Sivashinsky equation as an example of a spatiotemporally chaotic system.

  59. Author(s): J.L. Burch, R.E. Ergun, P.A. Cassak, J.M. Webster, R.B. Torbert, B.L. Giles, J.C. Dorelli, A.C. Rager, K.-J. Kwang, T.D. Phan, K.J. Genestreti, R.C. Allen, L.-J. Chen, S. Wang, D. Gershman, O. LeContel, C.T. Russell, R.J. Strangeway, F.D. Wilder, D.B. Graham, M. Hesse, J.F. Drake, M. Swisdak, et al.
    Publication: Geophys. Res. Lett. 45, 1237 (2018)
    Doi: 10.1002/2017GL076809

    Data from the NASA Magnetospheric Multiscale mission are used to investigate asymmetric magnetic reconnection at the dayside boundary between the Earth's magnetosphere and the solar wind. High-resolution measurements of plasmas and fields are used to identify highly localized (~15 electron Debye lengths) standing wave structures with large electric field amplitudes (up to 100 mV/m). These wave structures are associated with spatially oscillatory energy conversion, which appears as alternatingly positive and negative values of J · E. For small guide magnetic fields the wave structures occur in the electron stagnation region at the magnetosphere edge of the electron diffusion region. For larger guide fields the structures also occur near the reconnection X-line. This difference is explained in terms of channels for the out-of-plane current (agyrotropic electrons at the stagnation point and guide field-aligned electrons at the X-line).

  60. Author(s): Joseph L. Garrett, David A.T. Somers, Jeremy N. Munday
    Publication: Phys. Rev. Lett. 120, 040401 (2018)
    Doi: 10.1103/PhysRevLett.120.040401

    Complex interaction geometries offer a unique opportunity to modify the strength and sign of the Casimir force. However, measurements have traditionally been limited to sphere-plate or plate-plate configurations. Prior attempts to extend measurements to different geometries relied on either nanofabrication techniques that are limited to only a few materials or slight modifications of the sphere-plate geometry due to alignment difficulties of more intricate configurations. Here, we overcome this obstacle to present measurements of the Casimir force between two gold spheres using an atomic force microscope. Force measurements are alternated with topographical scans in the x−y plane to maintain alignment of the two spheres to within approximately 400 nm (∼1% of the sphere radii). Our experimental results are consistent with Lifshitz’s theory using the proximity force approximation (PFA), and corrections to the PFA are bounded using nine sphere-sphere and three sphere-plate measurements with spheres of varying radii.

  61. Author(s): M. Swisdak, J.F. Drake, L. Price, J.L. Burch, P.A. Cassak, T.-D. Phan
    Publication: Geophys. Res. Lett. 45, 5260 (2018)
    Doi: 10.1029/2017GL076862

    We analyze a high-resolution simulation of magnetopause reconnection observed by the Magnetospheric Multiscale mission and explain the occurrence of strongly localized dissipation with an amplitude more than an order of magnitude larger than expected. Unlike symmetric reconnection, wherein reconnection of the ambient reversed magnetic field drives the dissipation, we find that the annihilation of the self-generated, out-of-plane (Hall) magnetic field plays the dominant role. Electrons flow along the magnetosheath separatrices, converge in the diffusion region, and jet past the X-point into the magnetosphere. The resulting accumulation of negative charge generates intense parallel electric fields that eject electrons along the magnetospheric separatrices and produce field-aligned beams. Many of these features match Magnetospheric Multiscale observations.


  1. Author(s): Tao Cai, Subhojit Dutta, Shahriar Agh, Zhili Yang, Sanghee Nah, John T. Fourkas, Edo Waks
    Publication: Nano Lett. 17, 6564 (2017)
    Doi: 10.1021/acs.nanolett.7b02222

    Coupling of an atom-like emitter to surface plasmons provides a path toward significant optical nonlinearity, which is essential in quantum information processing and quantum networks. A large coupling strength requires nanometer-scale positioning accuracy of the emitter near the surface of the plasmonic structure, which is challenging. We demonstrate the coupling of single localized defects in a tungsten diselenide (WSe2) monolayer self-aligned to the surface plasmon mode of a silver nanowire. The silver nanowire induces a strain gradient on the monolayer at the overlapping area, leading to the formation of localized defect emission sites that are intrinsically close to the surface plasmon. We measured an average coupling efficiency with a lower bound of 26% ± 11% from the emitter into the plasmonic mode of the silver nanowire. This technique offers a way to achieve efficient coupling between plasmonic structures and localized defects of two-dimensional semiconductors.

  2. Author(s): Per Sebastian Skardal, Juan G. Restrepo, Edward Ott
    Publication: Chaos 27, 083121 (2017)
    Doi: 10.1063/1.4986957

    In the last decade, it has been shown that a large class of phase oscillator models admit low dimensional descriptions for the macroscopic system dynamics in the limit of an infinite number N of oscillators. The question of whether the macroscopic dynamics of other similar systems also have a low dimensional description in the infinite N limit has, however, remained elusive. In this paper, we show how techniques originally designed to analyze noisy experimental chaotic time series can be used to identify effective low dimensional macroscopic descriptions from simulations with a finite number of elements. We illustrate and verify the effectiveness of our approach by applying it to the dynamics of an ensemble of globally coupled Landau-Stuart oscillators for which we demonstrate low dimensional macroscopic chaotic behavior with an effective 4-dimensional description. By using this description, we show that one can calculate dynamical invariants such as Lyapunov exponents and attractor dimensions. One could also use the reconstruction to generate short-term predictions of the macroscopic dynamics.

  3. Author(s): Joshua Isaacs, Phillip Sprangle
    Publication: Appl. Phys. Lett. 110, 024101 (2017)
    Doi: 10.1063/1.4973642

    We propose and analyze a mechanism to accelerate protons in a low-phase-velocity wakefield. The wakefield is shock-excited by the interaction of two counter-propagating laser pulses in a plasma density gradient. The laser pulses consist of a forward-propagating short pulse (less than a plasma period) and a counter-propagating long pulse. The beating of these pulses generates a slow forward-propagating wakefield that can trap and accelerate protons. The trapping and acceleration is accomplished by appropriately tapering both the plasma density and the amplitude of the backward-propagating pulse. An example is presented in which the trapping and accelerating wakefield has a phase velocity varying from Vph ≈ 0   to ≈ 0.15 c   (∼10 MeV   proton  energy) over a distance of ∼1 cm. The required laser intensities, pulse durations, pulse energies, and plasma densities are relatively modest. Instabilities such as the Raman instability are mitigated because of the large plasma density gradients. Numerical solutions of the wakefield equation and simulations using turboWAVE are carried out to support our model.

  4. Author(s): Dominik Metzler, Chen Li, C. Steven Lai, Eric A. Hudson, Gottlieb S. Oehrlein
    Publication: J. Phys. D - Appl. Phys. 50, 254006 (2017)
    Doi: 10.1088/1361-6463/aa71f1

    The evaluation of a plasma-based atomic layer etching (ALE) approach for native oxide surface removal from Si substrates is described. Objectives include removal of the native oxide while minimizing substrate damage, surface residues and substrate loss. Oxide thicknesses were measured using in situ ellipsometry and surface chemistry was analyzed by x-ray photoelectron spectroscopy. The cyclic ALE approach when used for removal of native oxide SiO2 from a Si substrate did not remove native oxide to the extent required. This is due to the high reactivity of the silicon substrate during the low-energy (<40 eV) ion bombardment phase of the cyclic ALE approach which leads to reoxidation of the silicon surface. A modified process, which used continuously biased Ar plasma with periodic CF4 injection, achieved significant oxygen removal from the Si surface, with some residual carbon and fluorine. A subsequent H2/Ar plasma exposure successfully removed residual carbon and fluorine while passivating the silicon surface. The combined treatment reduced oxygen and carbon levels to about half compared to as received silicon surfaces. The downside of this process sequence is a net loss of about 40 Å of Si. A generic insight of this work is the importance of the substrate and final surface chemistry in addition to precise etch control of the target film for ALE processes. By a fluorocarbon-based ALE technique, thin SiO2 layer removal at the Ångstrom level can be precisely performed from an inert substrate, e.g., a thick SiO2 layer. However, from a reactive substrate, like Si, complete removal of the thin SiO2 layer is prevented by the high reactivity of low energy Ar+ ion bombarded Si. The Si surfaces are reoxidized during the ALE ion bombardment etch step, even for very clean and ultra-low O2 process conditions.

  5. Author(s): P.A. Cassak, K.J. Genestreti, J.L. Burch, T.-D. Phan, M.A. Shay, M. Swisdak, J.F. Drake, L. Price, et al.
    Publication: J. Geophys. Res. - Space Phys. 122, 11523 (2017)
    Doi: 10.1002/2017JA024555

    We use theory and simulations to study how the out-of-plane (guide) magnetic field strength modifies the location where the energy conversion rate between the electric field and the plasma is appreciable during asymmetric magnetic reconnection, motivated by observations (Genestreti et al., 2017). For weak guide fields, energy conversion is maximum on the magnetospheric side of the X line, midway between the X line and electron stagnation point. As the guide field increases, the electron stagnation point gets closer to the X line, and energy conversion occurs closer to the electron stagnation point. We motivate one possible nonrigorous approach to extend the theory of the stagnation point location to include a guide field. The predictions are compared to two-dimensional particle-in-cell (PIC) simulations with vastly different guide fields. The simulations have upstream parameters corresponding to three events observed with Magnetospheric Multiscale (MMS). The predictions agree reasonably well with the simulation results, capturing trends with the guide field. The theory correctly predicts that the X line and stagnation points approach each other as the guide field increases. The results are compared to MMS observations, Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations of each event, and a global resistive-magnetohydrodynamics simulation of the 16 October 2015 event. The PIC simulation results agree well with the global observations and simulation but differ in the strong electric fields and energy conversion rates found in MMS observations. The observational, theoretical, and numerical results suggest that the strong electric fields observed by MMS do not represent a steady global reconnection rate.

  6. Author(s): Toshihiro Taguchi, Thomas M. Antonsen, Jr., Kunioki Mima
    Publication: J. Plasma Phys. 83, 905830204 (2017)
    Doi: 10.1017/S0022377817000204

    Relativistic electron beam transport through a high-density, magnetized plasma is studied numerically and theoretically. An electron beam injected into a cold plasma excites Weibel and two-stream instabilities that heat the beam and saturate. In the absence of an applied magnetic field, the heated beam continues to propagate. However, when a magnetic field of particular strength is applied along the direction of beam propagation, a secondary instability of off-angle whistler modes is excited. These modes then couple nonlinearly creating a large amplitude parallel-propagating whistler that stops the beam. Here, we will show these phenomena in detail and explain the mechanism of whistler mediated beam stagnation.

  7. Author(s): Je-Hyung Kim, Shahriar Aghaeimeibodi, Christopher J. Richardson, Richard P. Leavitt, Dirk Englund, Edo Waks
    Publication: Nano Lett. 17, 7394 (2017)
    Doi: 10.1021/acs.nanolett.7b03220

    Scalable quantum photonic systems require efficient single photon sources coupled to integrated photonic devices. Solid-state quantum emitters can generate single photons with high efficiency, while silicon photonic circuits can manipulate them in an integrated device structure. Combining these two material platforms could, therefore, significantly increase the complexity of integrated quantum photonic devices. Here, we demonstrate hybrid integration of solid-state quantum emitters to a silicon photonic device. We develop a pick-and-place technique that can position epitaxially grown InAs/InP quantum dots emitting at telecom wavelengths on a silicon photonic chip deterministically with nanoscale precision. We employ an adiabatic tapering approach to transfer the emission from the quantum dots to the waveguide with high efficiency. We also incorporate an on-chip silicon-photonic beamsplitter to perform a Hanbury-Brown and Twiss measurement. Our approach could enable integration of precharacterized III–V quantum photonic devices into large-scale photonic structures to enable complex devices composed of many emitters and photons.

  8. Author(s): Martin Mittendorff, Shanshan Li, Thomas E. Murphy
    Publication: ACS Photon. 4, 316 (2017)
    Doi: 10.1021/acsphotonics.6b00751

    One of the major difficulties in the development of optoelectronic THz modulators is finding an active material that allows for large modulation depth. Graphene is a promising candidate because in the terahertz regime it behaves as a Drude metal with conductivity that can be electrostatically tuned through the application of a gate voltage. However, the maximum absorption incurred when a terahertz signal passes through a monolayer of graphene is still only on the order of 10–20%, even for the highest practically achievable carrier concentrations. We demonstrate here a THz modulator that overcomes this fundamental limitation by incorporating a graphene sheet on the surface of a passive silicon dielectric waveguide, in which the evanescent field penetrates the graphene sheet. By applying a gate voltage to the graphene sheet, a modulation depth of up to 50% was achieved. The performance of the modulator is confirmed through electromagnetic simulations, which give further insights into the spatial structure of the guided mode and polarization dependence of the modulation. We show both theoretically and experimentally that the modulation depth can be increased to over 90% by integrating the graphene sheet at the center of the waveguide.

  9. Author(s): Melissa Trepanier, Daimeng Zhang, Oleg Mukhanov, V.P. Koshelets, Philipp Jung, Susanne Butz, Edward Ott, Thomas M. Antonsen, Jr., Alexey V. Ustinov, Steven M. Anlage
    Publication: Phys. Rev. E 95, 050201(R) (2017)
    Doi: 10.1103/PhysRevE.95.050201

    Through experiments and numerical simulations we explore the behavior of rf SQUID (radio frequency superconducting quantum interference device) metamaterials, which show extreme tunability and nonlinearity. The emergent electromagnetic properties of this metamaterial are sensitive to the degree of coherent response of the driven interacting SQUIDs. Coherence suffers in the presence of disorder, which is experimentally found to be mainly due to a dc flux gradient. We demonstrate methods to recover the coherence, specifically by varying the coupling between the SQUID meta-atoms and increasing the temperature or the amplitude of the applied rf flux.

  10. Author(s): Sarthak Chandra, David Hathcock, Kimberly Crain, Thomas M. Antonsen, Jr., Michelle Girvan, Edward Ott
    Publication: Chaos 27, 033102 (2017)
    Doi: 10.1063/1.4977514

    We derive a mean-field approximation for the macroscopic dynamics of large networks of pulse-coupled theta neurons in order to study the effects of different network degree distributions and degree correlations (assortativity). Using the ansatz of Ott and Antonsen [Chaos 18, 037113 (2008)], we obtain a reduced system of ordinary differential equations describing the mean-field dynamics, with significantly lower dimensionality compared with the complete set of dynamical equations for the system. We find that, for sufficiently large networks and degrees, the dynamical behavior of the reduced system agrees well with that of the full network. This dimensional reduction allows for an efficient characterization of system phase transitions and attractors. For networks with tightly peaked degree distributions, the macroscopic behavior closely resembles that of fully connected networks previously studied by others. In contrast, networks with highly skewed degree distributions exhibit different macroscopic dynamics due to the emergence of degree dependent behavior of different oscillators. For nonassortative networks (i.e., networks without degree correlations), we observe the presence of a synchronously firing phase that can be suppressed by the presence of either assortativity or disassortativity in the network. We show that the results derived here can be used to analyze the effects of network topology on macroscopic behavior in neuronal networks in a computationally efficient fashion.

  11. Author(s): Alexander C. Kozen, Chuan-Fu Lin, Oliver Zhao, Sang Bok Lee, Gary W. Rubloff, Malachi Noked
    Publication: Chem. Mater. 29, 6298 (2017)
    Doi: 10.1021/acs.chemmater.7b01496

    Li metal is among the most attractive anode materials for secondary batteries, with a theoretical specific capacity > 3800 mAh g–1. However, its extremely low electrochemical potential is associated with high chemical reactivity that results in undesirable reduction of electrolyte species on the lithium surface, leading to spontaneous formation of a solid electrolyte interphase (SEI) with uncontrolled composition, morphology, and physicochemical properties. Here, we demonstrate a new approach to stabilize Li metal anodes using a hybrid organic/inorganic artificial solid electrolyte interphase (ASEI) deposited directly on the Li metal surface by self-healing electrochemical polymerization (EP) and atomic layer deposition (ALD). This hybrid protection layer is thin, flexible, ionically conductive, and electrically insulating. We show that Li metal protected by the hybrid protection layer gives rise to very stable cycling performance for over 300 cycles at current density 1 mA/cm2 and over 110 cycles at current density 2 mA/cm2, well above the threshold for dendrite growth at unprotected Li. Our strategy for protecting Li metal anodes by hybrid organic/inorganic ASEI represents a new approach to mitigating or eliminating dendrite formation at reactive metal anodes—illustrated here for Li—and may expedite the realization of a “beyond-Li-ion” battery technology employing Li metal anodes (e.g., Li–S).

  12. Author(s): Martin Mittendorff, Ryan J. Suess, Edward Leong, Thomas E. Murphy
    Publication: Nano Lett. 17, 5811 (2017)
    Doi: 10.1021/acs.nanolett.7b02931

    Photoconductive antennas are widely used for time-resolved detection of terahertz (THz) pulses. In contrast to photothermoelectric or bolometric THz detection, the coherent detection allows direct measurement of the electric field transient of a THz pulse, which contains both spectral and phase information. In this Letter, we demonstrate for the first time photoconductive detection of free-space propagating THz radiation with thin flakes of a van der Waals material. Mechanically exfoliated flakes of black phosphorus are combined with an antenna that concentrates the THz fields to the small flake (∼10 μm). Similar performance is reached at gating wavelengths of 800 and 1550 nm, which suggests that the narrow bandgap of black phosphorus could allow operation at wavelengths as long as 4 μm. The detected spectrum peaks at 60 GHz, where the signal-to-noise ratio is of the order of 40 dB, and the detectable signal extends to 0.2 THz. The measured signal strongly depends on the polarization of the THz field and the gating pulse, which is explained by the role of the antenna and the anisotropy of the black phosphorus flake, respectively. We analyze the limitations of the device and show potential improvements that could significantly increase the efficiency and bandwidth.

  13. Author(s): Edo Waks, Elizabeth A. Goldschmidt
    Publication: Science 357, 1354 (2017)
    Doi: 10.1126/science.aao2437

    Information networks must transmit data over long distances and store it for later retrieval. A quantum network operates in an analogous way but uses signals contained in a quantum system, such as single photons or atoms (1). Although rapid progress in the generation of quantum light provides hope for future realizations of quantum networks, the ability to store photons remains a critical limitation. A number of approaches offer the possibility to store photons (2), but they typically suffer from short storage times, low efficiencies, or large footprints that are incompatible with compact integrated devices. On page 1392 of this issue, Zhong et al. (3) report a major step toward the goal of storing light in a chip-scale atomic memory. They exploit the enhancement of optical cavities to transfer light efficiently to excitations in rare-earth ions. The device fits on a micrometer-sized footprint, allowing storage of light in a very tiny box.

  14. Author(s): Nightvid Cole, Thomas M. Antonsen, Jr.
    Publication: IEEE Trans. Plasma Sci. 45, 2945 (2017)
    Doi: 10.1109/TPS.2017.2759269

    A cyclotron resonance maser source using low-effective-mass conduction electrons in graphene, if successful, would allow for generation of far infrared (FIR) and terahertz (THz) radiation without requiring magnetic fields running into the tens of tesla. In order to investigate this possibility, we consider a situation in which electrons are effectively injected via pumping from the valence band to the conduction band using an IR laser source, subsequently gyrate in a magnetic field applied perpendicular to the plane of the graphene, and give rise to gain for an FIR/THz wave crossing the plane of the graphene. The treatment is classical, and includes on equal footing the electron interaction with the radiation field and the decay in electron energy due to collisional processes. Gain is found even though there is no inversion of the energy distribution function. Gain can occur for electron damping times as short as hundreds of femtoseconds.

  15. Author(s): Matt Landreman
    Publication: Nucl. Fusion 57, 046003 (2017)
    Doi: 10.1088/1741-4326/aa57d4

    Several fast methods for computing stellarator coil shapes are compared, including the classical NESCOIL procedure (Merkel 1987 Nucl. Fusion 27 867), its generalization using truncated singular value decomposition, and a Tikhonov regularization approach we call REGCOIL in which the squared current density is included in the objective function. Considering W7-X and NCSX geometries, and for any desired level of regularization, we find the REGCOIL approach simultaneously achieves lower surface-averaged and maximum values of both current density (on the coil winding surface) and normal magnetic field (on the desired plasma surface). This approach therefore can simultaneously improve the free-boundary reconstruction of the target plasma shape while substantially increasing the minimum distances between coils, preventing collisions between coils while improving access for ports and maintenance. The REGCOIL method also allows finer control over the level of regularization, it preserves convexity to ensure the local optimum found is the global optimum, and it eliminates two pathologies of NESCOIL: the resulting coil shapes become independent of the arbitrary choice of angles used to parameterize the coil surface, and the resulting coil shapes converge rather than diverge as Fourier resolution is increased. We therefore contend that REGCOIL should be used instead of NESCOIL for applications in which a fast and robust method for coil calculation is needed, such as when targeting coil complexity in fixed-boundary plasma optimization, or for scoping new stellarator geometries.

  16. Author(s): Gottlieb S. Oehrlein, Satoshi Hamaguchi, Achim Von Keudell
    Publication: J. Phys. D - Appl. Phys. 50, 490201 (2017)
    Doi: 10.1088/1361-6463/aa95c8

    State-of-the-art technologies are increasingly demanding materials and thin film processing technologies that offer control at atomistic length scales. These requirements are pushing plasma-based processing techniques towards fundamental limits with regard to modification, deposition, and etching of materials. The goal of achieving atomistic control in plasma–material interactions may be viewed as a grand challenge of low-temperature plasma science and technology. It involves the need to control the interaction of multiple particle fluxes that are characteristic of the plasma state, including electrons, ions, radicals, excited neutrals and photons, with surfaces and to regulate the consequences of these interactions towards desired atomistic outcomes.

    The challenge of atomistic control in plasma–material interactions provides the unifying perspective of this Special Issue. The collection of invited reviews and current research articles of the issue illustrates various aspects of the overall challenge. The presentation of a related number of coordinated topics was intended to (1) illustrate the achievements and state of the art of fundamental research and technical capabilities in different areas of application, (2) identify areas where we either lack sufficient understanding to achieve this goal or where our current plasma-surface interaction approaches provide insufficient control, and (3) clarify the scientific knowledge and potential advances required for low temperature plasma-based methods to be successful against the goal of achieving atomistic control in plasma–surface interactions.

  17. Author(s): Zhili Yang, Matthew Pelton, Maryna I. Bodnarchuk, Maksym V. Kovalenko, Edo Waks
    Publication: Appl. Phys. Lett. 111, 221104 (2017)
    Doi: 10.1063/1.5000248

    We demonstrate coupling of lead halide perovskite nanocrystals to a nanophotonic cavity. From photoluminescence measurements, we observe a factor of 10 enhancement in brightness from the cavity mode emission. We perform room temperature time-resolved lifetime measurements that demonstrate an average spontaneous emission rate enhancement of 2.9 for perovskite nanocrystals within the cavity as compared to those located on the unpatterned surfaces. Our method provides a way towards realizing efficient light emitters and low-threshold lasers, as well as fast nonlinear optical devices, using solution processable materials.

  18. Author(s): J.T. Dahlin, J.F. Drake, M. Swisdak
    Publication: Phys. Plasmas 24, 092110 (2017)
    Doi: 10.1063/1.4986211

    Magnetic reconnection is an important driver of energetic particles in many astrophysical phenomena. Using kinetic particle-in-cell simulations, we explore the impact of three-dimensional reconnection dynamics on the efficiency of particle acceleration. In two-dimensional systems, Alfvénic outflows expel energetic electrons into flux ropes where they become trapped and disconnected from acceleration regions. However, in three-dimensional systems these flux ropes develop an axial structure that enables particles to leak out and return to acceleration regions. This requires a finite guide field so that particles may move quickly along the flux rope axis. We show that greatest energetic electron production occurs when the guide field is of the same order as the reconnecting component: large enough to facilitate strong transport, but not so large as to throttle the dominant Fermi mechanism responsible for efficient electron acceleration. This suggests a natural explanation for the envelope of electron acceleration during the impulsive phase of eruptive flares.

  19. Author(s): Edward Leong, Ryan J. Suess, Andrei B. Sushkov, H. Dennis Drew, Thomas E. Murphy, Martin Mittendorff
    Publication: Opt. Exp. 25, 12666 (2017)
    Doi: 10.1364/OE.25.012666

    Two-dimensional black phosphorus is a new material that has gained widespread interest as an active material for optoelectronic applications. It features high carrier mobility that allows for efficient free-carrier absorption of terahertz radiation, even though the photon energy is far below the bandgap energy. Here we present an efficient and ultrafast terahertz detector, based on exfoliated multilayer flakes of black phosphorus. The device responsivity is about 1 mV/W for a 2.5 THz beam with a diameter of 200 μm, and is primarily limited by the small active area of the device in comparison to the incident beam area. The intrinsic responsivity is determined by Joule heating experiments to be about 44 V/W, which is in agreement with predictions from the Drude conductivity model. Time resolved measurements at a frequency of 0.5 THz reveal an ultrafast response time of 20 ps, making black phosphorus a candidate for high performance THz detection at room temperature.

  20. Author(s): M. Opher, J.F. Drake, M. Swisdak, B. Zieger, G. Toth
    Publication: Astrophys. J. Lett. 839, L12 (2017)
    Doi: 10.3847/2041-8213/aa692f

    Based on the difference between the orientation of the interstellar BISM and the solar magnetic fields, there was an expectation that the magnetic field direction would rotate dramatically across the heliopause (HP). However, the Voyager 1 spacecraft measured very little rotation across the HP. Previously, we showed that the BISM twists as it approaches the HP and acquires a strong T component (east–west). Here, we establish that reconnection in the eastern flank of the heliosphere is responsible for the twist. On the eastern flank the solar magnetic field has twisted into the positive N direction and reconnects with the southward pointing component of the BISM. Reconnection drives a rotational discontinuity (RD) that twists the BISM into the −T direction and propagates upstream in the interstellar medium toward the nose. The consequence is that the N component of BISM is reduced in a finite width band upstream of the HP. Voyager 1 currently measures angles (delta = sin-1(BN/B)) close to solar values. We present MHD simulations to support this scenario, suppressing reconnection in the nose region while allowing it in the flanks, consistent with recent ideas about reconnection suppression from diamagnetic drifts. The jump in plasma β (the plasma to magnetic pressure) across the nose of HP is much greater than in the flanks because the heliosheath β is greater there than in the flanks. Large-scale reconnection is therefore suppressed in the nose but not at the flanks. Simulation data suggest that BISM will return to its pristine value 10–15 au past the HP.

  21. Author(s): Zhili Yang, Matthew Pelton, Igor Fedin, Dmitri V. Talapin, Edo Waks
    Publication: Nature Commun. 8, 143 (2017)
    Doi: 10.1038/s41467-017-00198-z

    Colloidal semiconductor nanocrystals have emerged as promising active materials for solution-processable optoelectronic and light-emitting devices. In particular, the development of nanocrystal lasers is currently experiencing rapid progress. However, these lasers require large pump powers, and realizing an efficient low-power nanocrystal laser has remained a difficult challenge. Here, we demonstrate a nanolaser using colloidal nanocrystals that exhibits a threshold input power of less than 1 μW, a very low threshold for any laser using colloidal emitters. We use CdSe/CdS core-shell nanoplatelets, which are efficient nanocrystal emitters with the electronic structure of quantum wells, coupled to a photonic-crystal nanobeam cavity that attains high coupling efficiencies. The device achieves stable continuous-wave lasing at room temperature, which is essential for many photonic and optoelectronic applications. Our results show that colloidal nanocrystals are suitable for compact and efficient optoelectronic devices based on versatile and inexpensive solution-processable materials.

  22. Author(s): Kevin M. Daniels, M. Mehdi Jadidi, Andrei B. Sushkov, Anindya Nath, Anthony K. Boyd, Karthik Sridhara, H. Dennis Drew, Thomas E. Murphy, Rachael L. Myers-Ward, D. Kurt Gaskill
    Publication: 2D Mater. 4, 2 (2017)
    Doi: 10.1088/2053-1583/aa5c75

    Exploiting the underdeveloped terahertz range (~1012–1013 Hz) of the electromagnetic spectrum could advance many scientific fields (e.g. medical imaging for the identification of tumors and other biological tissues, non-destructive evaluation of hidden objects or ultra-broadband communication). Despite the benefits of operating in this regime, generation, detection and manipulation have proven difficult, as few materials have functional interactions with THz radiation. In contrast, graphene supports resonances in the THz regime through structural confinement of surface plasmons, which can lead to enhanced absorption. In prior work, the achievable plasmon resonances in such structures have been limited by multiple electron scattering mechanisms (i.e., large carrier scattering rates) which greatly broaden the resonance (>100 cm−1; 3 THz). We report the narrowest room temperature Drude response to-date, 30 cm−1 (0.87 THz), obtained using quasi-free standing bilayer epitaxial graphene (QFS BLG) synthesized on (0 0 0 1)6H–SiC. This narrow response is due to a 4-fold increase in carrier mobility and improved thickness and electronic uniformity of QFS BLG. Moreover, QFS BLG samples patterned into microribbons targeting 1.8–5.7 THz plasmon resonances also exhibit low scattering rates (37–53 cm−1). Due to the improved THz properties of QFS BLG, the effects of e-beam processing on carrier scattering rates was determined and we found that fabrication conditions can be tuned to minimize the impact on optoelectronic properties. In addition, electrostatic gating of patterned QFS BLG shows narrow band THz amplitude modulation. Taken together, these properties of QFS BLG should facilitate future development of THz optoelectronic devices for monochromatic applications.

  23. Author(s): Chuan-Fu Lin, Xiulin Fan, Alexander Pearse, Sz-Chian Liou, Keith Gregorczyk, Michal Leskes, Chunsheng Wang, Sang Bok Lee, Gary W. Rubloff, Malachi Noked
    Publication: Chem. Mater. 29, 8780 (2017)
    Doi: 10.1021/acs.chemmater.7b03058

    High-energy conversion electrodes undergo successive Li insertion and conversion during lithiation. A primary scientific obstacle to harnessing the potentially high lithium storage capabilities of conversion electrode materials has been the formation of insulating new phases throughout the conversion reactions. These new phases are chemically stable, and electrochemically irreversible if formed in large amounts with coarsening. Herein, we synthesized FeOF conversion material as a model system and mechanistically demonstrate that a thin solid electrolyte [lithium phosphorus oxynitride (LiPON)] atomic layer deposition-deposited on the composite electrode extends the Li insertion process to higher concentrations, delaying the onset of a parasitic chemical conversion reaction and rendering the redox reaction of the protected conversion electrode electrochemically reversible. Reversibility is demonstrated to at least 100 cycles, with the LiPON protective coating increasing capacity retention from 29 to 89% at 100 cycles. Pursuing the chemical mechanism behind the boosted electrochemical reversibility, we conducted electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and electrochemical measurements that unrevealed the suppression of undesired phase formation and extended lithium insertion of the coated electrode. Support for the delayed consequences of the conversion reaction is also obtained by high-resolution transmission electron microscopy. Our findings strongly suggest that undesired new phase formation upon lithiation of electrode materials can be suppressed in the presence of a thin protection layer not only on the surface of the coated electrode but also in the bulk of the material through mechanical confinement that modulates the electrochemical reaction.

  24. Author(s): Edward Ott, Thomas M. Antonsen, Jr.
    Publication: Chaos 27, 051101 (2017)
    Doi: 10.1063/1.4983470

    A common observation is that large groups of oscillatory biological units often have the ability to synchronize. A paradigmatic model of such behavior is provided by the Kuramoto model, which achieves synchronization through coupling of the phase dynamics of individual oscillators, while each oscillator maintains a different constant inherent natural frequency. Here we consider the biologically likely possibility that the oscillatory units may be capable of enhancing their synchronization ability by adaptive frequency dynamics. We propose a simple augmentation of the Kuramoto model which does this. We also show that, by the use of a previously developed technique [Ott and Antonsen, Chaos 18, 037113 (2008)], it is possible to reduce the resulting dynamics to a lower dimensional system for the macroscopic evolution of the oscillator ensemble. By employing this reduction, we investigate the dynamics of our system, finding a characteristic hysteretic behavior and enhancement of the quality of the achieved synchronization.

  25. Author(s): Min Zhou, Edward Ott, Thomas M. Antonsen, Jr., Steven M. Anlage
    Publication: Chaos 27, 103114 (2017)
    Doi: 10.1063/1.4986499

    Concepts from the field of wave chaos have been shown to successfully predict the statistical properties of linear electromagnetic fields in electrically large enclosures. The Random Coupling Model (RCM) describes these properties by incorporating both universal features described by Random Matrix Theory and the system-specific features of particular system realizations. In an effort to extend this approach to the nonlinear domain, we add an active nonlinear frequency-doubling circuit to an otherwise linear wave chaotic system, and we measure the statistical properties of the resulting second harmonic fields. We develop an RCM-based model of this system as two linear chaotic cavities coupled by means of a nonlinear transfer function. The harmonic field strengths are predicted to be the product of two statistical quantities and the nonlinearity characteristics. Statistical results from measurement-based calculation, RCM-based simulation, and direct experimental measurements are compared and show good agreement over many decades of power.

  26. Author(s): Anand Bahl, Jared K. Wahlstrand, Sina Zahedpour, Howard M. Milchberg, Miroslav Kolesik
    Publication: Phys. Rev. A 96, 043867 (2017)
    Doi: 10.1103/PhysRevA.96.043867

    The nonlinear polarization response and plasma generation produced by intense optical pulses, modeled by the metastable-electronic-state approach, are verified against space-and-time resolved measurements with single-shot supercontinuum spectral interferometry. This first of a kind theory-experiment comparison is done in the intensity regime typical for optical filamentation, where self-focusing and plasma generation play competing roles. Excellent agreement between the theory and experiment shows that the self-focusing nonlinearity can be approximated by a single resonant state. Moreover, we demonstrate that inclusion of the post-adiabatic corrections, previously tested only in theoretic models, provides a viable description of the ionization rate in real gases.

  27. Author(s): J.F. Drake, M. Swisdak, M. Opher, J.D. Richardson
    Publication: Astrophys. J. 837, 159 (2017)
    Doi: 10.3847/1538-4357/aa6304

    The misalignment of the solar rotation axis and the magnetic axis of the Sun produces a periodic reversal of the Parker spiral magnetic field and the sectored solar wind. The compression of the sectors is expected to lead to reconnection in the heliosheath (HS). We present particle-in-cell simulations of the sectored HS that reflect the plasma environment along the Voyager 1 and 2 trajectories, specifically including unequal positive and negative azimuthal magnetic flux as seen in the Voyager data. Reconnection proceeds on individual current sheets until islands on adjacent current lay