This paper theoretically proposes a multichannel nonlocal metasurface computer characterized by generalized sheet transition conditions (GSTCs) and surface susceptibility tensors. The study explores polarization- and angle-multiplexed metasurfaces enabling multiple and independent parallel analog spatial computations when illuminated by differently polarized incident beams from different directions. The proposed synthesis overcomes substantial restrictions imposed by the previous designs such as large architectures arising from the need for additional sub-blocks; slow responses; working for a certain incident angle or polarization; executing only a single mathematical operation; and, most importantly, supporting only the even-symmetric operations for normal incidences. The versatility of our design is demonstrated in a way that an ultracompact, integrable, and homogeneous metasurface-assisted platform can execute a variety of optical-signal-processing operations such as spatial differentiation and integration. It is demonstrated that a metasurface featuring nonreciprocal properties can be thought of as a new paradigm to break the even symmetry of reflection and perform both even- and odd-symmetric mathematical operations for input fields coming from a normal direction. Numerical simulations also illustrate different aspects of a multichannel edge detection scheme through projecting multiple images on the metasurface from different directions. Such appealing findings not only circumvent the major potential drawbacks of previous designs but may also offer an efficient, easy-to-fabricate, and flexible approach in wave-based signal processing, edge detection, image contrast enhancement, hidden object detection, and equation solving without a Fourier lens.

Optical nanofiber waveguides are widely used for near-field delivery and measurement of light. Despite their versatility and efficiency, nanofibers have a critical drawback—their inability to maintain light’s polarization state on propagation. Here, we design a directional coupler consisting of two crossed nanofibers to probe the polarization state at the waist region. Directionality of coupling occurs due to asymmetric dipolar emission or spin locking when the evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber system. We demonstrate that, by monitoring the outputs from the directional coupler, two nonorthogonal polarization states can be prepared at the nanofiber waist with a fidelity higher than 99%. Based on these states, we devise a simple and reliable method for complete control of the polarization along a nanofiber waveguide.

Transformation methods have stimulated many interesting applications of manipulating electromagnetic and acoustic waves by using metamaterials, such as super-lens imaging and cloaking. These successes are mainly due to the form-invariant property of the Maxwell equations and acoustic equations. However, the similar progress in manipulating elastic waves is very slow, because the elastodynamic equations are not form invariant. We show that the expression of the elastodynamic potential energy can almost retain its form after conformal mapping, if the longitudinal wave velocity is much larger than the transverse wave velocity, or if the wavelength can be shortened by converting the waves into surface modes. Based on these findings, it is possible to design and fabricate alternative devices with ease to manipulate elastic waves at will. One example presented in this paper is an efficient vibration isolator, which contains a 180-degree wave bender made of conventional rubbers. Compared with a conventional isolator of the same shape, similar static support stiffness, and smaller damping ratio, this isolator can further reduce wave transmissions by up to 39.9 dB in the range of 483–1800 Hz.

In most thin-film transistors, the concept of mobility edge needs to be redefined to take into account important constraints. We describe how the mobility edge concept has to be reinterpreted when applied to thin-film transistors. Charge carriers can be significantly localized due to strong scattering, altering the model of band transport in extended states so that, effectively, only a fraction of carriers can actually move in the extended states, in accordance with a statistical distribution of free paths. It is necessary to explicitly consider this effect in applying conventional semiconductor transport theories based on solutions to the Boltzmann transport equation (BTE). We are then able to apply the BTE with appropriate scattering mechanisms and obtain results that agree well with experiment for the case of a polymer thin-film transistor (TFT). This approach is very well suited to thin-film transistors based on polymer and organic semiconductors, and also many amorphous oxide semiconductors with room temperature mobilities in the range 1–20 ${\mathrm{cm}}^2$/(Vs).

We experimentally study the dynamical modes excited by spin current in spin Hall nano-oscillators based on $\mathrm{Pt}$/[$\mathrm{Co}$/$\mathrm{Ni}$] multilayers with perpendicular magnetic anisotropy. Both propagating spin-wave and localized solitonic modes of the oscillation are achieved and controlled by varying the applied magnetic field and current. At room temperature, the generation linewidth broadening associated with mode hopping is observed at currents close to the transition between different modes and in the mode-coexistence regimes. The mode hopping is suppressed at cryogenic temperatures, confirming that the coupling between modes is mediated by thermal-magnon-mediated scattering. We also demonstrate that coherent single-mode oscillations with a linewidth of 5 MHz can be achieved without applying an external magnetic field. Our results provide insight into the mechanisms controlling the dynamical coherence in nanomagnetic oscillators and guidance for optimizing their applications in spin-wave-based electronics.

We report the design of a diamond-based honeycomblike phononic network in which a mechanical resonator couples to three distinct phononic crystal waveguides. This two-dimensional (2D) phononic network extends an earlier study on one-dimensional (1D) phononic networks with closed mechanical subsystems. With a special design for the phononic band structures of the waveguides, any two neighboring resonators in the 2D network and the waveguide between them can form a closed mechanical subsystem, which enables nearest neighbor coupling and at the same time circumvents the scaling problems inherent in typical large mechanical systems. The nearest neighbor mechanical coupling allows the theoretical implementation of Molmer-Sorensen quantum gates between spins in adjacent mechanical resonators through phonon-assisted spin transitions. Honeycomblike phononic networks of spin qubits with nearest neighbor coupling can serve as an experimental platform for quantum computing and especially topological quantum error corrections.

Acoustics-based techniques are investigated to focus and bunch nanoparticle beams. This process allows us to overcome the prominent problem of the longitudinal and transverse mismatch of particle-stream and x-ray-beam size in single-particle and single-molecule imaging at x-ray free-electron lasers (XFELs). It also enables synchronized injection of particle streams at kilohertz repetition rates. Transverse focusing concentrates the particle flux to the size of the (sub)micrometer X-ray focus. In the longitudinal direction, focused acoustic waves can be used to bunch the particle to the same repetition rate as the x-ray pulses. The acoustic manipulation is based on simple mechanical recoil effects and could be advantageous over light-pressure-based methods, which rely on absorption. The acoustic equipment is easy to implement and can be conveniently inserted into current XFEL endstations. With the proposed method, data collection times could be reduced by a factor of ${10}^4$. This work does not just provide an efficient method for acoustic manipulation of streams of arbitrary gas-phase particles, but also opens up wide avenues for acoustics-based particle optics.

We demonstrate a single-mode acoustic waveguide that enables robust propagation of mechanical waves. The waveguide is a highly stressed silicon-nitride membrane that supports the propagation of out-of-plane modes. In direct analogy to rectangular microwave waveguides, there exists a band of frequencies over which only the fundamental mode is allowed to propagate, while multiple modes are supported at higher frequencies. We directly image the mode profiles using optical heterodyne vibration measurement, showing good agreement with theory. In the single-mode frequency band, we show low-loss propagation (approximately 1 $\mathrm{dB}/\mathrm{cm}$) for an approximately 5-MHz mechanical wave. This design is well suited for acoustic circuits interconnecting elements such as nonlinear resonators or optomechanical devices for signal processing, sensing, or quantum technologies.

We undertake an experimental study on the early detection of combustion-driven thermoacoustic instability using a method combining complex networks and machine learning. The probability distribution of the transition patterns in ordinal partition transition networks significantly captures the subtle changes in the combustion state during a transition from combustion noise with a low amplitude to thermoacoustic combustion instability with a high amplitude. A feature space consisting of the principal component plane estimated from the probability distribution of the transition patterns, which is obtained by a support vector machine, allows us to detect a precursor of thermoacoustic combustion instability.

In this work, we present efficient coding metasurfaces for acoustic-wave field manipulations. Distinct from the conventional metasurfaces usually having a complex configuration along the surface, our proposed metasurfaces are compact and composed of bipartite units: hornlike helical structures and air. Two kinds of hornlike helical units, defined as logical units “1” and “1/2,” are employed to provide phase differences of $\pi$ and $\pi /2$ compared to air (logical unit “0”). To demonstrate such a simple yet efficient design possessing various fascinating functionalities, we first realize acoustic splitting with suppressed side lobes by suitably arranging logical units 1 and 0. Then, acoustic bending is further achieved with logical units 1/2 and 0. Experiments are conducted to validate these acoustic field manipulations with a broadband characteristic, whose results are consistent with the analytical predictions and simulations. Because of the advantages of the broadband feature and compact size, our proposed coding metasurfaces could contribute to designing devices for manipulating acoustic propagation in desired manners.

We fabricate fully epitaxial $\mathrm{Fe}/\mathrm{Zn}\mathrm{O}/\mathrm{Mg}\mathrm{O}/\mathrm{Fe}$ magnetic tunnel junctions (MTJs) with a bilayer tunnel barrier in which $\mathrm{Zn}\mathrm{O}$ has a metastable rock-salt crystal structure. We observe a high magnetoresistance ratio up to 96% at room temperature (RT) and find that these MTJs have asymmetric current-voltage characteristics, and their rectifying performances are largely dependent on the magnetization alignments of the $\mathrm{Fe}$ electrodes. Diode responsibilities at a zero-bias voltage (${\beta }_0$), which is an important performance index for harvesting applications, are observed up to 1.3 A/W at RT in the antiparallel alignment of the magnetizations while maintaining rather low resistance-area products (RA) of a few tens of kΩ $\mu {\mathrm{m}}^2$. Even with the same top and bottom electrodes ($\mathrm{Fe}$), the obtained ${\beta }_0$ values are comparable to those of reported high-performance tunnel diodes consisting of amorphous bilayer tunnel barriers with polycrystalline dissimilar electrodes. This strongly suggests that the epitaxial $\mathrm{Zn}\mathrm{O}/\mathrm{Mg}\mathrm{O}$ bilayer tunnel barrier is effective for enhancing the ${\beta }_0$ without significant increase in the RA. In addition, we demonstrate that a zero-bias anomaly in the tunnel conductance, which originates from the magnon excitations at the $\mathrm{Fe}$/barrier interfaces, plays a crucial role in observed spin-dependent diode performance. The results indicate that a fully epitaxial MTJ with a bilayer tunnel barrier is a promising candidate to establish a high-performance high-frequency rectifying system.

The electronic transport and photoelectric properties of hydrogenated borophene ${\mathrm{B}}_4{\mathrm{H}}_4$, which were realized in a recent experiment by Nishino, et al. [J. Am. Chem. Soc. 139, 13761 (2017)] are systematically investigated using the density functional theory and nonequilibrium Green's function methods. We find that ${\mathrm{B}}_4{\mathrm{H}}_4$ exhibits a perfect current-limiting effect and has high (along the zigzag direction) and low (along the armchair one) optional levels due to its strong electrical anisotropy. Moreover, ${\mathrm{B}}_4{\mathrm{H}}_4$ can generate sizable photocurrents under illumination, with strong photoelectronic response to blue or green light along the zigzag or armchair direction. Our work demonstrates that ${\mathrm{B}}_4{\mathrm{H}}_4$ is promising for the applications of current limiter and photodetectors.

$\mathrm{Nb}$ superconducting radio-frequency (SRF) cavities are observed to break down and lose their high-$Q$ superconducting properties at accelerating gradients below the limits imposed by theory. The microscopic origins of SRF cavity breakdown are still a matter of some debate. To investigate these microscopic issues, temperature- and power-dependent local third-harmonic response is measured on bulk $\mathrm{Nb}$ and $\mathrm{Nb}$ thin-film samples using a novel near-field magnetic microwave microscope between 2.9 and 10 K and 2 and 6 GHz. Both periodic and nonperiodic response as a function of applied rf field amplitude are observed. We attribute these features to extrinsic and intrinsic nonlinear responses of the sample. The rf-current-biased resistively shunted junction (RSJ) model can account for the periodic response and fits very well to the data using reasonable parameters. The nonperiodic response is consistent with vortex semiloops penetrating into the bulk of the sample once sufficiently high rf magnetic field is applied and the data can be fit to a time-dependent Ginzburg-Landau (TDGL) model of this process. The fact that these responses are measured on a wide variety of $\mathrm{Nb}$ samples suggests that we are capturing the generic nonlinear response of air-exposed $\mathrm{Nb}$ surfaces.

The availability of large amounts of data and the necessity of processing it efficiently have led to the rapid development of machine-learning techniques. To name a few examples, artificial-neural-network architectures are commonly used for financial forecasting, speech and image recognition, robotics, medicine, and even research. Direct hardware for neural networks is highly sought for overcoming the von Neumann bottleneck of software implementations. Reservoir computing (RC) is a recent and increasingly popular bioinspired computing scheme that holds promise for efficient temporal information processing. We demonstrate the applicability and performance of RC in a general complex Ginzburg-Landau lattice model, which adequately describes the dynamics of a wide class of systems, including coherent photonic devices. In particular, we propose that the concept can be readily applied in exciton-polariton lattices, which are characterized by unprecedented photonic nonlinearity, opening the way to signal processing at rates of the order of $1\mathrm{Tbit}{\mathrm{s}}^{-1}$.

We report on an experimental study of a laser regime in erbium-doped whispering-gallery-mode (WGM) microspheres under modal-coupling between the co- and counterpropagating modes. The evidence of modal coupling is observed in the relative intensity noise spectrum of several WGM lasers. Cross-correlation measurements are carried out in order to analyze precisely the emission regimes. It is shown that depending on the material constituting the WGM resonator, frequency-locked bidirectional emission or a self-modulated regime could be reached. The control of the laser emission regime of WGM microlasers is of great importance in the aim of applications in microwave optics or optical sensor miniaturization.

Dispersive gate sensing (DGS) uses radio-frequency (rf) reflectometry to locally probe the quantum capacitance of a gate electrode. Applying DGS to heterostructure-based qubit devices, we report the repeated observation of anomalous signals that we attribute to pockets of charge in the potential landscape likely under, and surrounding, the surface gates that define quantum-dot qubits. Interestingly, these charge pockets appear to evade detection with conventional charge sensors but manifest strongly in the response of the gate sensor. Configuring a quantum point contact (QPC) as a highly localized heat source, we show how these charge pockets likely form close to the end of the gate electrodes, in close proximity to gate-defined qubits. The presence of uncontrolled charge may lead to offsets in gate voltage and further contribute to charge noise that produces decoherence in semiconductor qubits.

We demonstrate a tunnel field-effect transistor based on a lateral heterostructure patterned from an $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ electron gas. Charge is injected by tunneling from the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$/$\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ contacts and the current through a narrow channel of insulating $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ is controlled via an electrostatic side gate. Drain-source $I$-$V$ curves are measured at low and elevated temperatures. The transistor shows strong electric-field-dependent and temperature-dependent behavior, with a steep subthreshold slope as small as $10\mathrm{mV}/\mathrm{dec}$ and a transconductance as high as approximately $22\mu \mathrm{A}/\mathrm{V}$. A fully consistent transport model for the drain-source tunneling reproduces the measured steep subthreshold slope.

Phonon reflection and transmission at the interfaces plays a fundamental role in cryogenic particle detectors, in which the optimization of the phonon signal at the sensor (in case of phonon-mediated detectors) or the minimization of the heat transmission (when the detection occurs in the sensor itself) is of primary importance to improve sensitivity. Nevertheless, the mechanisms governing the phonon physics at the interfaces are still not completely understood. The two more successful models, the acoustic mismatch model (AMM) and diffuse mismatch model (DMM) are not able to explain all the accumulated experimental data and the measurement of the transmission coefficients between the materials remains a challenge. Here, we use measurements of the athermal phonon flux in aluminum kinetic inductance detectors (KIDs) deposited on silicon substrates following a particle interaction to validate a Monte Carlo (MC) phonon simulation. We apply the Mattis-Bardeen theory to derive the phonon pulse energy and timing from the KID signal and compare the results with the MC for specular AMM and DMM reflection, finding a remarkably good agreement for specular, while diffuse reflection is clearly disfavored. For an aluminum film of 60 nm and a silicon substrate of $380\mu \mathrm{m}$, we obtain transmission coefficients $\mathrm{Si}$-$\mathrm{Al}$ in the range 0.3–0.55 and $\mathrm{Si}$-Teflon in the range 0.1–0.15.

Optical detection of solid-state spin state has been widely used in research in quantum-information processing. The signal contrast is crucial for various applications. Here we demonstrate a time-gated fluorescence-detection method for enhancing the spin-state signal contrast of the nitrogen-vacancy (N-V) center in diamond. By adjusting the delay between the time gate and the excitation-laser pulse, we increase both the signal contrast and the signal-to-noise ratio for N-V spin detection. An enhancement ratio of approximately 1.9 times is achieved for the signal contrast. Using time-gated fluorescence detection, we extract the N-V-center information from high-level background, and demonstrate high-contrast quantum imaging of a nanoparticle’s stray magnetic field. Without any additional manipulation of the quantum state, we expect that this method can be used to improve the performance of various applications with the N-V center.

We demonstrate a compact (0.25 L) system for laser cooling and trapping atoms from a heated dispenser source. Our system uses a nanofabricated diffraction grating to generate a magneto-optical trap (MOT) using a single input laser beam. An aperture in the grating allows atoms from the dispenser to be loaded from behind the chip, increasing the interaction distance of atoms with the cooling light. To take full advantage of this increased distance, we extend the magnetic field gradient of the MOT to create a Zeeman slower. The MOT traps approximately ${10}^6{}^7\mathrm{Li}$ atoms emitted from an effusive source with loading rates greater than ${10}^6{\mathrm{s}}^{-1}$. Our design is portable to a variety of atomic and molecular species and could be a principal component of miniaturized cold-atom-based technologies.