Breaking reciprocity with spatiotemporal modulation provides an opportunity to design unprecedented optical, acoustic, and mechanical waveguides. A main challenge is to physically realize continuum-based metamaterials whose properties can be rapidly tuned in both space and time at the length and time scales of the propagated waves. We design a tunable elastic metamaterial by embedding in a beam a set of permanent magnets, and placing oscillating electrical coils coaxially adjacent to each magnet. By programming in space and time the ac input of the coils, the magnet-coil effective coupling stiffness is modulated along with the resonance frequency. Distinctly nonreciprocal flexural wave propagation is then experimentally observed. In addition, robust tunability of unidirectional band gaps and wave energy bias are quantitatively analyzed by applying different modulation current amplitudes, material damping coefficients, and modulation frequencies. Both simplified analytical and finite-element-method-based numerical models of the modulated metamaterial are suggested and analyzed in support of the experimental work. Specifically, unidirectional frequency conversions and band gaps due to the second-order mode interactions are discussed for the first time when the large modulation amplitude is implemented. The suggested prototype sheds light on nonreciprocal waveguiding, which could be applied in advanced wave diodes, phononic logic, energy localization, trapping, and harvesting.

The dynamics of a ${90}^{\circ }$ domain wall in an antiferromagnetic nanostrip driven by the current-induced spin-orbit torque are theoretically examined in the presence of thermal fluctuations. A soliton-type equation of motion is developed on the basis of energy balance between the driving forces and dissipative processes in terms of the domain-wall velocity. Comparison with micromagnetic simulations in the deterministic conditions shows good agreement in both the transient and steady-state transport. When the effects of thermal fluctuations are included via a stochastic treatment, the results clearly indicate that the dispersion in the domain-wall position can be controlled electrically by tailoring the strength and duration of the driving current mediating the spin orbital torque in the antiferromagnet. More specifically, the standard deviation of the probability distribution function for the domain-wall movement can be tuned widely while maintaining the average position unaffected. Potential applications of this unusual functionality include probabilistic computing such as Bayesian learning.

The antiferroelectric-ferroelectric phase transition in ${\mathrm{Pb}\mathrm{Zr}\mathrm{O}}_3$-based oxides is of both fundamental and practical importance. In ceramics in which such a transition readily occurs, the antiferroelectric and the ferroelectric phases often coexist in individual grains with a coherent interphase interface. In this work, the electric biasing in situ transmission electron microscopy technique is employed to directly observe a unique microstructural dynamic when ferroelectric and antiferroelectric domains are driven by a moderate electric field to interact. It is found that, under monotonic loading, the ferroelectric domain grows until it is blocked by the ferroelectric-antiferroelectric interface. At the same time, a kink is formed on the interface at the contact point. The interaction of the growing domain with the interface is interpreted in terms of depolarization field-assisted phase transition, which is supported by our phase-field simulation. Upon further bipolar cycling, the ferroelectric domain becomes less mobile and no longer reaches the ferroelectric-antiferroelectric interface, indicative of electric fatigue of the ferroelectric phase.

The major challenge to improve deterministic single-ion sources is to control the position and momentum of each ion. On the basis of the extra information given by the electron created in a photoionization process, the trajectory of the correlated ion can be corrected with use of a fast real-time feedback system. In this paper, we report on a proof-of-principle experiment that demonstrates the performance of this feedback control with individual cesium ions. The electron produced is detected with a time- and position-sensitive detector, the information from which is used to quickly infer the position of the corresponding ion. Then the feedback system drives the ion trajectory through steering plates, and the individual ion can thus be sent to any dedicated location. This enables us to perform deterministic patterning and achieve a factor-of-1000 reduction in spot area. The single-ion feedback control is versatile and can be applied to different kinds of ion sources. It provides a powerful tool to optimize the ion beam and offers a complete portfolio for quantum systems and applications in materials science.

We present an ultrahigh gain, narrow bandwidth, and low noise optical amplifier via the radiation pressure induced by the four-wave mixing (FWM) effect based on a cavity optomechanical system in the unresolved sideband regime. It offers a favorable platform to amplify a weak signal field and simultaneously generate a conjugate field with tunable gain and bandwidth. We achieve an ultranarrow bandwidth (9 Hz) and ultrahigh gain (40 dB) optical field amplification at a weak pump power (10 µW). Due to the unresolved sideband regime, the system supports simultaneous multimode amplification. The presented optomechanical FWM mechanism-based amplifier can provide standard quantum-noise-limit amplification, in principle, and is applicable to a broad range of the electromagnetic spectrum ranging from the microwave band to the ultraviolet band.

Spin and valley are two coupled degrees of freedom relevant for low-energy electrons in heavy group-IV monolayers with strong spin-orbit couplings, such as silicene, germanene, and stanene. Using these materials, we propose that a spin-diode effect driven by longitudinal bias can be realized in a ferromagnetic/antiferromagnetic junction, where an interlayer electric field with strength $E_z$ is applied in the antiferromagnetic region. This effect shows a unique bipolar-unipolar transition strongly depending on $E_z$. When $E_z=0$, only electrons from one valley contribute to electric current, with one spin only moving forward and the opposite spin only moving backward, thus corresponding to a bipolar spin diode. When $E_z$ is increased, the active valley can be switched to the other valley, and only electrons of one spin orientation are involved in transport and only move forward, corresponding to a unipolar spin diode. This transition is attributed to the characteristic band-to-band tunneling mechanism of spin-valley matching. We also find that, by reversing the direction of the interlayer electric field, the bipolar or unipolar regime of the spin diode does not change as a result of the symmetry in transport. By electrically adjusting the longitudinal bias and $E_z$, we further prove that the signals produced by the spin diode are strong enough to be observed even when the magnetic exchange fields are on the order of 1 meV, which is fully available in experimentally reported samples with $\mathrm{Eu}$ ($\mathrm{Gd}$)-intercalated materials or those in proximity to ferromagnetic insulators. Our findings on the valley-mediated controllability and switching between the bipolar and unipolar regimes of the spin-diode operation suggest promising applications of heavy group-IV monolayers in improvement of reprogrammable spin logic and nonvolatile memory.

Current-induced spin-orbit torque (SOT) driven magnetization switching has relied mostly on 4d and 5d heavy metals with strong spin-orbit coupling to generate large spin currents to deliver SOT and a $\mathrm{Mg}\mathrm{O}$ layer to acquire perpendicular magnetic anisotropy (PMA). We demonstrate 3d $\mathrm{Cr}$ metal can generate PMA and deliver SOT switching with more prowess than $\mathrm{Ta}$. In certain $\mathrm{Cr}$-based heterostructures, even field-free SOT switching has been achieved due to the subtle microstructure in the layered structure. The field-free SOT switching device based on 3d $\mathrm{Cr}$ provides significant advantages for next generation high-efficiency and low-cost nonvolatile spintronic devices.

The nondestructive detection and evaluation of crystallographic properties of nanocrystals is of great significance for both fundamental physics research and further development of high-performance functional devices employing nanostructured materials. Synchrotron radiation-based CXD using a nanoscale x-ray beam is shown to be a powerful tool to explore the crystallographic properties of $\mathrm{In}\mathrm{P}$ nanocrystals (NCs) selectively grown on $\mathrm{Si}$ nanotip wafers. CXD characterization clearly clarifies, with atomic sensitivity and without complex sample preparation, the crystallographic properties of the selected $\mathrm{In}\mathrm{P}$ NC such as the structure of the facets, the strain, the existence of defects (stacking faults and microtwins), and the size of the defected crystallites. Several selected $\mathrm{In}\mathrm{P}$ NCs explored by CXD reveal homogeneous structures. The CXD results are in good agreement with electron microscopy. These results not only confirm that nanoheteroepitaxy is a promising approach to monolithically integrate high-quality III-V compounds on silicon wafers, but also opens a pathway to nondestructively explore the crystallinity of materials on the nanometer scale, particularly in nano-electronic and nano-optoelectronic devices.

Two-dimensional (2D) semiconductors, graphene, ${\mathrm{Mo}\mathrm{S}}_2$, and black phosphorus, have attracted tremendous interest for their unique mechanical, electronic, optical, and magnetic properties. However, few of the previously studied systems are perfect for practical electronic and optoelectronic applications due to limitations such as zero band gap, low carrier mobility, severe instability, or low absorption coefficient. Here, we predict an alternative ultrathick 2D semiconducting ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ with high mobility ($2.782\times {10}^4{\mathrm{cm}}^2\cdot {\mathrm{V}}^{-1}\cdot {\mathrm{s}}^{-1}$) and large absorption coefficient ($1.29\times {10}^6$). Intriguingly, the electronic band gap of 2D ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ exhibits an indirect-direct transition with increasing thickness, which is opposite to that observed in many 2D transition metal disulfides. Importantly, compared with the monolayer, bilayer ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ possesses stronger light harvesting, higher mobility, and larger on-state current by about one order of magnitude due to electronic states’ rehybridization with charge accumulation in the interlayer region. All these features suggest 2D ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ as a promising competitor for future high-performance infrared electronic and optoelectronic applications.

Universal unitary photonic devices can apply arbitrary unitary transformations to a vector of input modes and provide a promising hardware platform for fast and energy-efficient machine learning using light. We simulate the gradient-based optimization of random unitary matrices on universal photonic devices composed of imperfect tunable interferometers. If device components are initialized uniform randomly, the locally interacting nature of the mesh components biases the optimization search space toward banded unitary matrices, limiting convergence to random unitary matrices. We detail a procedure for initializing the device by sampling from the distribution of random unitary matrices and show that this greatly improves convergence speed. We also explore mesh architecture improvements such as adding extra tunable beam splitters or permuting waveguide layers to further improve the training speed and scalability of these devices.

There has been recent interest in neuromorphic photonics, a field with the promise to access pivotal and unexplored regimes of machine intelligence. Progress has been made on isolated neurons and analog interconnects; nevertheless, this renewal of interest has yet to produce a demonstration of a silicon photonic neuron capable of interacting with other like neurons. We report a modulator-class photonic neuron fabricated in a conventional silicon photonic process line. We demonstrate the behaviors of transfer-function configurability, fan-in, inhibition, time-resolved pulse processing, and, crucially, autaptic cascadability—a sufficient set of behaviors for a device to act as a neuron participating in a network of like neurons. The silicon photonic modulator neuron constitutes the final piece needed to make photonic neural networks fully integrated on currently available silicon photonic platforms.

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.