Elastic metagratings that can efficiently manipulate the propagation of bulk longitudinal and transverse waves are highly desired in various applications, such as geological exploration and nondestructive detection. Here, a class of elastic metagratings consisting of a one-dimensional array of elliptical hollow cylinders carved in a steel background is proposed and studied. With the help of elastic-wave-diffraction analysis and a genetic-algorithm-based optimization approach, these intelligently designed metagratings can reflect longitudinal and transverse waves in a highly efficient and spatially separated way. Various wave-front-manipulation functionalities, such as specular reflection for longitudinal waves and anomalous reflection for transverse waves, can be simultaneously achieved with an almost perfect efficiency. Interestingly, different functionalities over both longitudinal and transverse waves can be realized by using the same cylinders with different rotation angles. Due to the nonresonant characteristics, the exhibited functionality shows robustness over both working frequencies and unit-cell configurations, which are beneficial for practical applications.

Prototypes for hot-carrier solar cells based on type-II $\mathrm{InAs}/{\mathrm{AlAs}}_{0.16}{\mathrm{Sb}}_{0.84}$ multiple quantum wells are examined with ac photoconductivity as a function of lattice temperature and photoexcitation energy to determine the photoexcited charge-carrier transport. These samples previously exhibited an excitation energy onset of a metastable regime in their short-time charge-carrier dynamics that potentially improves their applicability for hot-carrier photovoltaic cells. The transport results illustrate that the ac photoconductivity is larger in the regime corresponding to the metastability as a result of higher excitation photocarrier densities. In this excitation regime, the ac photoconductivity is accompanied by slightly lower carrier mobility, arising from the plasmalike nature of carriers scattered by Auger recombination. Outside of this regime, higher mobility is observed as a result of a lower excitation density that is more readily achievable by solar concentration. Additionally, at ambient temperatures, more scattering events are accompanied by slightly lower mobility, but the excitation dependence indicates that this is accompanied by an ambipolar diffusion length that is greater than half a micron. These transport properties are consistent with good quality in-organic elemental and III-V semiconductor solar cells and far exceed those of organic and perovskite materials. The transport results complement the dynamics observed in type-II $\mathrm{InAs}/{\mathrm{AlAs}}_{0.16}{\mathrm{Sb}}_{0.84}$ and can guide the design of hot-carrier solar cells based on these and related materials.

In this work, we propose the design of a flexoelectric sensor, which can measure sound-pressure gradient directly. It is found that the sensitivity of sound-pressure gradient of the flexoelectric sensor has a wide band for low working frequencies. Our theoretical and experimental results indicate that the position of sound sources can be predicted based on the measured sound-pressure gradients of three flexoelectric sensors, which are perpendicular to each other. Moreover, different from the previous design for a piezoelectric sensor array, only three flexoelectric sensors are needed to determine the orientation of sound sources. The flexoelectric sensor proposed here may provide an alternative way for the pressure-gradient measurement.

The equivalence between the natural minimization of energy in a dynamic system and the minimization of an objective function characterizing a combinatorial optimization problem offers a promising approach to design dynamic-system-inspired computational models and solvers for such problems. For instance, the ground-state energy of coupled electronic oscillators, under second-harmonic injection, can be directly mapped to the optimal solution of the maximum-cut problem. However, prior work has focused on a limited set of such problems. Therefore, in this work, we formulate computing models based on synchronized oscillator dynamics for a broad spectrum of combinatorial optimization problems ranging from the Max-K-Cut (the general version of the maximum-cut problem) to the traveling-salesman problem. We show that synchronized oscillator dynamics can be engineered to solve these different combinatorial optimization problems by appropriately designing the coupling function and external injection to the oscillators. Our work marks a step forward towards expanding the functionalities of oscillator-based analog accelerators and furthers the scope of dynamic system solvers for combinatorial optimization problems.

A single Bessel beam (BB) enables the scalable optical writing of waveguide devices with multiple inputs and outputs in a nonlinear medium. For this purpose, we experimentally propagate a diffracting zero-order BB in a strontium barium niobate photorefractive crystal. Such a configuration can be used for the self-writing of complex waveguiding structures. By tailoring different key parameters such as the diffraction phenomenon through the BB size, the nonlinearity strength through the applied electric field, and the photoinduced refractive-index modulation depth through input beam intensity, our optical platform enables not only the generation of optical splitters but also an all-optical control of the output intensity levels. We also analyze the stability in space and time of the different photoinduced channels. We show that adding a background illumination onto the medium permits a steady-state photoinduced refractive-index modulation with up to nine output channels. These results enrich the research on scalable optical writing techniques for the realization of complex interconnects and enlarge further possibilities for all-optical switching.

Graphene and transition-metal-dichalcogenide- (TMD) based van der Waals heterostructures in field-effect-transistor (FET) architecture exhibits extremely high sensitivity to optical radiation due to transit and physical separation of the photogenerated carriers across the heterointerface. Both the sensitivity and speed of these detectors depend on the kinetics of charge transfer, but their interdependency at room temperature (T), where these detectors would be most useful, remains largely unexplored. Here we systematically measure the T dependence of the magnitude (gain) and timescale (bandwidth) of photoresponse in graphene-TMD heterostructures well up to the room T. The gain-bandwidth product is found to be strongly dependent on the power of optical illumination and increases with decreasing power (P), becoming as large as $1\mathrm{M}\mathrm{Hz}$ in the low-P limit. We find that thermally activated back transfer of charge from graphene to the TMD determines the response time of the detector at higher temperatures under continuous illumination. Our experiment reveals the impact of charge-transfer pathways on the performance in a broad class of graphene-TMD detectors.

Acoustic manipulations of microparticles have significant implications in physics, chemistry, biology, and biomedicine. For large Mie particles, both the scattering force and the gradient force affect the trapping and manipulation behaviors, and thus, a passive acoustic tweezer (AT) for three-dimensionally manipulating a Mie particle remains challenging. Here, a passive AT based on a hybrid acoustic artificial plate is proposed to generate a hybrid acoustic focused petal beam (HAFPB) for capturing and manipulating a Mie microparticle in three dimensions. In the HAFPB, a lateral acoustic focused petal beam and an axial bifocal acoustic beam (BAB) combine to create a central zero-intensity zone for particle trapping, which can be adjusted by modulating the topological charge of the HAFPB. The acoustic radiation forces (ARFs) acting on large Mie particles with different diameters are investigated. The ARFs on the polystyrene sphere are greatly influenced by the particle size. If the particle is larger than the central zero-intensity zone, an AT with a larger topological charge and a stronger BAB is required for trapping and manipulating this larger particle. Finally, the experiments demonstrate that the AT can stably trap and manipulate a large Mie particle in three dimensions in water.

In a moving acoustic medium, sound waves travel differently with and against the fluid flow. This well-established acoustic effect is backed by the intuition that the fluid velocity bias imparts momentum on the propagating acoustic waves, thus violating reciprocity. Based on this conception, fluid flow that is transverse to the wave direction of propagation will not break reciprocity. In this paper we contrast this common wisdom and theoretically show that the interplay between transverse mean flow and transverse structural gliding asymmetry can yield strong nonreciprocity and even, surprisingly, one-way acoustic waveguiding. To demonstrate that, we analyze a waveguide that comprises a few adjacent acoustic subdiffraction chains, i.e., linear chains of acoustic scatterers with monopolar or dipolar response. The structure is embedded within a medium with mean flow velocity that is transverse to the waveguide axis. We find the symmetry breaking conditions under which nonreciprocity is obtained, and we show how, under transverse mean flow, with Mach numbers as low as 0.02, one-way propagation of the acoustic wave is obtained on a sub-wavelength-thick acoustic waveguide. To demonstrate the phenomenon, we introduce two models. One in which the scatterers are assumed to be pointlike and the flow is homogeneous, and a second, in which we accommodate the issue of possible wakes created by the presence of scatterers inside the flow. The latter setup exhibits no direct interaction of the scatterers with the flow, by placing the waveguide in a quiescent medium adjacent to a flowing medium. Our results push forward the understanding of acoustic nonreciprocity, and may open another venue for the design of nonreciprocal acoustic wave devices for various applications.

We report, the use of laser-plasma-driven x rays to reveal the three-dimensional (3D) structure of a highly atomizing water spray. Soft x rays approximately 5 keV are generated by means of a laser-plasma accelerator. Transmission radiography measurements are performed at different angles, by rotating a multihole injector. Using computer tomography, the local liquid volume distribution and its spatial variation are retrieved in 3D, showing up to 55% liquid fraction at the nozzle outlet, which decreases to below 7% within only 1 mm. The resolution of the liquid volume fraction is 0.5% while the spatial resolution of the radiographic images is $11.5\mu \mathrm{m}$. The x-ray source used here provides successful measurements of liquid mass distribution over a relatively large volume and is very promising for the analysis of a variety of challenging transient spray systems, e.g., the injection of liquid synthetic and biofuels used for future clean-combustion applications.

Plasmons in two-dimensional electron systems (2DESs) feature ultrastrong confinement and are expected to efficiently mediate the interactions between light and charge carriers. Despite these expectations, the electromagnetic detectors exploiting 2D plasmon resonance have been so far inferior to their nonresonant counterparts. Here, we theoretically analyze the origin of these failures, and suggest a proper niche for 2D plasmonics in electromagnetic wave detection. We find that a confined 2DES supporting plasmon resonance has an upper limit of absorption cross section, which is identical to that of simple metallic dipole antenna. Small size of plasmonic resonators implies their weak dipole moments and impeded coupling to free-space radiation. Achieving the “dipole limit” of the absorption cross section in isolated 2DES is possible either at unrealistically long carrier momentum relaxation times, or at resonant frequencies below units of terahertz. We further show that amendment of even small metal contacts to 2DES promotes the coupling and reduces the fundamental mode frequency. The contacted resonators can still have deep-subwavelength size. They can be merged into compact arrays of detectors responding resonantly to multiple frequencies. Such arrays may find applications in multichannel wireless communications, hyper-spectral imaging, and energy harvesting.

This study examines Bragg’s band gap and its mechanical tuning in a stretch-buckled kirigami sheet with “zig-zag” distributed parallel cuts. When stretched beyond a critical threshold, the kirigami buckles out of plane and generates a three-dimensional periodic architecture. Our theoretical calculation, numerical simulation, and experiments confirm the transverse elastic wave-propagation band gaps and their correlation to stretching. This result opens an avenue of using kirigami as a simple and effective approach for creating and adapting periodicity for wave-propagation control.

A band-to-band tunneling FET (TFET) with an atomical two-dimensional (2D) channel is a potential candidate for the next-generation electronic device in view of its steep subthreshold swing and low power consumption. However, how to establish a precise physical model between the band property of 2D channel materials and the device performance of 2D TFETs is the key to accelerate their practical applications. Herein, through high-throughput first-principles calculations, we study the tunneling transport properties of 44 representative 2D materials with four kinds of crystal system. Particularly, we propose a well-defined and striking exponential scaling law $(\mathrm{SS}=A{\mathrm{e}}^{B{m}_r}+C)$ between the subthreshold swings (SSs) and reduced effective masses $(m_r)$ of 10-nm TFETs. According to the exponential model, 2D orthorhombic and trigonal crystal TFETs with the steep SS hold the reduced effective masses of more than 0.1 and $0.2m_0$, which could inhibit tunneling leakage current. These insights provide guidance for material screening in the construction of post-Moore 2D low-power transistors.

Unidirectional magnetoresistance (UMR) in a ferromagnetic bilayer due to the spin Hall effect (SHE) provides a facile means of probing in-plane magnetization to avoid complex magnetic tunnel junctions. However, the UMR signal is very weak and usually requires a lock-in amplifier for detection, even in a bilayer involving $\mathrm{Ta}$ or $\mathrm{Pt}$ with a large spin Hall angle (SHA). Here, we report a type of UMR, termed the anomalous UMR (AUMR), in a single $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ layer without any adjacent SHE layers, where the UMR signal is about 10 times larger than that in $\mathrm{Ta}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ structures and can be detected by using conventional dc multimeters in the absence of lock-in amplifiers. We further demonstrate that the extracted AUMR, by excluding thermal contributions, shows reversal signs for the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ and $\mathrm{Ni}\text{−}\mathrm{Fe}$ single layers with opposite SHAs, indicating that the AUMR may originate from the self-generated spin accumulation interacting with magnetization through the giant-magnetoresistance-like mechanism. These results suggest that the AUMR contributes UMR signals larger than the interfacial spin Hall UMR in the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$-based systems, providing a convenient and reliable approach to detect in-plane magnetization for two-terminal spintronic devices.

The ability to measure the passage of electrical current with high spatial and temporal resolution is vital for applications ranging from inspection of microscopic electronic circuits to biosensing. The ability to image such signals passively and remotely is of great importance, in order to measure without invasive disruption of the system under study or the signal itself. A recent approach to achieving this utilizes point defects in solid-state materials; in particular, nitrogen-vacancy centers in diamond. Acting as a high-density array of independent sensors, addressable opto-electronically and highly sensitive to factors including temperature and magnetic field, these are ideally suited to microscopic wide-field imaging. In this work, we demonstrate simultaneous spatially and temporally resolved recovery signals from a microscopic lithographically patterned circuit. Through application of a lock-in amplifier camera, we demonstrate micrometer-scale imaging resolution with a millimeter-scale field of view with simultaneous spatially resolved submillisecond (up to 3500 frames ${\mathrm{s}}^{-1}$) recovery of dc to kilohertz alternating and broadband pulsed-current electrical signals, without aliasing or undersampling. We demonstrate as examples of our method the recovery of synthetic signals replicating digital pulses in integrated circuits and signals that would be observed in a biological neuronal network in the brain.

We propose a scheme for the generation of optimal squeezed states for Ramsey interferometry. The scheme consists of an alternating series of one-axis twisting pulses and rotations, both of which are straightforward to implement experimentally. The resulting states show a metrological gain proportional to the Heisenberg limit. We demonstrate that the Heisenberg scaling is maintained even when placing constraints on the amplitude of the pulses implementing the one-axis twisting and when taking into account realistic losses due to photon scattering.

We show the physical realization and experimental demonstration of an exceptional point of sixth-order degeneracy in a triple-ladder (or three-way) microwave waveguide realized using three coupled microstrips on a grounded dielectric substrate. This three-way waveguide supports six Bloch eigenmodes, and all coalesce into a degenerate single eigenmode at a given frequency. The three-way waveguide is gainless, and this exceptional point is associated with a vanishing-group velocity and its multiple derivatives. Indeed, the $\omega -k$ dispersion diagram that we call the sixth-order degenerate band edge (6DBE) has six coalescing branches. We provide experimental verification of a sixth-order exceptional point by evaluating the degenerate wave number–frequency-dispersion diagram from the measurement of scattering parameters of a six-port unit cell. We also show the resonant behavior of a cavity made of the three-way waveguide with finite length. The unique properties of 6DBE can be exploited to design innovative high-Q resonators, oscillators, filters, and pulse-shaping devices.