This work presents an investigation of carrier transport in $\mathrm{Ga}\mathrm{N}$-based light-emitting diodes in the subthreshold forward-bias regime where tunneling processes are relevant. A quantum kinetic theory of trap-assisted tunneling is developed within the framework of the nonequilibrium Green’s function formalism. Based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure, the model provides access to spectral quantities, such as the local density of states and the current density, which are essential to understand the nature of the tunneling process. The quantum nonradiative recombination rates can be reproduced by the conventional Shockley-Read-Hall theory, provided that the classical charge is replaced with the correct quantum charge, which means that trap-assisted tunneling can be described with drift-diffusion solvers complemented with appropriate quantum corrections for the calculation of the local density of states. The subthreshold $I$-$V$ characteristics and ideality factors predicted by the quantum kinetic model are in agreement with measurements.

Two-dimensional (2D) semiconductors have attracted tremendous interest as natural passivation and atomically thin channels could facilitate continued transistor scaling. However, air-stable 2D semiconductors with high performance are quite elusive. Recently, an extremely-air-stable ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ monolayer was successfully fabricated [Hong et al., Science 369, 670 (2020)]. To further reveal its potential application in sub-5-nm metal-oxide-semiconductor field-effect transistors (MOSFETs), there is an urgent need to develop integrated circuits. Here, we report first-principles quantum-transport simulations on the performance limits of n- and p-type sub-5-nm monolayer (ML) ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ MOSFETs. We find that the on-state current in the ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ MOSFETs can be effectively manipulated by the length of gate and underlap, as well as the doping concentration. Very strikingly, we also find that for the n-type devices the optimized on-state currents can reach up to 1390 and 1025 µA/µm for high-performance and low-power (LP) applications, respectively, both of which satisfy the International Technology Roadmap for Semiconductors (ITRS) requirements. The optimized on-state current can meet the LP application (348 µA/µm) for p-type devices. Finally, we find that the ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ MOSFETs have an ultralow subthreshold swing and power-delay product, which have the potential to realize high-speed and low-power consumption devices. Our results show that ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ is an ideal 2D channel material for future competitive ultrascaled devices.

Super-resolution acoustic imaging with state-of-the-art spatial resolution (λ/50), with λ being the wavelength, is showcased with a holey-structured metalens. However, the imaging mechanism under unity transmission based on Fabry-Perot resonances means the metalens fundamentally suffers from narrow bandwidth and limited deep-subwavelength contrast, and therefore further advancement of deep-subwavelength imaging has been stalled. Here we break the barriers for deep-subwavelength acoustic imaging comprehensively in spatial resolution, resolving contrast, and working bandwidth, by exploiting field enhancement inside the metalens. A microscopic model is established to theoretically reveal the underlying physics for escalated deep-subwavelength acoustic imaging. For a proof-of-concept, the imaging performance of the proposed method is numerically proven and experimentally demonstrated. Specifically, a breakthrough resolution below λ/100 is achieved while resolving contrast is improved by at least 6.5 times and working bandwidth is broadened to approximately 25% of the operating frequency. Furthermore, pulsed acoustic imaging on the deep-subwavelength scale is showcased, which is an important step towards the practical application of the ultrahigh-resolution acoustic imaging technique. We believe the work presented here may greatly benefit a variety of fields in acoustics, such as visualizing subcutaneous structures in medical diagnosis and characterizing subsurface flaws in industrial nondestructive evaluation.

The Fano resonance is a widespread wave-scattering phenomenon associated with an ultrasharp line shape, which just serves a narrow working frequency range around the interference frequency, rendering the realization of Fano-based applications extremely challenging. Here, we present and experimentally verify a mechanism of acoustic consecutive Fano resonances (ACFRs) with a symmetric profile for broadband sound attenuation, and extend to a practical implementation of a tunable low-frequency double-helix metasilencer. Based on the ACFRs’ dependence on material parameters in the bilayer metamaterial model, we employ an inverse design using Bayesian machine learning to search the optimal broadband insulating performance with a rapid convergence speed (15 iterations). For practical requirement, we extend the ACFRs’ prototype to a continuously tunable double-helix metastructure for broadband low-frequency sound attenuation. This broadband effect can be interpreted by the dual-band single-negativity property. A good agreement between numerical simulation and experiment evidences the effectiveness of the proposed metasilencer with tunable sound attenuation (>90%) in 425–865 Hz and high ventilation (>80%) at various double-helix combinations. Our proposed ACFRs’ mechanism and its associated metastructure would open routes to promising acoustic metamaterial-based applications, such as filtering, switching, and sensing, and beyond.

Stimulated parametric down-conversion is a nonlinear optical process in which the orbital angular momentum of light is conserved under most practical conditions. However, there are instances in which the topological charge is not compatible with the remaining modal structure of the stimulated idler beam. As a result, the stimulated down-converted light diffracts and gives rise to Laguerre-Gaussian-like beams with nonzero radial indices. We present a theoretical and experimental analysis demonstrating this fundamental aspect of nonlinear processes with light beams possessing orbital angular momentum.

We present an electro-optic modulation technique that enables a single laser diode to realize a cold-atom source and a quantum inertial sensor based on matter-wave interferometry. Using carrier-suppressed dual-single-sideband modulation, an in-phase and quadrature modulator generates two optical sidebands from separate radio-frequency (rf) signals. These sidebands are controlled independently in frequency, phase, and power using standard rf components. Our laser source exhibits improved rejection of parasitic sidebands compared to those based on phase modulators, which generate large systematic shifts in atom interferometers. We measure the influence of residual laser lines on an atom-interferometric gravimeter and show agreement with a theoretical model. We estimate a reduction of the systematic shift by 2 orders of magnitude compared to previous architectures and reach a long-term sensitivity of $15\mathrm{n}g$ on the gravitational acceleration with an interrogation time of only $T=20\mathrm{ms}$. Finally, we characterize the performance of our integrated laser system and show that it is suitable for mobile sensing applications, including gravity surveys and inertial navigation.

The kinetic inductance (KI) of superconducting devices can be exploited for reducing the footprint of linear elements as well as for introducing nonlinearity to the circuit. We characterize the linear and nonlinear properties of a multimode resonator fabricated from amorphous tungsten silicide ($\mathrm{WSi}$), with a fundamental frequency of $f_1=172$ MHz. We show how the multimode structure of the device can be used to extract the different quality factors and to aid the nonlinear characterization. In the linear regime, the footprint is reduced by a factor of approximately $2.9$ with standard lateral dimensions, with no significant degradation of the internal quality factor compared to a similar $\mathrm{Al}$ device. In the nonlinear regime, we observe self-positive frequency shifts at low powers, which can be attributed to saturation of tunneling two-level systems. The cross-mode nonlinearities are described well by a Kerr model with a self-Kerr coefficient on the order of $|K_{11}|/2\pi \approx 1.5\times {10}^{-7}$ Hz per photon. These properties, together with a reproducible fabrication process, make $\mathrm{WSi}$ a promising candidate for creating linear and nonlinear circuit quantum electrodynamics elements.

An optical microcavity provides a prominent platform for single-nanoparticle detection with ultrahigh sensitivity. Recently, microcavity sensors working at the exceptional point have caught great attention due to their ability to enhance sensitivity, but suffer from simultaneously amplified noise. Here, we propose employing a nonlinear microcavity to enhance the sensitivity of single-nanoparticle detection based on spontaneous chiral symmetry breaking. It is found that sensors operating at the symmetry-breaking threshold experience a gigantic enhancement in sensitivity, which is caused by the square-root response to perturbation in a lossless microcavity. Through the analysis for a realistic microcavity, a 30-fold sensitivity enhancement is demonstrated by working at the threshold, and the enhancement is also confirmed by numerical simulation. Furthermore, the noise performance is analyzed to be superior in thermorefractive noise and quantum noise performance in comparison to a conventional microcavity-sensing scheme. Merging the spontaneous chiral symmetry-breaking effect with practical sensing applications, the results pave a universal way for high-performance microcavity sensing with enhanced sensitivity and reduced noise.

We present a superconductor-semiconductor materials system that is both scalable and monolithically integrated on a silicon substrate. It uses selective-area growth of $\mathrm{Al}$-$\mathrm{In}\mathrm{As}$ hybrid structures on a planar III-V buffer layer, grown directly on a high-resistivity silicon substrate. We characterize the electrical properties of this materials system at millikelvin temperatures and observe a high average field-effect mobility of $\mu \approx 3200{\mathrm{cm}}^2\mathrm{/Vs}$ for the $\mathrm{In}\mathrm{As}$ channel and a hard induced superconducting gap. Josephson junctions exhibit a high interface transmission, $\mathcal{T}\approx 0.75$, a gate-voltage-tunable switching current with a product of critical current and normal state resistance, $I_C{R}_N\approx 83\mu \mathrm{V}$, and signatures of multiple Andreev reflections. These results pave the way for scalable and high-coherence gate-voltage-tunable transmon devices and other superconductor-semiconductor hybrids fabricated directly on silicon.

In this work, we explore the stability of magnetic skyrmions confined in a disk geometry by analyzing how to switch a skyrmionic state in a circular disk into a uniformly magnetized state when applying an external magnetic field. The technologically highly relevant energy barrier between the skyrmion state and the uniformly magnetized state is a key parameter needed for lifetime calculations. In an infinite sample, this relates to the out-of-plane rupture field against the skyrmion-core direction, while in confined geometries the topological charge can also be changed by interactions with the sample edges. We find that annihilating a skyrmion with an applied field in the direction of the core magnetization—we call this expulsion—the energy barrier to the uniform state is generally around one order of magnitude lower than the annihilation via the rupture of the core in the disk center, which is observed when the applied field is acting in the direction opposite to the core magnetization. For the latter case a Bloch point (BP) needs to be nucleated to change the topological charge to zero. We find that the former case can be realistically calculated using micromagnetic simulations but that the annihilation via rupture, involving a Bloch point, needs to be calculated with the Heisenberg model because the high magnetization gradients present during the annihilation process cannot be accurately described within the micromagnetic framework.

We report a thermal-light “ghost” imaging without conventional intensity correlation calculation. Typically, ghost imaging is achieved with correlation calculation not only for entangled photons but also for thermal-light sources. In this paper, we set only thresholds for the bucket detector and the reference detector, and count the realizations when their signals are above or below the thresholds simultaneously. The distribution of the counts shows the image of our target. With proper threshold restrictions on the two detectors, we demonstrate both positive and negative images. Our work broadens the ghost-imaging reconstruction method, and provides a potential approach to ghost imaging with binary detectors in low-light or photon-counting regimes and a broader spectrum.

We present a very-compact bicomponent-permanent-magnet design capable of generating 1.25 T in a small volume, significantly above the 0.6 T available from a single uniformly magnetized permanent magnet. In addition to the enhanced maximum field, our design drastically limits the stray field present around a standard permanent magnet. These features make it suitable for retrofitting existing experiments with a substantial magnetic field, in particular, scanning probes and optical, Raman, and photoemission spectroscopy, in diverse environments, from ambient to ultrahigh vacuum and over a wide temperature range.

Chiral, topologically protected, photonic surface states can be found at the boundary between gyrotropic photonic crystals where a changing magnetic field induces different topology across the interface. Typically, photonic crystals with either a suitable band structure on both sides of the interface to provide a band gap and evanescent decay of the surface states away from the interface, or an outer layer with engineered material properties is required. In this paper, we show the emergence of topological, unidirectional surface states at the termination of finite gyrotropic photonic crystals with a simple square lattice and ${\mathcal{C}}_4$ rotational symmetry bounded by a vacuum, eliminating the need for an outside layer to enable chiral surface modes. We start from an infinite, time-reversal-symmetry-breaking photonic crystal with a band gap associated with bands with nonzero Chern numbers, different from all-zero Chern numbers in air. We then modify the photonic crystal to move this band gap below the light line, while maintaining the Chern-number discontinuities. Band-structure calculations for a supercell approximating a photonic crystal finite in the direction normal to the surface demonstrate the existence, dispersion, and chirality of the surface mode. Extensive direct scattering calculations for a point source and spatial Fourier analysis further reveal a unidirectional free-space topological surface state, which propagates counterclockwise around the surface of a finite photonic crystal, providing a nearly foolproof way to cross-check the surface-mode band structure unaffected by backscattering from local defects. Additionally, scattering simulations allow an independent characterization of the state dispersion and unveil the robustness of the topological plasmonic mode propagation around the ${90}^{\circ }$ bends of the structure, being due to only radiation leakage. In contrast to buried topological surface states, the observed surface modes at the photonic crystal–air interface have the advantage of being accessible to the outside world, allowing one to take advantage of the defect-tolerant backscattering-free surface modes to engineer emission from photonic crystal surfaces into arbitrary free-space beam shapes and directions.

This paper proposes an alternative technique for the design of miniaturized waveguide filters based on locally resonant metamaterials (LRMs). We implement ultrasmall metamaterial filters (metafilters) by exploiting a subwavelength (sub-λ) guiding mechanism in evanescent hollow waveguides, which are loaded by small resonators. In particular, we use composite pin-pipe waveguides (CPPWs) built from a hollow metallic pipe loaded by a set of resonant pins, which are spaced by deep-subwavelength distances. We demonstrate that, in such structures, multiple resonant scattering nucleates a sub-λ mode with a customizable bandwidth below the induced hybridization band gap (HBG) of the LRM. The sub-λ guided mode and the HBG, respectively, induce pass and rejection bands in a finite-length CPPW, creating a filter, the main properties of which are largely decoupled from the specific arrangement of the resonant inclusions. To guarantee compatibility with existing technologies, we propose a subwavelength method to match the small CPPW filters to standard waveguide interfaces, which we call a metaport. Finally, we build and test a family of low- and high-order ultracompact aluminum CPPW filters in the X and $K_{u}$ bands (10–18 GHz). Our measurements demonstrate the customizability of the bandwidth and the robustness of the passband against geometrical scaling. The three-dimensional printed prototypes, which are 1 order of magnitude smaller and lighter than traditional filters and are also compatible with standard waveguide interfaces, may find applications in future satellite systems and 5G infrastructures.

Our experiments demonstrate that alloying the cubic-phase $\mathrm{Yb}\mathrm{N}$ into the wurtzite-phase $\mathrm{Al}\mathrm{N}$ results in clear mechanical softening and enhanced electromechanical coupling of $\mathrm{Al}\mathrm{N}$. First-principle calculations reproduce experimental results well, and predict a maximum 270% increase in electromechanical coupling coefficient caused by (1) an enhanced piezoelectric response induced by the local strain of $\mathrm{Yb}$ ions and (2) a structural flexibility of the $(\mathrm{Yb},\mathrm{Al})\mathrm{N}$ alloy. Extensive calculations suggest that the substitutional neighbor $\mathrm{Yb}$-$\mathrm{Yb}$ pairs in wurtzite $\mathrm{Al}\mathrm{N}$ are energetically stable along the $c$ axis, and avoid forming on the basal plane of the wurtzite structure due to the repulsion between them, which explains why $(\mathrm{Yb},\mathrm{Al})\mathrm{N}$ films with high $\mathrm{Yb}$ concentrations are difficult to fabricate in our sputtering experiments. Moreover, the neighbor $\mathrm{Yb}$-$\mathrm{Yb}$ pair interactions also promote structural flexibility of $(\mathrm{Yb},\mathrm{Al})\mathrm{N}$, and are considered a cause for mechanical softening of $(\mathrm{Yb},\mathrm{Al})\mathrm{N}$.

Topological materials for classical waves, e.g., electromagnetic and acoustic waves, have attracted growing interest, mainly due to the robustness, low loss, and new artificial degree of freedom conferred by their boundaries. Surface acoustic waves (SAWs), as widely used information carriers of microdevice relevance, are ubiquitous in today’s wireless communication and sensing networks. Herein, we report the implementation of a SAW topological insulator based on a monolithically integrated platform. By using a miniature acoustic resonator array working tens of megahertz on a piezoelectric half-space, we successfully endow electrically pumped Rayleigh-type SAWs with the “spin-momentum locking” feature, enabling solid-state acoustic waves on the “one-dimensional interface of the two-dimensional surface on the three-dimensional volume” detour arbitrarily and pass through defects and intersections with much smaller losses than those incurred with any other solutions. These revolutionary topological SAWs may open an avenue for monolithic electronic-(photonic)-phononic circuits with ultrahigh performance and advanced functionalities in, e.g., future mobile communicating, sensing, and quantum-information processing.

Developing nanomechanical oscillators for ultrasensitive force detection is very useful in exploring science. We report our achievement of ultrasensitive detection of the external force regarding the radiofrequency electric field by a nanosensor made of a single trapped ${}^{40}{\mathrm{Ca}}^{+}$ ion under injection locking, where squeezing is additionally applied to detection of the smallest force in the ion trap. The employed ion is confined stably in a surface electrode trap and works as a phonon laser that is very sensitive to the external disturbance. The injection locking drove the ion’s oscillation with phase synchronization, yielding the force detection with sensitivity of 347 $\pm 50\mathrm{yN}/\sqrt{\mathrm{Hz}}$. Further with 3-dB squeezing applied on the oscillation phase variance, we achieve a successful detection of the smallest force to be $86.5\pm 70.1$ yN.

Topological notions in physics have become a powerful perspective that leads to the discoveries of interface states. In this work, we present a scheme to steer waves by leveraging the properties of valley interface states (VISs) in a valley photonic crystal (VPC). Due to the chiral characteristics, the VISs deterministically possess either positive or negative dispersion relation, which can cause an obliquely incident wave to undergo a lateral shift. By stacking multiple VPC interfaces, the VIS-induced lateral shifts can relay in the transmitted wave towards the far side of incidence. By selectively engaging one type of VISs, the resultant outgoing wave appears to have undergone negative refraction. While this refraction can be flipped to positive by selectively exciting the other type of VISs with positive dispersion. This finding is verified in microwave experiments. Our scheme opens application scenarios for topological systems in wave manipulations.

The key for realizing fault-tolerant quantum computation lies in maintaining the coherence of all qubits so that high-fidelity and robust quantum manipulations on them can be achieved. One of the promising approaches is to use geometric phases in the construction of universal quantum gates, due to their intrinsic robustness against certain types of local noise. However, due to limitations in previous implementations, the noise-resilience feature of nonadiabatic holonomic quantum computation (NHQC) still needs to be improved. Here, in combination with the dynamical-correction technique, we propose a general protocol of universal NHQC with simplified control, which can greatly suppress the effect of the accompanied X errors, retaining the main merit of geometric quantum operations. Numerical simulation shows that the performance of our gate can be much better than that of previous protocols. Remarkably, when incorporating a decoherence-free subspace encoding for the collective dephasing noise, our scheme can also be robust against the involved Z errors. In addition, we also outline the physical implementation of the protocol, which is insensitive to both X and Z errors. Therefore, our protocol provides a promising strategy for scalable fault-tolerant quantum computation.

Quantum computation based on geometric phase is generally believed to be more robust against certain errors or noises than the conventional dynamical strategy. However, the gate error caused by the decoherence effect is inevitable, and thus faster gate operations are highly desired. Here, we propose a nonadiabatic holonomic quantum-computation (NHQC) scheme with detuned interactions on a $\mathrm{\Delta }$-type three-level system, which combines the time-optimal control technique with the time-independent detuning adjustment to further accelerate universal gate operations, so that the gate time can be greatly shortened within the hardware limitation, and thus high-fidelity gates can be obtained. Meanwhile, our numerical simulations show that the gate robustness is also stronger than previous schemes. Finally, we present an implementation of our proposal on superconducting quantum circuits, with a decoherence-free subspace encoding, based on the experimentally demonstrated parametrically tunable coupling technique, which simplifies previous investigations. Therefore, our protocol provides a more promising alternative for future fault-tolerant quantum computation.