In this work, we propose the use of contact resonances, controlled via an external magnetic field, as a tunable platform to manipulate the dispersion of surface acoustic waves (SAWs). We exploit the analogy between surface acoustic waves in a semi-infinite medium and edge waves in a plate, to realize a compact experimental setup and to demonstrate our tuning strategy. The setup consists of a set of ferromagnetic bead resonators in contact with thin, permanent magnets and positioned at the free edge of an elastic plate. An additional set of magnets, placed at a controlled and variable distance from the beads, is used to alter the contact stiffness and natural frequencies of the bead resonators. We exploit resonances to open large-frequency band gaps via edge-wave hybridization and implement our tuning strategy to shift their frequency ranges. We predict the tuned dispersive properties of hybridized edge waves via numerical models and experimentally reconstruct them via laser vibrometry, finding excellent agreement. The use of magnetic interaction and contact mechanics as a tuning strategy for SAW systems could pave the way toward programmable devices for sensing and electronic components.

The drastic difference between complex transmission coefficients for ${\mathrm{TE}}_{11}$ and ${\mathrm{TH}}_{11}$ GHz modes propagating through subwavelength gradient barriers in a metallic waveguide is examined. Amplitude-phase structures of the transmitted ${\mathrm{TE}}_{11}$ and ${\mathrm{TH}}_{11}$ modes are described in the framework of an exactly solvable model of these barriers. A sharp contrast in transmission is found between degenerate ${\mathrm{TE}}_{11}$ and ${\mathrm{TH}}_{11}$ modes distinguished by their polarization structures due to polarization-dependent tunneling and nonlocal energy transport through thin gradient dielectric barriers.

In recent years different setups based on external cavities have been developed, in order to enhance the performance of pulsed semiconductor mode-locked lasers, namely the peak power and width of the emitted pulses. Depending on the operating conditions and the cavity configuration, pulse cluster solutions emerge with a nonidentical temporal interpulse spacing, which limit the performance of such devices. In this work we present a system of multidelay differential equations to describe the dynamics of a passively mode-locked vertical-external-cavity surface-emitting laser with V-shaped cavity geometry, that allows for an effective modeling and detailed studies of parameter dependencies. We apply numeric integration as well as path-continuation methods to understand the underlying bifurcation scenarios and hence the parameter regions of stable operation. Our investigations indicate that pulse cluster solutions emerge along the fundamental periodic solution branch with a critical influence of the cavity round-trip time on the number of pulses in the cluster. We find regions of multistability of higher-order pulse clusters and predict how different types of dynamics can be favored by tuning the cavity geometry.

Recently, metamaterials-inspired diffraction gratings (or metagratings) have demonstrated unprecedented efficiency in wavefront manipulation by means of relatively simple structures. Conventional one-dimensional (1D) gratings have a profile modulation in one direction and a translation symmetry in the other. In 1D metagratings, the translation invariant direction is engineered at a subwavelength scale, which allows one to accurately control polarization line currents and, consequently, the scattering pattern. In bright contrast to metasurfaces, metagratings cannot be described by means of surface impedances (or local reflection and transmission coefficients). In this paper, we present a simulation-based design approach to construct metagratings in the “unit cell by unit cell” manner. It represents an analog of the local periodic approximation that has been used to design space-modulated metasurfaces and allows one to overcome the limitations of straightforward numerical optimization and semianalytical procedures that have been used to date to design metagratings. Electric and magnetic metagrating structures responding to, respectively, transverse electric and transverse magnetic incident plane waves are presented to validate the proposed design approach.

In this work, our first-principles calculations reveal that direct wave-vector excitation (i.e., interband transitions between different wavenumbers without phonon assistance) can occur in the indirect-band-gap semiconductor silicon. The wave-vector excitation is successfully induced by irradiation of a silicon thin film with an optical near-field (ONF). As a result, the absorption-band-edge energy $E_{\mathrm{edge}}$ shifts to a lower photon energy of 1.6 eV for ONF excitation from $E_{\mathrm{edge}}$ of 2.1 eV for the conventional excitation by propagating far-field light. The direct wave-vector excitation is caused by the sufficiently large components of wave vectors inherent in the ONF, and thus does not require phonon assistance. For a realistic silicon system, it is clarified that the wave-vector excitations are determined by the energy difference between the valence and conduction bands and occur irrespective of the initial and final wave vectors.

Bismuth vanadate–graphitic carbon nitride (${\mathrm{Bi}\mathrm{VO}}_4/\mathrm{g}$-${\mathrm{C}}_3{\mathrm{N}}_4$) heterostructures combining the advantages of bismuth vanadate and graphitic carbon nitride have recently emerged as a promising material system for photocatalytic applications. Further performance improvement will require in-depth understanding and fine tuning the near-edge electronic properties of these heterostructures. We investigate the electronic properties, in particular, the band-edge states, that control the optical and transport properties of ${\mathrm{Bi}\mathrm{VO}}_4/\mathrm{g}$-${\mathrm{C}}_3{\mathrm{N}}_4$ nanoheterostructures using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional approach. Our results demonstrate that ${\mathrm{Bi}\mathrm{VO}}_4/\mathrm{g}$-${\mathrm{C}}_3{\mathrm{N}}_4$ heterostructures have a desired type-II band alignment, which may greatly facilitate rapid separation of photogenerated carriers. We find that the ${\mathrm{Bi}\mathrm{VO}}_4$ subsystem is responsible for the robust and strong optical absorption observed experimentally. Our calculated absorption edge of about 2.5 eV agrees remarkably well with the experimental value of 2.45 eV. We also investigate the effect of an external electric field on the band-edge states, band alignment, and the optical and fundamental band gaps of the ${\mathrm{Bi}\mathrm{VO}}_4/\mathrm{g}$-${\mathrm{C}}_3{\mathrm{N}}_4$ heterostructures. The band-edge states of the ${\mathrm{Bi}\mathrm{VO}}_4$ and $\mathrm{g}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ subsystems respond very differently to the applied electric field, resulting in electric field-tunable band offsets and band gap of the ${\mathrm{Bi}\mathrm{VO}}_4/\mathrm{g}$-${\mathrm{C}}_3{\mathrm{N}}_4$ heterostructures. The contrasting response of the fundamental and optical gaps to an electric field suggests an alternative avenue for future experimental optimization of the optical absorption and carrier separation dynamics of these heterostructures for photoelectrochemical catalysis applications.

We experimentally demonstrate a random distributed feedback fiber laser generating cylindrical vector beams via a mode-selective coupler and a fiber ring resonator with 50-m-long single-mode fiber. Because of random distributed feedback provided by the single-mode fiber, multimode interactions are suppressed in the cavity, leading to modeless output. Azimuthally and radially polarized beams can both be obtained by adjustment of the polarization controllers. Speckle contrast measurements are made to confirm the effectiveness of the designed laser in reducing laser speckle. Apart from the low cost and simple technology of such a laser, the cylindrical vector beams with modeless behavior may have applications in areas such as laser radars, free-space communication, remote sensing, and biomedical imaging.

We present an in situ microscopic technique to calibrate phase-only liquid-crystal-based spatial light modulators (LC SLM). The technique relies on the spatial structure of focused fields that are commonly encountered in optical microscopy. To retrieve the phase response curve of the LC SLM, we modulate the phase of one half of the incident beam, record the resulting variations of the focused fields at the focal plane of the microscope objective, and perform a correlation of those variations in the corresponding experimental and theoretical intensity distributions of the focused fields. We establish the validity of the technique by comparing its performance to the well-known two-beam interference technique for calibrating a phase-only LC SLM. Our technique is general, robust, and directly applicable to any microscopy set up that utilizes a LC SLM in the excitation path.

We discuss a noninvasive technique to detect glucose changes with enhanced sensitivity based on parity-time (PT) symmetry. We detect glucose level changes within the skin by measuring the frequency shift in the electromagnetic resonance induced within a PT-symmetric system that sandwiches the tissue sample under analysis. Even though the sample itself is lossy, and therefore, resonances would be damped, the introduction of balanced gain and loss enables an efficient sensing mechanism that bypasses the conventional limitations of passive sensing schemes. Our results indicate that the resonance shift can be made fairly linear with respect to the glucose concentration variations, and the expected accuracy is large. We also investigate a realistic system to implement the noninvasive PT-symmetric glucose sensor using loop antennas and negative impedance converters, exploring its sensitivity with respect to design errors and disorder.

We present a design for a superconducting, on-chip circulator composed of dynamically modulated transfer switches and delays. Design goals are set for the multiplexed readout of superconducting qubits. Simulations of the device show that it allows for low-loss circulation (insertion loss $<0.35$ dB and isolation $>20$ dB) over an instantaneous bandwidth of 2.3 GHz. This design improves on the bandwidth of previous superconducting circulators by more than an order of magnitude, making it ideal for integration with broadband quantum-limited amplifiers.

By combining first-principles simulations including an on-site Coulomb-repulsion term and Boltzmann theory, we demonstrate how the interplay of quantum confinement and epitaxial strain allows one to selectively design $n$- and $p$-type thermoelectric response in $({\mathrm{La}\mathrm{Ni}\mathrm{O}}_3{)}_3/({\mathrm{La}\mathrm{Al}\mathrm{O}}_3{)}_1(001)$ superlattices. In particular, varying strain from $-4.9\mathrm{\%}$ to $2.9\mathrm{\%}$ tunes the $\mathrm{Ni}$ orbital polarization at the interfaces from $-6\mathrm{\%}$ to $3\mathrm{\%}$. This is caused by an electron redistribution among $\mathrm{Ni}$ $3d_{x^2-y^2}$- and $3d_{z^2}$-derived quantum-well states, which respond differently to strain. Owing to this charge transfer, the position of emerging cross-plane-transport resonances can be tuned relative to the Fermi energy. For moderate compressive strains of $-1.5\mathrm{\%}$ and $-2.8\mathrm{\%}$, the cross-plane Seebeck coefficient reaches approximately $-60$ and $100\mu \mathrm{V}/$K at around room temperature, respectively. This provides a compelling mechanism to tailor thermoelectric materials. Finally, we demonstrate the robustness of the proposed concept with respect to oxygen-vacancy formation.

We report the current-driven dynamics of frustrated skyrmions in an antiferromagnetically exchange-coupled bilayer system, where the bilayer skyrmion consists of two monolayer skyrmions with opposite skyrmion numbers $Q$. We show that the in-plane current-driven bilayer skyrmion moves in a straight path, while the out-of-plane current-driven bilayer skyrmion moves in a circular path. It is found that the in-plane current-driven mobility of a bilayer skyrmion is much better than the monolayer one at a large ratio of $\beta /\alpha$, where $\alpha$ and $\beta$ denote the damping parameter and nonadiabatic spin-transfer torque strength, respectively. Also, the out-of-plane current-driven mobility of a bilayer skyrmion is much better than the monolayer one when $\alpha$ is small. We also reveal that one bilayer skyrmion (consisting of monolayer skyrmions with $Q=\pm 2$) can be separated into two bilayer skyrmions (consisting of monolayer skyrmions with $Q=\pm 1$) driven by an out-of-plane current. Our results may be useful for designing skyrmionic devices based on frustrated multilayer magnets.

High-electron-mobility transistors (HEMTs) based on ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{Ga}\mathrm{N}$ heterostructures have great potential for applications in power electronics and radio frequency applications. Operating under large bias and electric fields, hot electrons are present in the channel where they can activate preexisting benign defects that cause scattering or carrier trapping, resulting in device degradation, such as threshold-voltage shifts and transconductance degradation. We report a comprehensive investigation of the electron transport properties of wurtzite-phase $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Al}\mathrm{N}$ and of an ${\mathrm{Al}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{N}/\mathrm{Ga}\mathrm{N}$ HEMT by solving the Boltzmann transport equation with a synchronous ensemble Monte Carlo technique and employing first-principles electronic properties, including the full energy bands, phonon dispersions, and electron-phonon scattering rates. We find that, for electron collisions with the highest-energy optical phonon mode, nonpolar scattering by optical phonons contributes comparable to polar scattering. Polar scattering with acoustic phonons, that is, piezoelectric scattering, is found to be as important as the polar scattering with optical phonons for low-energy electrons at room temperature. We compare the calculated high-field transport characteristics of bulk $\mathrm{Ga}\mathrm{N}$ with previously reported results. We also calculate the nonpolar acoustic deformation potential, nonpolar optical deformation potential, and high-field transport characteristics of bulk $\mathrm{Al}\mathrm{N}$. We find that inclusion of piezoelectric scattering results in a low-field electron mobility of approximately $450{\mathrm{cm}}^2$/(V s), which is very close to the experimental value. Simulation results are presented for an ${\mathrm{Al}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{N}/\mathrm{Ga}\mathrm{N}$ HEMT, including electric field, average carrier kinetic energy, and drift velocity in the channel. Finally, we present the carrier energy distribution function in the conducting channel, which is key to accurately determine hot-carrier-caused device degradation and identify possible routes to an improved device design.

Whispering-gallery-(WG) mode resonators are machined from a boule of single-crystal isotopically pure silicon-28. Before machining, the as-grown rod is measured in a cavity, with the best Bragg confined modes exhibiting microwave $Q$ factors on the order of a million for frequencies between 10 and 15 GHz. After machining the rod into smaller cylindrical WG-mode resonators, the frequencies of the fundamental mode families are used to determine the relative permittivity of the material to be $11.488\pm 0.024$ near 4 K, with the precision limited only by the dimensional accuracy of the resonator. However, the machining degraded the $Q$ factors to below $4\times {10}^4$. Raman spectroscopy is used to optimize postmachining surface treatments to restore high-$Q$-factors. This is an enabling step for the use of such resonators for hybrid quantum systems and frequency-conversion applications, as silicon-28 also has very low phonon losses, can host very narrow linewidth spin ensembles, and is a material commonly used in optical applications.

Controlling spin-to-orbital angular momentum (OAM) conversion plays an important role in many optical and wireless communication applications. While independent control of different circular-polarized (CP) wavefronts could offer an ultimate degree of freedom in designing advanced multifunctional spin devices, most metasurfaces only provide dependent wavefront control. We propose a reflective dual-helicity decoupled coding metasurface to completely realize independent control of OAM vortices for two orthogonal helicities. The element combines both the propagation phase and geometric phase, thus overcoming the inherent limitation encountered by conventional geometric phase elements whose phase responses are constrained to be opposite values for different CP wavefronts. Several design examples for independently generating OAM vortices in the microwave region are presented where a free combination of different OAM modes, helicity, as well as complex spatial beam editing can be fully achieved. Experiments show good agreements with the full-wave simulations, successfully verifying the design theory. The proposed method could offer an alternative platform for designing high-performance devices with independent CP wavefront manipulations, which may enhance the information capacity of metasurfaces and trigger versatile electromagnetic (em) wave function integrations for advanced compact systems.

Metamaterials have generated considerable research interest in manipulating light polarization states, but many proposals suffer from the issue of narrow bandwidth and/or low transmission efficiency. We show that a classical multiple-beam interference mechanism can be employed to modulate the phase dispersion of transmitted waves in metamaterials, which motivates us to establish a general strategy to achieve a constant cross-polarization phase difference by independently controlling the phase dispersion for the two orthogonal polarizations. We derive a criterion under which the cross-polarization phase difference across the metamaterial devices can be maintained at a nearly constant value over a wide frequency band. The codesigned metamaterial quarter-wave plate presents an excellent polarization conversion performance over a broad frequency band. We also employ full-wave simulations to demonstrate a broadband quarter-wave plate at telecom and terahertz frequencies based on the proposed design strategy. Our findings can stimulate making high-performance broadband optical devices based on various metamaterials in different frequency domains, and impact numerous photonic applications.

Combining solid-state single-photon emitters (SPEs) with nanophotonic platforms is a key goal in integrated quantum photonics. In order to realize functionality in potentially scalable elements, suitable SPEs have to be bright, stable, and widely tunable at room temperature. In this work, we show that selected SPEs embedded in a few-layer hexagonal boron nitride ($h$-BN) meet these demands. In order to show the wide tunability of these SPEs we employ an atomic force microscope (AFM) with a conductive tip to apply an electrostatic field to individual $h$-BN emitters sandwiched between the tip and an indium-tin-oxide-coated glass slide. A very large and reversible Stark shift of $(5.5\pm 0.3)\mathrm{nm}$ at a zero-field wavelength of 670 nm is induced by applying just 20 V, which exceeds the typical resonance linewidths of nanodielectric and even nanoplasmonic resonators. Our results help to further understand the physical origin of SPEs in $h$-BN as well as for practical quantum photonic applications where wide spectral tuning and on/off resonance switching are required.

Under conditions where the angular momentum of a ferromagnetic particle is dominated by intrinsic spin, applied torque is predicted to cause gyroscopic precession of the particle. If the particle is sufficiently isolated from the environment, a measurement of spin precession can potentially yield sensitivity to torque beyond the standard quantum limit. Levitation of a micron-scale ferromagnetic particle above a superconductor is a possible method for near-frictionless suspension enabling observation of ferromagnetic particle precession and ultrasensitive torque measurements. We experimentally investigate multiple instances of a micron-scale ferromagnetic particle levitated above a superconducting niobium surface. We find that the levitating particle is trapped in a potential minimum associated with residual magnetic flux pinned by the superconductor and, using an optical technique, characterize the dynamics of the particle in such a trap.

Numerous studies have reported performance enhancement of thermophotovoltaic (TPV) systems when an emitter is separated by nanoscale gaps from a TPV cell. Although $p$-$n$-junction-based TPV cells have been widely used for near-field TPV systems, Schottky-junction-based near-field TPV systems have drawn attention recently with the advantage of easy fabrication. However, existing studies mostly focused on the generated photocurrent only in the metal side due to the fact that required energy for the metal-side photocurrent (i.e., Schottky barrier height) is smaller than the band-gap energy. Here, we suggest a precise performance analysis model for Schottky-junction-based near-field TPV systems, including photocurrent generation on the semiconductor side by considering the transport of minority carriers within the semiconductor. It is found that most of the total photocurrent in Schottky-junction-based near-field TPV systems is generated in the semiconductor side. We also demonstrate that further enhancement in photocurrent generation can be achieved by reabsorbing the usable photon energy in the metal with the help of a backside reflector. The present work provides a design guideline for Schottky-junction-based near-field TPV systems, taking into account three types of photocurrents.

Many Heusler compounds are predicted to be ferromagnetic half metals in the bulk, which makes them promising compounds for spintronics. However, for devices the transport spin polarization at specific interfaces requires optimization. We show that investigations of the unidirectional magnetoresistance provide an alternative approach to access this quantity. Based on a Wheatstone-bridge design we probe the unidirectional magnetoresistance of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$/($\mathrm{Ag}$, $\mathrm{Cu}$, or $\mathrm{Cr}$)($0.5$ nm)/$\mathrm{Pt}$ (or $\mathrm{Ta}$) multilayers and separate the spin-dependent unidirectional spin Hall magnetoresistance from other contributions. We demonstrate that by the insertion of a thin epitaxial $\mathrm{Ag}$ layer the spin-dependent contribution is doubled corresponding to a significant increase of the transport spin polarization, which is discussed in the framework of highly spin-polarized interface states.