We explore and systematically compare the morphological, structural, and optical properties of $\mathrm{Ga}\mathrm{N}$/($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PA MBE) on freestanding $\mathrm{Ga}\mathrm{N}$$(0001)$ and $\mathrm{Ga}\mathrm{N}$$(000\overline{1})$ substrates. Samples of different polarity are found to be comparable in terms of their morphological and structural perfection and exhibit essentially identical quantum well widths and $\mathrm{Al}$ content. Regardless of the crystal orientation, the exciton decay in the MQWs at $10$ K is dominantly radiative and the photoluminescence (PL) energy follows the quantum confined Stark effect for different quantum well widths. A prominent free-to-bound transition involving interface shallow donors is, however, visible for the N-polar MQWs. At room temperature, in contrast, the exciton decay in all samples is dominated by nonradiative recombination taking place at point defects, presumably $\mathrm{Ca}$ or $V_{\mathrm{N}}$ located at the bottom QW interface. Remarkably, the N-polar MQWs exhibit a higher PL intensity and longer decay times than the $\mathrm{Ga}$-polar MQWs at room temperature. This improved internal quantum efficiency is attributed to the beneficial orientation of the internal electric field that effectively reduces the capture rate of minority carriers by interface trap states.

We present metal-dielectric metasurfaces fabricated from metal periodical nanograting deposited on a dielectric substrate. The metasurface consists of a modulated dielectric, which is covered by a thin silver layer. The metasurface operates as an open plasmon resonator. The theory of plasmons excited in the open resonator formed by a metal nanograting is presented. The large local electromagnetic field is predicted for optical frequencies. The excitation of plasmons is experimentally demonstrated in the metasurface designed on a 4-in. $\mathrm{Si}$ wafer. The enhancement of the local electric field results in surface-enhanced Raman scattering (SERS). To investigate the SERS effect, the metasurface is covered with a thin layer of the 4-mercaptophenylboronic acid that molecules form covalent bonds with the silver nanolayer. The enhancement of the Raman scattering serves as a proof of concept. We obtain a detection limit of 230 nM for molecules of 4-mercaptophenylboronic acid.

Over the years, an enormous effort has been made to establish nitrogen vacancy (N-$V$) centers in diamond as easily accessible and precise magnetic field sensors. However, most of their sensing protocols rely on the application of bias magnetic fields, preventing their usage in zero- or low-field experiments. We overcome this limitation by exploiting the full spin $S=1$ nature of the N-$V$ center, allowing us to detect nuclear spin signals at zero and low field with a linearly polarized microwave field. As conventional dynamical decoupling protocols fail in this regime, we develop robust pulse sequences and optimize pulse pairs, which allow us to sense temperature and weak ac magnetic fields and achieve an efficient decoupling from environmental noise. Our work allows for much broader and simpler applications of N-$V$ centers as magnetic field sensors in the zero- and low-field regime and can be further extended to three-level systems in ions and atoms.

The effective modulation of acoustic fields is the most important property of acoustic metasurfaces. The realization of full-space wavefront control can significantly enhance the functionality of metasurfaces; however, the existing solutions to this problem are limited by the coupled modulations of the transmitted and reflected wavefronts. In this study, we demonstrate the possibility of controlling transmitted and reflected acoustic wavefronts in a decoupled manner with a passive structure. Simulated analyses of the parameter dependences of the transmission and reflection phases reveal that these phases can be combined arbitrarily within a range of structural parameters. Meanwhile, tunable designs increase the flexibility and simplicity of the modulation of acoustic waves. Using such a tunable structure, a transmission-reflection-integrated (TRI) metasurface is designed. By applying a single TRI metasurface, multiple independent functions are simultaneously realized in the transmitted and reflected regions, which is further confirmed by pancratic multifocal focusing (performed both experimentally and theoretically) and holographic imaging simulations. The simulated, calculated, and experimental data obtained demonstrate efficient wavefront control and excellent functional-integration performance of the TRI metasurface. In this paper, we propose a decoupled method for the simultaneous manipulation of reflected and transmitted acoustic waves, which can enhance the spatial utilization and functionality of acoustic devices.

Electrical control of conjugate degrees of freedom in multiferroics provides the advantage of reducing energy consumption to femto- and even attojoules per switch in spintronics and memory devices. This is achieved through the development of technologies that make it possible to fabricate artificial materials with constantly improving properties. Here, we present the design, physics, and characteristics of a composite multiferroic spintronic emitter, which provides electrical control of the emitted terahertz (THz) wave polarization. The effect is due to electrical control of the magnetization in a high-quality magnetostrictive superlattice, ${\mathrm{Tb}\mathrm{Co}}_2/\mathrm{Fe}\text{−}\mathrm{Co},$ deposited on an anisotropic piezoelectric substrate. In our approach, several mechanisms are realized in the system simultaneously: the strain-mediated coupling of the magnetic and piezoelectric subsystems, which operate in the range of the spin-reorientation transition of the magnetic superlattice, and THz-wave generation in the superlattice by an optical femtosecond pulse. This provides flexibility and control of the set of parameters. We determine the magnetoelectric parameter, which is responsible for THz polarization control. Our results offer a significant fundamental insight into the physics of composite multiferroic systems that can be used for applications of multiferroicity, primarily for THz spintronic emitters. We believe that our findings represent a decisive step towards technologies for other types of spintronic and memory devices.

Atomic force microscopy is a tool for characterizing surface acoustic waves, in particular for high frequencies, where the wavelength is too short to be resolved by laser interferometry. A caveat is, that the cantilever deflection is not equal to the amplitude of the surface acoustic wave. We show that the energy transfer from the moving surface to the cantilever instead leads to a deflection exceeding the surface modulation. We present a method for an accurate determination of surface acoustic wave amplitudes based on comparing force-curve measurements with the equation of motion of a driven cantilever. We demonstrate our method for a standing surface acoustic wave on a $\mathrm{Ga}\mathrm{As}$ crystal confined in a focusing cavity with a resonance frequency near 3 GHz.

This work presents a theoretical investigation of an active photonic grating of the parity-time- ($PT$) symmetric architecture. The analytical study of the free-space mode propagation from the grating structure indicates the unique bifurcation property due to the $PT$-symmetry modulation. It is shown that both the gain-loss contrast and the lattice constant parameters are critical factors to modulate the active photonic system in between the $PT$-symmetry to the symmetry-broken phases. Furthermore, numerical simulations via the rigorous coupled-wave analysis (RCWA) method discover the existence of a unique spectral singularity phenomenon in this $PT$ grating structure, which corresponds to a nontrivial single mode and near-zero bandwidth photonic resonant emission. Also, the guiding procedure for fulfilling spectral singularity modes is found to be related to the unique formation of the scattering matrix applied in the $PT$-symmetric photonic gratings. This theoretical work takes a fresh look into the active $PT$-symmetric photonic gratings focusing on the discovery of nontrivial free-space emission modes, which could contribute to the development of high-performance surface-emitting semiconductor devices.

We study the negative couplings amid local resonances of photonic metasurfaces and their radiation polarizations. In our analysis, we discover fully circularly polarized, wave-vector variant, radiational eigenstates that are attributed to interorbit coupling-induced negative couplings. Our theoretical model is exemplified via a guided resonance dielectric metasurface that possesses type-II non-Hermitian Dirac points, from where the circular polarization lines emanate. The designed metasurface carries circularly polarized radiation eigenstates in both the zeroth and the first diffraction orders, allowing spin-selective light deflections. The high quality factor and field enhancement of the designed metasurface could lead to applications for spin-selective sensing, beam control, and nonlinear optics. Our findings open a gateway for the design of near-field coupling-assisted metasurfaces.

Modern optoelectronic devices rely on cavity electrodynamics concepts for improved performances, embedding the active medium in an optical cavity to enhance the light-matter coupling. This coupling rate is usually small compared to the electronic and photon energies. Despite several demonstrations, devices operating with much larger light-matter coupling strength, in the so-called strong light-matter coupling regime, are yet to be demonstrated as viable, practical candidates. One of the main technological obstacles that hamper their dissemination is the comprehension of the carrier current extraction and injection from and into strongly coupled light-matter states. Here, we study this fundamental phenomenon in midinfrared quantum cascade detectors (QCDs) operating in the moderate to strong light-matter coupling regime. They operate around $\lambda =10\mu \mathrm{m}$ with a minimum Rabi splitting of 9.3 meV. A simple model based on the usual description of transport in QCDs does not reproduce the polaritonic features in the photocurrent spectra. On the contrary, a more refined approach, based on the semiclassical coupled modes theory, is capable of reproducing both optical and electrical spectra with excellent agreement. By correlating absorption and photoresponse with the simulations, we demonstrate that—in this system—resonant tunneling from the polaritonic states appears to be the predominant extraction mechanism. The dark intersubband states do not have a significant role in the process, contrary to what happens in electrically injected polaritonic emitters.

We realize and model an internal-state Rydberg atom interferometer for measurement of phase and intensity of radio-frequency (rf) electromagnetic waves. A phase reference is supplied to the atoms via a modulated laser beam, enabling atomic measurement of the rf wave’s phase without a rf reference wave. The rf and optical fields give rise to closed interferometric loops within the atoms’ internal Hilbert space. In our experiment, we construct interferometric loops in the state space $\{6P_{3/2},90S_{1/2},91S_{1/2},90P_{3/2}\}$ of cesium and employ them to measure phase and intensity of a 5-GHz rf wave in a room-temperature vapor cell. Electromagnetically induced transparency on the $6S_{1/2}$ to $6P_{3/2}$ transition serves as an all-optical interferometer probe. The rf phase is measured over a range of $\pi$, and a sensitivity of 2 mrad is achieved. Measurements of rf phase and amplitude at submillimeter optical spatial resolution are demonstrated.

Silicon nitride ($\mathrm{Si}\mathrm{N}$) drumhead resonators offer a promising platform for thermal sensing owing to their high mechanical quality factor and the high temperature sensitivity of their resonance frequency. As such, gaining an understanding of heat transport in $\mathrm{Si}\mathrm{N}$ resonators as well as their noise limitations are of interest, both of which are goals of the present work. We first present measurements of radiative heat transport in $\mathrm{Si}\mathrm{N}$ membranes, which we use for benchmarking two recently proposed theoretical models. We measure the characteristic thermal response time of square $\mathrm{Si}\mathrm{N}$ membranes with a thickness of 90 ± 1.7 nm and side lengths from 1.5 to 12 mm. A clear transition between radiation- and conduction-dominated heat transport is measured, in close correspondence with theory. In the second portion of this work, we use our experimentally validated heat transport model to provide a closed-form expression for thermal fluctuation-induced frequency noise in $\mathrm{Si}\mathrm{N}$ membrane resonators. We find that, for large-area $\mathrm{Si}\mathrm{N}$ membranes, thermal fluctuations can be greater than thermomechanical contributions to frequency noise. For the specific case of thermal radiation sensing applications, we also derive the noise-equivalent power resulting from thermal fluctuation-induced frequency noise, and we show in which conditions it reduces to the classical detectivity limit of thermal radiation sensors. Our work therefore provides a path towards achieving thermal radiation sensors operating at the unattained fundamental detectivity limit of bolometric sensing. We also identify questions that remain when attempting to push the limits of radiation sensing; in particular, the effect of thermal fluctuation noise in closed-loop frequency tracking schemes remains to be clarified.

Vacuum-packed particle (VPP) systems have been recently used in the development of smart structures due to their ability to actively change material stiffness through the mechanism of granular jamming. By altering the strength of the vacuum pressure applied to the particles, the material can be proportionally and reversibly transitioned from a liquidlike low-stiffness state to a stiff state. The ability to control material stiffness in this way opens up different possibilities for the design of morphing and smart structures by allowing them to soften during deformation to reduce actuation energy requirements and to then stiffen once the desired shape has been achieved to provide a zero-energy holding mechanism. The following research describes two useful models that predict the mechanical response of VPP beams under flexural loading. Firstly, four-point bending tests with digital image correlation strain mapping are performed in order to measure the axial and transverse strains in a bending beam made from vacuum-packed particles. The test results show a nonlinear mechanical response, including a change in beam thickness with deformation, that motivated an analytical model of the structure incorporating a nonlinear material stress model based on the Mohr-Coulomb failure envelope. In addition, finite-element simulations are implemented using a Johnson-Cook model extension to predict the response under loading of three-dimensional beam elements at different vacuum pressure levels. Lastly, the models are compared to the experimental results, indicating good agreement. Both methods are shown to be useful for predicting the variation of stiffness of vacuum-packed particle beams.

The anisotropic nature of the electrical conductance in low-symmetry two-dimensional materials is of great interest for emerging devices, as the existence of anisotropy potentially offers an additional degree of freedom. The comprehensive modulation of the conductance anisotropy in electronic devices is highly desired. However, so far, only tuning of the magnitude has been reported. Here, we report the comprehensive electrical modulation of conductance anisotropy in low-symmetry ${\mathrm{Re}\mathrm{S}}_2$ transistors. Device design ensures two comparable processes, i.e., thermionic emission and carrier drift, involved in device transport, which have opposite in-plane anisotropic distributions of conductance. Application of a gate voltage could switch over the dominance between these two processes, therefore, reversing the polarization directions of the maximum conductance. Systematic investigations also show the strong geometrical and temperature dependences of gate modulation. Conductance anisotropy can be tuned from a fourfold difference, with the maximum polarization in the direction of the light effective mass, to a sevenfold difference polarized in the perpendicular direction, i.e., the direction of the heavy effective mass. The demonstrated modulation facilitates the conductance anisotropy as an additional degree of freedom for exploring functionalities for future electronics in low-symmetry semiconductors.

We study the behavior of hydrated graphite oxide (GO) at high temperatures using thermally-accelerated molecular dynamics simulations based on ab initio density-functional theory. Our results suggest that GO, a viable candidate for water treatment and desalination membranes, is more heat resilient than currently used organic materials. The system we consider to represent important aspects of thermal processes in highly disordered GO is a hydrated GO bilayer in a vacuum. Our study provides microscopic insight into reactions involving water and functional epoxy-$\mathrm{O}$ and $\mathrm{OH}$ groups bonded to graphene layers, and also describes the swelling of the structure by water vapor pressure at elevated temperatures. We find the system withstands simulation temperatures up to approximately $2500\mathrm{K}$ before the graphitic layers start decomposing, implying the possibility of cleaning biofouling residue from a GO-based membrane by heating it in an inert-gas atmosphere.

Unlike their electromagnetic and acoustic counterparts, elastic waves involve different wave modes. The interplay and the coupling among them increase the complexity of the problem while also offering a larger space for wave manipulation. Elastic bulk wave conversion in an elastic metamaterial has recently shown great promise in medical ultrasound and nondestructive testing. Unlike the transmission-type conversion, however, reflective elastic mode conversion has been explored less in terms of analysis and design, despite the enormous possibilities that it might offer for energy trapping and dissipation. In this work, we develop a theoretical framework for constructing elastic anisotropic metasurfaces that can enable reflective longitudinal-to-transverse (L-to-T) and transverse-to-longitudinal (T-to-L) wave conversions. We capitalize on the mechanism of multiple reflective mode conversion to achieve broadband, subwavelength, and near perfect sound absorption in the underwater environment. The reflective scattering properties of the metasurfaces are systematically exploited for incident longitudinal or transverse waves. The conversion mechanism is rooted in reflective Fabry-Perot (FP) resonance, whose occurrence conditions and features are predicted for prescribed effective parameters of the metasurface. We then establish an inverse-design framework for conceiving an underwater coating system formed by a viscoelastic rubber layer and the metasurface. A series of metasurfaces allowing for customized mode conversions are realized for delivering broadband low-frequency and high-efficiency underwater sound absorption. Specifically, an ultrathin rubber-metasurface layer in which the metasurface with a thickness of approximately λ/70 can lead to nearly 100% sound absorption. Furthermore, we demonstrate that a persistently high absorption (over 80%) can be obtained in a rather robust manner within a wide range of wave incidence angle from −60° to 60°. More importantly, high-efficiency acoustic absorption exceeding 75% can be readily achieved through multiple mode conversions within the ultrabroadband range featuring a relative bandwidth of 119%. We reveal the combined FP resonance mechanism of underwater sound absorption, i.e., the FP resonance of the metaconverter, which determines the L-to-T and T-to-L conversion ratio, and the FP resonance of the rubber-metasurface layers, which enhances the wave attenuation inside the rubber. The proposed reflective multiple mode-conversion mechanism and metasurface design methodology open a route towards a class of elastic-wave-based devices with promising potential for underwater applications.

The demands on adaptive optics are increasing to achieve high performance and meet the requirements for accuracy and reliability with these systems. At present, common technologies for actuator arrays are highly developed and designed in complex arrangements leading to bulky devices. In this work, we propose an approach for designing modular actuation arrays by means of kirigami metasheets realigned in parallel and leveled in plane. The study focuses on the numerical analysis of different cut patterns to couple lift-off motions within a thin, scalable sheet. A design of a modular actuation array is explained and illustrated in the context of a deformable mirror. The optimal lift-off positions within the array are calculated for this example by a proposed positioning algorithm, which makes use of common methods in adaptive optics such as Zernike polynomial fitting. The algorithm is validated by several simulations calculating the membrane deflection, and the overall concept is discussed.