Nonreciprocal signal operation is highly desired for various acoustic applications, where protection from unwanted backscattering can be realized, so that transmitting and receiving signals are processed in full-duplex mode. Here, we present the realization of a class of nonreciprocal circulators based on simply structured acoustic metagratings, which consist only of a few solid cylinders and a steady fluid flow with low velocity. These nonreciprocal metagratings are intelligently designed via a diffraction analysis of the linearized potential flow equation and a genetic-algorithm-based optimization process. Unitary reflection efficiency between desired ports of the circulators is demonstrated through full-wave numerical simulations, confirming nonreciprocal and robust circulation of the acoustic signal over a broad range of flow velocity magnitudes and profiles. Our design provides a feasible degree of tunability, including switching from reciprocal to nonreciprocal operation and reversing the handedness of the circulator, presenting a convenient and efficient approach for the realization of nonreciprocal acoustic devices from wavelength-thick metagratings. It may find applications in various scenarios, including underwater communication, energy harvesting, and acoustic sensing.

Electron paramagnetic resonance (EPR) is a versatile tool to probe spin physics in organic semiconductor materials. A common method used to detect the spin-½ paramagnetic resonance in organic light-emitting diodes (OLEDs) is to measure the device resistance under EPR conditions, i.e., to record electrically detected magnetic resonance (EDMR). Here, we present ultralow-frequency EDMR experiments on OLEDs that exhibit a qualitatively new line shape because of a quasistatic magnetic field effect: the modulation of the static ultrasmall field-effect magnetoresistance arising from the magnetic field amplitude $B_1$ of the radio frequency (rf) radiation. The disappearance of spin-½ Zeeman resonances of individual charge carriers in the OLED, i.e., the resonances at magnetic fields where the Zeeman splitting matches the photon energy of the incident rf radiation, coincides with the emergence of the quasistatic effect. We discuss the origin of this quasistatic magnetic field effect, its characteristic line shape in terms of the magnetic field dependence, the influence of experimental parameters, and the application potential with regards to EDMR experiments. The EDMR line shape can be inferred numerically from the magnetoresistance measurements. This approach enables a unique means of determining the drive-field strength $B_1$ in EDMR under driving conditions where alternative methods employing an analysis of the Zeeman resonance—such as power broadening and Rabi flopping—are not applicable.

This is a comment on the assumptions, model, and calculations in the paper by Farhat et al. [Phys. Rev. Appl. 11, 044089 (2019)] on cloaking of thermal waves in solids. The differences between the two thermal flux laws considered in the latter paper are also critically discussed, specifically showing that the chosen model does not correspond to the Maxwell-Cattaneo hyperbolic (wave) theory of heat transfer.

The surface phonon-polariton heat capacity of polar nanofilms is analytically determined and analyzed as a function of their thickness and temperature. It is shown that the polariton heat capacity increases with the square of the nanofilm thickness, such that for thin enough nanofilms at sufficient low temperature $T$, its value becomes independent of the material properties and is given by $C=1.15{\epsilon }_0{k}_B{(k_{B}T/\hslash c)}^2$, where ${\epsilon }_0$ and $c$ are the respective relative permittivity of the surrounding medium and light speed in vaccum, while $k_B$ and $2\pi \hslash$ are the Boltzmann and Planck constants, respectively. The photonlike nature of surface phonon polaritons is found to be responsible for their main contribution to the material heat capacity. As a result of the polariton speed close to $c$ and hence much higher than that of phonons, the polariton heat capacity of ${\mathrm{Si}\mathrm{O}}_2$, SiC, and $\mathrm{SiN}$ is found to be several orders of magnitude smaller than its corresponding phonon counterpart. Surface phonon polaritons are thus not expected to increase the expansion coefficient of polar nanofilms, which favors their utilization as effective heat dissipators.

We report on the photodetection properties of a graphene-oxide-semiconductor (GOS) diode by measuring its current-voltage characteristics under illumination with light-emitting diodes (LEDs). We demonstrate that a GOS structure, with graphene used as a transparent gate electrode to form an inversion layer at the oxide-semiconductor interface, can function as a GOS field-effect transistor operable at low temperatures down to 1.5 K. By investigating the gate tunneling current in a GOS diode with a transistor structure, we find that the dark current is below approximately 0.1 nA at $T=150$ K, which is almost two orders of magnitude lower than that in a graphene-semiconductor (GS) Schottky-diode photodetector. Notably, the GOS diode shows a bilateral photoresponse in both forward- and reverse-bias regimes under LED illumination. The photocurrent responsivity $R$ reaches approximately 100 mA/W at $T=150$ K with a low bias voltage of approximately $-0.6$ V, which is one order of magnitude lower than that applied in a GS Schottky-diode photodetector. We propose that a GOS heterostructure can be made to behave as a $p$-$i$-$n$ or an $n$-$i$-$p$ diode by manipulating the polarity of the bias voltage applied to the graphene gate. In addition, we quantitatively simulate the key features of the dark current by taking into account the associated bipolar current in graphene along with band-to-trap tunneling and trap-assisted tunneling processes. Our theoretical model sheds light on the mechanism of the bilateral photoresponse of the GOS photodetector. Our work paves the way to engineering hybrid devices made of two-dimensional materials and conventional semiconductors with CMOS integrability.

We consider a single electron confined within a quantum wire in a system of two electrostatically induced quantum dots defined by nearby gates. A time-varying electric field, of single gigahertz frequency, perpendicular to the quantum wire, is used to induce Rashba coupling and enable spin-dependent resonant tunneling of the electron between two adjacent potential wells with fidelity over 99.5%. This effect can be used for high-fidelity all-electric electron-spin initialization or readout in a spin-based quantum computer. In contrast to other spin-initialization methods, our technique can be performed adiabatically without an increase in the energy of the electron. Our simulations are supported by realistic self-consistent time-dependent Poisson-Schrödinger calculations.

Ultrafast magnetization switching induced by a single femtosecond laser pulse, under no applied magnetic field has attracted a lot of attention in the last 10 years because of its high potential for low-energy and ultrafast memory applications. Single-pulse helicity-independent switching has mostly been demonstrated for $\mathrm{Gd}$-based materials. It is now necessary to optimize the pulse duration and the energy needed to switch a $\mathrm{Gd}$-$\mathrm{Fe}$-$\mathrm{Co}$ magnet depending on the alloy thickness and composition. Here we experimentally report state diagrams showing the magnetic state obtained after one single pulse depending on the laser pulse duration and fluence for various $\mathrm{Gd}$-$\mathrm{Fe}$-$\mathrm{Co}$ thin films with different compositions and thicknesses. We demonstrate that these state diagrams share similar characteristics: the fluence window for switching narrows for longer pulse duration and for the considered pulse-duration range the critical fluence for single-pulse switching increases linearly as a function of the pulse duration while the critical fluence required for creating a multidomain state remains almost constant. Calculations based on the atomistic spin model qualitatively reproduce the experimental state diagrams and their evolution. By studying the effect of the composition and the thickness on the state diagram, we demonstrate that the best energy efficiency and the longest pulse duration for switching are obtained for composition around the magnetic compensation.

Using magneto-optical Faraday and Kerr measurements, we investigate the magnetic and magneto-optical properties of a thick $\mathrm{Bi}$-substituted gadolinium iron garnet film over a broad range of wavelengths (250–850 nm) and temperatures (150–300 K), including the magnetization compensation point, $T_M$. We observe an exchange-bias-like effect in the vicinity of $T_M$. By slightly changing the sample temperature, we can precisely tune the bias field, which reaches a magnitude 6 times higher than the coercive field. We explain this phenomenon by considering the short-range superexchange interaction and a change in the magnetic behavior when moving from the surface to the bulk of the film. This finding may lead to the development of single-film magneto-optical devices based on the exchange-bias effect.

Electrically small antennas are characterized by large quality factors, which yield limited gain-bandwidth products, as a result of the Bode-Fano limit. This bound implies a trade-off between the antenna’s footprint and radiation features, hindering wireless applications that require compact, broadband, and efficient antennas. Here, building on a previous theoretical analysis of parametric matching networks [H. Li, A. Mekawy, and A. Alù, Phys. Rev. Lett. 123, 164102 (2019)], we demonstrate how parametric phenomena can overcome this trade-off, offering a pathway to realize stable non-Foster wideband antennas that go beyond the restrictions of passive systems. We demonstrate our approach in a planar small loop antenna loaded by a time-varying capacitor that oscillates around twice the radiation frequency, showing that it can result in wideband radiation enhancement, exceeding the limitations of passive scenarios.

Atoms in magnetically sensitive sublevels constitute a natural sensor for absolute magnetic field measurements, which are explored here to form an optical Ramsey interferometer for determining the magnetic field distribution. An optical Ramsey atom interferometer provides a convenient way to map a magnetic field in vacuum. In comparison with the often used Raman spectroscopy method, a Ramsey interferometer can not only improve the sensitivity, but also dramatically suppress the contribution of the ac Stark effect. Furthermore, in this work, the magnetically sensitive atoms are velocity-selected, which improves the spatial resolution for the magnetometer as well as the contrast of the Ramsey fringes. A short-term sensitivity of $1.4{\mathrm{nT/Hz}}^{1/2}$ for magnetic field measurement is finally achieved even with an interrogation time as short as $500\mu \mathrm{s}$. The presented approach for absolute magnetic field measurement is expected to find wide applications in atom-interferometry-based precision measurements.

A splitting of the fundamental optical modes in micro- and nanocavities comprising semiconductor heterostructures is commonly observed. Given that this splitting plays a role in light-matter interaction and hence quantum technology applications, a method for controlling the mode splitting is useful. In this work we use an open microcavity composed of a “bottom” semiconductor distributed Bragg reflector (DBR) incorporating a $n$-$i$-$p$ heterostructure, paired with a “top” curved dielectric DBR. We measure the mode splitting as a function of wavelength across the stopband. We demonstrate a reversible in situ technique to tune the mode splitting by applying uniaxial stress to the semiconductor DBR. The method exploits the photoelastic effect of the semiconductor materials. We achieve a maximum tuning of approximately $11$ GHz. The stress applied to the heterostructure is determined by observing the photoluminescence of quantum dots embedded in the sample, converting a spectral shift to a stress via deformation potentials. A thorough study of the mode splitting and its tuning across the stopband leads to a quantitative understanding of the mechanism behind the results.

We report the fabrication of an array of highly scaled $\mathrm{Ge}$-nanowire-based (radius ∼ 25 nm) vertical metal-oxide-semiconductor devices that can operate as voltage-tunable quantum dots (VTQDs) at room temperature. The electrons in such nanowires experience geometrical confinement in the radial direction, whereas they can be confined axially by tuning the applied bias to manipulate the quantum states. Such three-dimensional confinement of electrons is confirmed from the steplike responses in the room-temperature capacitance-voltage (C-V) characteristics at relatively low frequency (200 kHz). Each step is observed to encompass convolution of the quantized states occupying about six electronic charges. Such ultrasmall capacitance (∼aF) is measured by exploring a technique that utilizes the method of removing frequency dispersion. Details of such carrier confinement are analyzed in the current work by theoretically modeling the device’s transport properties based on the nonequilibrium Green function formalism.

The negatively charged nitrogen vacancy (N-$V^{-}$) center in diamond has shown great success in nanoscale, high-sensitivity magnetometry. Efficient fluorescence detection is crucial for improving the sensitivity. Furthermore, integrated devices enable practicable sensors. Here, we present an integrated architecture which allows us to create N-$V^{-}$ centers a few nanometers below the diamond surface, and at the same time covering the entire mode field of femtosecond-laser-written type-II waveguides. We experimentally verify the coupling efficiency, showcase the detection of magnetic resonance signals through the waveguides and perform proof-of-principle experiments in magnetic field and temperature sensing. The sensing task can be operated via the waveguide without direct light illumination through the sample, which is important for magnetometry in biological systems that are sensitive to light. In the future, our approach will enable the development of two-dimensional sensing arrays facilitating spatially and temporally correlated magnetometry.

The electron- and hole-transport properties in cadmium zinc telluride selenide (CZTS) crystals are studied using a laser-induced transient-current technique with pulsed and dc bias. The internal electric field profile and velocity of surface recombination are determined by Monte Carlo simulations of electron and hole transient currents combined with a numerical solution of the drift-diffusion equation coupled with Poisson’s equation. Electron and hole drift mobilities of ${\mu }_e$ = 830 ${\mathrm{cm}}^2$/Vs and ${\mu }_h$ = 40 ${\mathrm{cm}}^2$/Vs, respectively, are determined. We also develop a simple technique for evaluating surface recombination directly from measured current waveforms without the need for numerical simulation. The good quality of the prepared detector at pulsed bias, with electron- and hole-mobility-lifetime products of $(\mu \tau {)}_e$ = 1.9 × ${10}^{-3}$ ${\mathrm{cm}}^2$/V and $(\mu \tau {)}_h$ = 1.4 × ${10}^{-4}$ ${\mathrm{cm}}^2$/V, respectively, are observed. The formation of a positive space charge, originating from hole injection combined with a recombination level, is found. We observe a significant position dependence of the lifetime of electrons and holes in dc bias due to hole injection. The experiment is successfully fitted by a simple model dominated by a single deep recombination level with an energy of $E_t=E_C-0.73\mathrm{eV}$; concentration of 7.3 × ${10}^{11}$ ${\mathrm{cm}}^{-3}$; and electron- and hole-capture cross sections of 3.5 × ${10}^{-14}{\mathrm{cm}}^2$ and 6.5 × ${10}^{-14}{\mathrm{cm}}^2$, respectively.

Nonlinear electrical properties, such as negative differential resistance (NDR), are essential in numerous electrical circuits, including memristors. Several physical origins have been proposed to lead to the NDR phenomena in semiconductor devices over the last more than half a century. Here, we report NDR behavior formation in randomly oriented graphenelike nanostructures up to 37 K and high on-current density up to ${10}^5\mathrm{A}/{\mathrm{cm}}^2$. Our modeling of the current-voltage characteristics, including the self-heating effects, suggests that strong temperature dependence of the low-bias resistance is responsible for the nonlinear electrical behavior. Our findings open opportunities for the practical realization of the on-demand NDR behavior in nanostructures of two- and three-dimensional material-based devices via heat management in the conducting films and the underlying substrates.

We use square and rectangular phononic crystals to create experimental realizations of complex topological phononic circuits. The exotic topological transport observed is wholly reliant upon the underlying structure that must belong to either a square or rectangular lattice system and not to any hexagonal-based structure. The phononic system we use consists of a periodic array of square steel bars that partitions acoustic waves in water over a broadband range of frequencies (about $0.5$ MHz). An ultrasonic transducer launches an acoustic pulse that propagates along a domain wall, before encountering a nodal point, from which the acoustic signal partitions towards three exit ports. Numerical simulations are performed to clearly illustrate the highly resolved edge states as well as corroborate our experimental findings. To achieve complete control over the flow of energy, we need to create power division and redirection devices. The tunability afforded by our designs, in conjunction with the topological robustness of the modes, will lead to incorporation into acoustical devices.

Photonic measurement-based quantum computation (MBQC) is a promising route towards fault-tolerant universal quantum computing. A central challenge in this effort is the huge overhead in the resources required for the construction of large photonic clusters using probabilistic linear-optics gates. Although strong single-photon nonlinearity ideally enables deterministic construction of such clusters, it is challenging to realise in a scalable way. Here we explore the prospects of using moderate nonlinearity (with conditional phase shifts smaller than $\pi$) to boost photonic quantum computing and significantly reduce its resources’ overhead. The key element in our scheme is a nonlinear router that preferentially directs photonic wavepackets to different output ports depending on their intensity. As a relevant example, we analyze the nonlinearity provided by Rydberg blockade in atomic ensembles, in which the trade-off between the nonlinearity and the accompanying loss is well understood. We present protocols for efficient Bell measurement and GHZ-state preparation—both key elements in the construction of cluster states, as well as for the cnot gate and quantum factorization. Given the large number of entangling operations involved in fault-tolerant MBQC, the increase in success probability provided by our protocols already at moderate nonlinearities can result in a significant reduction in the required resources.

When a dynamic medium in which the laser propagates changes its refractive index in time, the laser changes its frequency while keeping its wavevector unchanged to fullfil the dispersion relation. This is usually applied to upshift the laser frequency with ionizing plasma. We propose an alternative technique to modify light frequency. A transient plasma grating can be generated by two identical counterpropagating laser pulses via strongly coupled stimulated Brillouin scattering (SC SBS). The rapid evolution of the plasma grating affects the wave-dispersion relation and a band gap develops around the laser frequency, dependent on the grating amplitude. As a result, the lasers convert their frequency downward to the low edge of the band gap, while a free-traveling laser converts its frequency to both the upper and lower edge of the band gap. Depending on the considered setup, practical applications of this technique include either laser-frequency downshift or spectral splitting can be exploited. The former can be used for Raman amplification in plasma and the latter for dual-color x-ray generation by Thomson and/or Compton scattering.