We demonstrate the parametric amplification of an acoustic magnon mode induced by an optical magnon mode in synthetic antiferromagnets, which was achieved by using the all-optical pump-probe time-resolved magneto-optical Kerr effect. The acoustic and optical modes with low and high frequencies, respectively, are clearly observed under different field directions and pump-laser powers. For a relatively high laser power, the acoustic mode shows a temporal increase in amplitude in the time domain; this is observed when the acoustic mode frequency is approximately half the frequency of the optical mode. Correspondingly, we also observe a large enhancement in the spectral density of the acoustic mode in the frequency domain. These data are supported by a numerical simulation using a macrospin model; furthermore, the optical mode amplitude threshold for achieving the acoustic mode amplification is also discussed. The parametric effect in synthetic antiferromagnets demonstrated here can be applied to nanoscale parametric amplifiers and oscillators of magnons, which are the building blocks for spintronic and magnonic computing beyond von Neumann architectures.

Study of the physics of quantum electronic transport has not tackled the problems raised by quantum-dot intermediate-band solar cells. Our study shows that this physics imposes design rules for the intersubband transition. We develop an analytical model that correctly treats, from a quantum point of view, the trade-off between the absorption, the recombination, and the electronic transport occurring in this transition. Our results clearly indicate that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor. For that, we propose assuming the dot in a tunnel shell whose main characteristics can be obtained by a simple analytical formula. Moreover, we show that in a realistic case, the energy transition needs to be larger than only 0.27 eV to obtain a quasi-Fermi-level-splitting. This quite small value designates the quantum-dot solar cell as a serious candidate to be an efficient intermediate-band solar cell. This work gives a framework to design efficient intersubband transitions and opens new opportunities for quantum-dot intermediate-band solar cells.

We show that $\mathrm{N}$-polar $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ heterostructures exhibit significant $\mathrm{N}$ deficiency at the bottom $(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ interface, and that these $\mathrm{N}$ vacancies are responsible for the trapping of holes observed in unoptimized $\mathrm{N}$-polar $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ high electron mobility transistors. We arrive at this conclusion by performing positron annihilation experiments on $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ heterostructures of both $\mathrm{N}$ and $\mathrm{Ga}$ polarity, as well as state-of-the-art theoretical calculations of the positron states and positron-electron annihilation signals. We suggest that the occurrence of high interfacial $\mathrm{N}$ vacancy concentrations is a universal property of nitride semiconductor heterostructures at net negative polarization interfaces.

A well-known principle for periodical production of high-power ultrashort optical pulses is related to passive mode locking realized when a saturable absorber is incorporated into the laser resonator. A similar effect can be implemented in two-section electronic oscillators comprising an amplifier and a saturable absorber, which should be applicable in the microwave frequency band and suitable for high-power operation. In this paper, we analyze in detail the possibility of realizing a saturable absorber based on cyclotron resonance interaction in the regular waveguide with initially rectilinear electron beam, when saturation is caused by the relativistic dependence of the gyrofrequency on electron energy. Maximum contrast between the absorption of a high-power signal and of a weak signal is achieved in the group synchronism regime when the particle velocity is close to the wave's group velocity. Within the frame of a time-domain self-consistent model, we demonstrate that when such an absorber is installed in the feedback loop of an electron generator powered by existing Ka-band helical-waveguide gyrotron traveling-wave tube, periodical generation of subnanosecond pulses can be provided with peak power up to several hundred kW and sub-GHz repetition frequency. We discuss the influence of deviation from the group synchronism regime on the parameters of generated pulses. Experimental feasibility of the considered scheme is verified based on three-dimensional particle-in-cell simulations.

It has been shown that the spin Hall effect from heavy transition metals can generate sufficient spin-orbit torque and further produce current-induced magnetization switching in the adjacent ferromagnetic layer. However, if the ferromagnetic layer has in-plane magnetic anisotropy, probing such switching phenomenon typically relies on tunneling magnetoresistance measurement of nano-sized magnetic tunnel junctions, differential planar Hall voltage measurement, or Kerr-imaging approaches. We show that in magnetic heterostructures with spin Hall metals, there exist current-induced in-plane spin Hall effective fields and unidirectional magnetoresistance that will modify their anisotropic magnetoresistance behavior. We also demonstrate that by analyzing the response of anisotropic magnetoresistance under such influences, one can directly and electrically probe magnetization switching driven by the spin-orbit torque, even in micron-sized devices. This pump-probe method allows for efficient and direct determination of key parameters from spin-orbit torque switching events without lengthy device fabrication processes.

Bose-Einstein condensates (BECs) represent one of the very few manifestations of purely quantum effects on a macroscopic level. Recently, a new type of condensate has been observed—the photon BEC, where light in a dye-filled cavity thermalizes with dye molecules under the influence of an external driving laser, condensing to the lowest-energy mode. Here, we consider medium-induced symmetry breaking in a photon BEC and show that it can be used as a quantum sensor. The introduction of polarizable objects such as chiral molecules lifts the degeneracy between cavity modes of different polarizations. Even a tiny imbalance is imprinted on the condensate polarization, in a “winner-takes-it-all” effect. When used as a sensor for enantiomeric excess, the predicted sensitivity exceeds that of contemporary methods based on circular dichroism. Our results introduce a symmetry-breaking mechanism that is independent of the external pump and demonstrate that the photon BEC can be used for practical purposes.

Spin-orbit torque (SOT) has been studied extensively in a heavy-metal (HM)/ferromagnet (FM) bilayer structure, where a HM is an essential ingredient because a spin current is generated via the spin Hall effect within the HM layer and/or the Rashba-Edelstein effect from the HM/FM interface. Here, we report the observation of SOT in a ferrimagnetic $\mathrm{Tb}\text{−}\mathrm{Co}$ single layer with perpendicular magnetic anisotropy without a HM layer. Using harmonic Hall voltage measurements, we investigate the SOT-induced dampinglike effective field $(B_{\mathrm{DL}})$ in a $\mathrm{Tb}\text{−}\mathrm{Co}$ layer; the sign of $B_{\mathrm{DL}}$ is opposite from that of a $\mathrm{Pt}/\mathrm{Tb}\text{−}\mathrm{Co}$ bilayer, and the magnitude of $B_{\mathrm{DL}}$ increases as the $\mathrm{Tb}\text{−}\mathrm{Co}$ composition approaches its magnetization compensation point. Moreover, we analyze the elemental composition of $\mathrm{Tb}\text{−}\mathrm{Co}$ as a function of film thickness using scanning transmission electron microscopy and electron energy-loss spectroscopy, indicating that the sign and magnitude of the SOT are virtually insensitive to the vertical composition gradient within the $\mathrm{Tb}\text{−}\mathrm{Co}$ layer. Our results demonstrate that the bulk spin-orbit interaction within the $\mathrm{Tb}\text{−}\mathrm{Co}$ layer itself plays a major role in generating SOT in a $\mathrm{Tb}\text{−}\mathrm{Co}$ single layer.

Oxidation strongly influences the properties of magnetic layers employed in spintronic devices. We study the effect of oxidation on the structural, magnetic, and electrical properties as well as current-induced spin-orbit torques (SOTs) in $\mathrm{Pt}$/$\mathrm{Co}$/${\mathrm{Al}\mathrm{O}}_x$, $\mathrm{Pt}$/${\mathrm{Co}\mathrm{O}}_x$/$\mathrm{Co}$/${\mathrm{Al}\mathrm{O}}_x$, and ${\mathrm{Pt}\mathrm{O}}_x$/$\mathrm{Co}$/${\mathrm{Al}\mathrm{O}}_x$ layers. We show how the saturation magnetization, perpendicular magnetic anisotropy, anomalous Hall resistance, and SOT are systematically affected by the degree of oxidation of both the $\mathrm{Pt}$/$\mathrm{Co}$ and $\mathrm{Co}$/$\mathrm{Al}$ interfaces. Oxidation of the $\mathrm{Co}$/$\mathrm{Al}$ interface results in a 21% and 42% variation of the dampinglike and fieldlike SOT efficiencies, which peak at 0.14 and 0.07, respectively. The insertion of a paramagnetic ${\mathrm{Co}\mathrm{O}}_x$ layer between $\mathrm{Pt}$ and $\mathrm{Co}$ maintains a very strong perpendicular magnetic anisotropy and improves the dampinglike and fieldlike SOT efficiencies, up to 0.26 and 0.20, respectively. In contrast with recent reports, we do not find that the oxidation of $\mathrm{Pt}$ leads to a significant enhancement of the torques. Rather, we find that oxygen migrates from $\mathrm{Pt}$ to the $\mathrm{Co}$ and $\mathrm{Al}$ layers, leading to a time-dependent oxidation profile and an effective spin Hall conductivity that decreases with increasing oxygen concentration. Finally, we study current-induced switching in $\mathrm{Pt}$/$\mathrm{Co}$/${\mathrm{Al}\mathrm{O}}_x$ with different degrees of oxidation and find a linear relationship between the critical switching current and the effective magnetic anisotropy controlled by the oxidation of $\mathrm{Al}$. These results highlight the importance of interfaces and oxidation effects on the SOT and magnetotransport properties of heavy metal/ferromagnet/oxide trilayers and provide information on how to improve the SOT efficiency and magnetization-switching characteristics of these systems.

Conventional sound barriers impede airflow at the same time. Recent advances in acoustic metasurfaces provide a solution for air-permeable barriers utilizing the Fano-like interference. While the mechanism of Fano-like interference implies that such a realized device serves a narrow working frequency range around every destructive-interference frequency. Considering the fact that noise usually covers a wide frequency range, designing a broadband acoustic barrier is still a challenge. Here, we theoretically design a planar-profile and subwavelength-thickness (approximately $\lambda /8$) acoustic ventilation barrier prohibitive for sound in a broad range. Our design is a metasurface consisting of a central hollow orifice and two surrounding helical pathways with varying pitch. Thanks to the hornlike helical pathways, the response strength from the monopolar and dipolar modes of the system almost keeps balance in the frequency range of interest, leading to an effective blocking of more than $90\mathrm{\%}$ of incident energy in the range of 900–1418 Hz. Experiments are conducted to validate the proposed design, whose results are consistent with the analytical predictions and simulations. The underlying working mechanism ensures the metasurface is capable of handling broadband sound coming from various directions. Our design has potential in air-permeable yet sound-proofing applications, such as simultaneously natural ventilation and noise reduction in green buildings.

We demonstrate a method for simultaneously measuring the spin polarizations of noble-gas and alkali-metal atoms. Under a low magnetic field, the polarized alkali-metal and noble-gas spin ensembles exhibit novel dynamics in the self-compensation and resonance regimes, which can be used to measure the polarizations. We perform this method on a $\mathrm{K}$-$\mathrm{Rb}$-${}^{21}$$\mathrm{Ne}$ comagnetometer with various operation temperatures and pump-light intensities. The polarization of alkali-metal electrons from 0.245 to 0.964 and the polarization of noble-gas nuclear spins from 0.01 to 0.065 are measured with relative uncertainties less than $12.4\mathrm{\%}$. The measured polarizations are consistent with the theoretical model and are also verified by the traditional polarization-measurement method.

We propose an optical Boolean function generator based on an electro-optical nonlinear feedback loop. Theoretically, all possible Boolean functions with optical input and output can be implemented by passing through enough iterations. By changing the control signal, the optical circuit can be configured to any desired Boolean function without adjusting the physical setup. Optical components with high bandwidth can improve the computing speed markedly. The impact of noise on the system is considered in detail. The results show that a substantial number of reliable Boolean functions can be obtained in the presence of noise contamination. Compared with traditional optical digital computation systems, the proposed scheme exhibits a huge advantage in multifunctional computing and flexible configuration.

Optical resonant cavities play an important role in electromagnetic wave control; they can confine electromagnetic waves and improve the interaction between light and matter. However, because of the limitations of the standing-wave formation conditions for Fabry-Perot-type resonance, the miniaturization of optical resonant cavities formed using traditional materials is difficult. Recently, the miniaturization of three-dimensional optical resonant cavities has been demonstrated based on electric hyperbolic metamaterials (HMMs); their isofrequency contour takes the form of an open hyperboloid because the principal components of the permittivity tensor have opposite signs. Here, based on the permeability tensor, we propose theoretically and verify experimentally a planar magnetic hyperbolic cavity with a subwavelength scale, $(\lambda /12)\times (\lambda /17)$, using a circuit-based HMM in the microwave regime. Furthermore, the anomalous scaling laws in the circuit-based magnetic hyperbolic cavities are studied. As the frequency increases, the mode order decreases, which is markedly different from traditional cavities. It is also possible to realize size-independent cavity modes based on the HMMs by design. In addition, by considering a composite structure that contains two hyperbolic cavities, the coupling of two hyperbolic cavity modes in the near-field regime is demonstrated. The circuit-based hyperbolic cavities not only extend previous research work on hyperbolic cavities to magnetic HMMs, but they also have a planar structure that is easier to integrate and has a smaller loss. Finally, the hyperbolic cavities may enable their use in some microwave-related applications, such as in high-sensitivity sensors, resonance imaging, and miniaturized narrowband filters.

Rare-earth-doped crystals have numerous applications ranging from frequency metrology to quantum information processing. To fully benefit from their exceptional coherence properties, the effect of mechanical strain on the energy levels of the dopants—whether it is a resource or perturbation—needs to be considered. We demonstrate that by applying uniaxial stress to a rare-earth-doped crystal containing a spectral hole, we can shift the hole by a controlled amount that is larger than the width of the hole. We deduce the sensitivity of $\mathrm{Eu}$${}^{3+}$ ions in an $\mathrm{Y}$${}_2$$\mathrm{Si}\mathrm{O}$${}_5$ matrix as a function of the crystal site and the crystalline axis along which the stress is applied.

Using a microwave-regime cyclic three-state configuration, enantiomer-selective state transfer (ESST) is carried out through two-path interference between a direct one-photon coupling and an effective two-photon coupling. A phase difference of $\pi$ in the one-photon process between the two enantiomers makes the interference constructive for one enantiomer but destructive for the other. Therefore only one enantiomer is excited into a higher rotational state, while the other remains in the ground state. The scheme allows flexibility in the pulse waveforms and the time order of the two paths. We simulate the scheme for a sample of cyclohexylmethanol (${\mathrm{C}}_7{\mathrm{H}}_{14}\mathrm{O}$) molecules. The simulation results show that robust high-fidelity ESST can be obtained when experimental concerns are considered. Finally, we propose to employ the ESST scheme to implement enantioseparation and determine enantiomeric excesses.

Antiferromagnetic materials (AFMs) have recently been attracting interest in research on spintronics due to their zero stray magnetic field, high anisotropy, and spin-orbit coupling. In this context, the bimetallic AFM ${\mathrm{Mn}}_2\mathrm{Au}$ has drawn attention because it exhibits unique properties and its Néel temperature is very high ($T_N=1500\mathrm{K}$). Here, we report investigations of spin pumping and the inverse spin Hall effect (ISHE) in a ${\mathrm{Mn}}_2\mathrm{Au}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ bilayer system using ferromagnetic resonance. We find a large spin Hall angle ${\theta }_{\mathrm{SH}}=0.22$, with a spin Hall conductivity ${\sigma }_{\mathrm{SH}}=1.46\times {10}^5(\hslash /2e){\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$, comparable to that of $\mathrm{Pt}$. Further, we evaluate the effective spin-mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow }=3.27\times {10}^{18}{\mathrm{m}}^{-2}$ and intrinsic spin-mixing conductance $g_r^{\uparrow \downarrow }=8.83\times {10}^{18}{\mathrm{m}}^{-2}$, which are higher than the previously reported value ($1.40\times {10}^{18}{\mathrm{m}}^{-2}$ for ${\mathrm{Mn}}_2\mathrm{Au}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$).

Switching on and off acoustic waves through metamaterials has potential in noise canceling, underwater detection, and communication. However, traditional acoustic designs are challenging in manipulating underwater acoustic waves when the device thickness is less than wavelength. Here we report an alternative design of an underwater metasurface-based acoustic switcher to achieve this goal. The switching mechanism is revealed by combing acoustic diffraction of grating with mode conversion of double-layer PMMA plates. The device is tuned to control wave transmission by changing grating angle. Furthermore, we experimentally fabricate the metasurface acoustic switcher. The broadband-switching performance is realized to control underwater target detection and to produce binary digital encoding for acoustic waves. The proposed metasurface acoustic switcher offers the advantages of broadband performance and thin structure, which promises the opportunity for designing next-generation broadband-switching devices in underwater acoustic detection and communication.

Nonreciprocal propagation of sound, that is, the different transmission of acoustic waves traveling in opposite directions, is a challenging requirement for the realization of devices such as acoustic isolators and circulators. Here, we demonstrate efficient nonreciprocal transmission of surface acoustic waves (SAWs) propagating in opposite directions in a $\mathrm{Ga}\mathrm{As}$ substrate coated with an epitaxial ${\mathrm{Fe}}_3\mathrm{Si}$ film. The nonreciprocity arises from the acoustic attenuation induced by the magnetoelastic (ME) interaction between the SAW strain field and spin waves in the ferromagnetic film, which depends on the SAW propagation direction and can be controlled via the amplitude and orientation of an external magnetic field. The acoustic-transmission nonreciprocity, defined as the difference between the transmitted acoustic powers for forward and backward propagation at the ME resonance, reaches values of up to 20%, which are, to our knowledge, the largest values of nonreciprocity reported for SAWs traveling in a semiconducting piezoelectric substrate covered by a ferromagnetic film. The experimental results are well accounted for by a model for the ME interaction, which also shows that the nonreciprocity can be further enhanced by optimization of the sample design. These results make ${\mathrm{Fe}}_3\mathrm{Si}/\mathrm{Ga}\mathrm{As}$ a promising platform for the realization of efficient nonreciprocal SAW devices.

We report a first-principles study of the driven dissipative dynamics for Kerr oscillators in the mesoscopic regime. This regime is characterized by large Kerr nonlinearity, realized here using the nonlinear kinetic inductance of a large array of Josephson junctions. The experimentally measured nonlinear resonance lineshapes of the junction array modes show significant deviations from steady-state numerical predictions, and necessitate time-dependent numerical simulations indicative of strong measurement-induced dephasing in the system arising from the large cross-Kerr effect between array modes. Analytical and numerical calculations of switching rate corroborate this by showing the emergence of a slow time scale, which is much longer than the linear decay rate and is set by fluctuation-induced switching times in the bistable regime. Furthermore, our analysis shows that the usual quantum-activated escape treatment is inadequate for prediction of the switching rates at large frequency shifts caused by strong nonlinearities, necessitating a quantum treatment that utilizes the full system Liouvillian. Based on our analysis, we identify a universal crossover parameter that delineates the regimes of validity of semiclassical and quantum descriptions, respectively. Our work shows how dynamical switching effects in strongly nonlinear systems provide a platform to study quantum-to-classical transitions.