Integrating multiple functionalities into a single metasurface is becoming of great interest for future intelligent communication systems. While such devices have been extensively explored for reciprocal functionalities, in this work, we integrate a wide variety of nonreciprocal applications into a single platform. The proposed structure is based on spatiotemporally modulated impedance sheets supported by a grounded dielectric substrate. We show that, by engineering the excitation of evanescent modes, nonreciprocal interactions with impinging waves can be configured at will. We demonstrate a plethora of nonreciprocal components, such as wave isolators, phase shifters, and circulators, on the same metasurface. This platform allows switching between different functionalities by modifying only the pumping signals (harmonic or nonharmonic), without changing the main body of the metasurface structure. This solution opens the door for future real-time reconfigurable and environment-adaptive nonreciprocal wave controllers.

We investigate a solid-state reversible alkali-ion battery (AIB) capable of regulating the density of alkali atoms in a vacuum system used for the production of laser-cooled atoms. The cold-atom sample can be used with in-vacuum chronoamperometry as a diagnostic for the voltage-controlled electrochemical reaction that sources or sinks alkali atoms into the vapor. In a combined reaction-diffusion-limited regime, we show that the number of laser-cooled atoms in a magneto-optical trap can be increased both by initially loading the AIB from the vapor for longer and by using higher voltages across the AIB when atoms are subsequently sourced back into the vapor. The time constants associated with the change in atom number in response to a change in AIB voltage are in the range of 0.5–40 s. The AIB alkali reservoir is demonstrated to survive oxidization during atmospheric exposure, simplifying reservoir loading prior to vacuum implementation as a replacement for traditional resistively heated dispensers. The AIB capabilities may provide an improved atom-number stability in next-generation atomic clocks and sensors, while also facilitating fast loading and increased interrogation times.

The principle of reciprocity underpins one of the fundamental effects in optics that signifies symmetric transmission with respect to the interchange of the source and observation points. Breaking reciprocity, however, enables additional functionalities and greatly enriches the applications of photonics in terms of nonreciprocal devices. Here, a realistic nanoscale cascaded-cavity system that envisages nonreciprocal optical isolation and efficient wavelength conversion based on spatiotemporally modulated index of refraction are proposed and numerically demonstrated. The on-chip, linear, magnetic-free nonreciprocal isolation and dynamically controllable wavelength conversion can be a promising candidate for silicon nanophotonic and optoelectronic devices.

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.