We introduce a microwave-assisted spectroscopy technique to determine the relative ratio of fluorescence emitted by nitrogen-vacancy (N-$V$) centers in diamond that are negatively charged ($\mathrm{N}\text{−}{V}^{-}$) and neutrally charged ($\mathrm{N}\text{−}{V}^0$) and present its application to studying spin-dependent ionization in N-$V$ ensembles and enhancing N-$V$-magnetometer sensitivity. Our technique is based on selectively modulating the $\mathrm{N}\text{−}{V}^{-}$ fluorescence with a spin-state-resonant microwave drive to isolate, in situ, the spectral shape of the $\mathrm{N}\text{−}{V}^{-}$ and $\mathrm{N}\text{−}{V}^0$ contributions to an N-$V$-ensemble sample’s fluorescence. As well as serving as a reliable means to characterize the charge state, the method can be used as a tool to study spin-dependent ionization in N-$V$ ensembles. As an example, we apply the microwave technique to a high-N-$V$-density diamond sample and find evidence for an additional spin-dependent ionization pathway, which we present here alongside a rate-equation model of the data. We further show that our method can be used to enhance the contrast of optically detected magnetic resonance (ODMR) on N-$V$ ensembles and may lead to significant sensitivity gains in N-$V$ magnetometers dominated by technical noise sources, especially where the $\mathrm{N}\text{−}{V}^0$ population is large. With the high-N-$V$-density diamond sample investigated here, we demonstrate an up to 4.8-fold enhancement in the ODMR contrast. We also propose a second postprocessing method of increasing the ODMR contrast in shot-noise-limited applications. The techniques presented here may also be applied to other solid-state defects, as long as their fluorescence can be selectively modulated by means of a microwave drive. We demonstrate this utility by applying our method to isolate room-temperature spectral signatures of the V2-type silicon vacancy from an ensemble of V1 and V2 silicon vacancies in $4H$ silicon carbide.

The existing challenges in designing infrared illusion are related to complex working environments, multifold heat-transfer modes, and fabrication difficulties. Recent research focuses mainly on solving one of these challenges. In this work, we consider them synthetically and propose a class of restructurable metasurfaces to show both illusion viewed in infrared light and similarity in when viewed in visible light. We consider the three basic modes of heat transfer (omnithermotics) in theoretical designs and use radiation-cavity effects in experimental manufacture. We also make it feasible to tune the surface temperature and emissivity synergistically. Besides, such metasurfaces can work in temperature-varying backgrounds and transient states. This scheme may not only provide a platform for the design of adaptable thermal illusion but may also show robustness under multifrequency detection.

We report a comprehensive study of low-power octave-bandwidth single-soliton microresonator frequency combs in both the 1550-nm and 1064-nm bands. Our experiments utilize fully integrated silicon-nitride Kerr microresonators and we demonstrate direct soliton generation with widely available distributed-Bragg-reflector lasers that provide less than 40 mW of chip-coupled laser power. We report measurements of soliton thermal dynamics and demonstrate how rapid laser-frequency control, consistent with the thermal time scale of a microresonator, facilitates stabilization of octave-bandwidth soliton combs. Moreover, since soliton combs are completely described by the fundamental linear and nonlinear dynamics of the intraresonator field, we demonstrate the close connection between modeling and generation of octave-bandwidth combs. Our experiments advance the development of self-referenced frequency combs with integrated-photonics technology and comb-laser sources with a tens-of-terahertz pulse bandwidth across the near infrared.

Natural nonlinear materials, e.g., biological materials and polymers, are mechanically weak. It has been a major challenge to develop a nonlinear material with potential mechanical applications. Here, we develop a nonlinear elastic metamaterial with giant and tailorable-nonreciprocal elastic moduli. The metamaterial is designed with a microstructural axial asymmetry, which activated nonlinear microstructural deformations in the axial direction and microstructural residual moments. This gives a robust load dependence of the nonlinear elastic modulus (i.e., secant modulus). The nonlinear elastic modulus is observed decreasing or increasing depending on, both, the direction and magnitude of the applied load. The nonlinear elastic modulus is increased by approximately $400\mathrm{\%}$ upon switching the load direction and the activation of a microstructural buckling. Thus, the nonlinear elastic modulus is generally nonreciprocal, and the metamaterial disobeys the Maxwell-Betti reciprocal theorem. This metamaterial is demonstrated by means of experimental and analytical mechanics. Various material samples are experimentally tested and their nonreciprocal elastic moduli are measured. In addition, the relation between the material’s nonreciprocity and its topological mechanics is defined. The developed metamaterial can find many important mechanical applications such as making mechanical resonators with enhanced mass sensitivity.

In antiferromagnetic spintronics, it is essential to separate the resistance modifications of purely magnetic origin from other effects generated by current pulses intended to switch the Néel vector. We investigate the magnetoresistance effects resulting from magnetic-field-induced reorientations of the staggered magnetization of epitaxial antiferromagnetic ${\mathrm{Mn}}_2\mathrm{Au}(001)$ thin films. The samples are exposed to 60-T magnetic field pulses along different crystallographic in-plane directions of ${\mathrm{Mn}}_2\mathrm{Au}(001)$, while their resistance is measured. For the staggered magnetization aligned via a spin-flop transition parallel to the easy [110] direction, an anisotropic magnetoresistance of $\simeq -0.15\mathrm{\%}$ is measured. In the case of a forced alignment of the staggered magnetization parallel to the hard [100] direction, evidence for a larger anisotropic magnetoresistance effect is found. Furthermore, transient resistance reductions of $\simeq 1\mathrm{\%}$ are observed, which we associate with the annihilation of antiferromagnetic domain walls by the magnetic field pulses.

The switching of magnetization by electric current pulse in the ${\phi }_0$ Josephson junction formed by ordinary superconductors and a magnetic noncentrosymmetric interlayer is studied. The ground state of this junction is characterized by the finite phase difference ${\phi }_0$, which is proportional to the strength of the spin-orbit interaction and the exchange field in the normal metal. Based on the Landau-Lifshits-Gilbert and resistively shunted junction model equations we build an analytical description of the magnetization dynamics induced by an arbitrary current pulse. We formulate the criteria for magnetization reversal and, using the obtained results, the form and duration of the current pulse are optimized. The analytical and numerical results are in excellent agreement at $Gr{I}_p\gg 1$, where $G$ is a Josephson-to-magnetic energy ratio, $r$ is a strength of spin-orbit interaction, and $I_p$ is a value of the current pulse. The analytical result allows one to predict magnetization reversal at the chosen system parameters and explains the features of magnetization reversal in the $G$-$r$ and $G$-$\alpha$ diagrams, where $\alpha$ is the Gilbert damping. We propose to use such a ${\phi }_0$ Josephson junction as a memory element, with the information encoded in the magnetization direction of the ferromagnetic layer.

Spin-hydrodynamic coupling is a recently discovered method to directly generate electricity from an electrically conducting fluid flow in the absence of Lorentz forces. This method relies on a collective coupling of electron spins—the internal quantum-mechanical angular momentum of the electrons—with the local vorticity of a fluid flow. In this work, we experimentally investigate the spin-hydrodynamic coupling in circular- and noncircular-capillary pipe flows and extend a previously obtained range of Reynolds numbers to smaller and larger values, $20<\mathrm{Re}<21500$, using the conducting liquid-metal alloy ($\mathrm{Ga}$,$\mathrm{In}$)$\mathrm{Sn}$ as the working liquid. In particular, we provide experimental evidence for the linear dependence of the generated electric voltage with respect to the bulk-flow velocity in the laminar regime of the circular pipe flow as predicted by Matsuo et al. [Phys. Rev. B. 96, 020401 (2017)]. Moreover, we show analytically that this behavior is universal in the laminar regime regardless of the cross-sectional shape of the pipe. Finally, the proposed scaling law by Takahashi et al. [Nat. Phys. 12, 52 (2016)] for the generated voltage in turbulent circular pipe flows is experimentally evaluated at Reynolds numbers higher than in previous studies. Our results verify the reliability of the proposed scaling law for Reynolds numbers up to $\mathrm{Re}=21500$ for which the flow is in a fully developed turbulent state.

Real-time monitoring of minute quantities of charge plays an important role in quantum-physics research and electrical measurements within modern high-end scientific instruments. High-precision charge detection approaching the single-electron level at room temperature in analog or digital electronics is limited due to the considerable thermal noise. Herein, we propose a method of charge measurement with a resolution of 0.17 e/√Hz at room temperature by resonant electrometry based on tracking the quasidigital frequency output of a highly force-sensitive oscillator. Real-time charge monitoring by 67 electrons per step is performed. We demonstrate a charge prebiased scheme for physically manipulating the quadratic nature of charge sensing of the oscillator into parabolic and linear forms with dramatic improvements in metrics such as sensitivity and resolution. Theoretical models for describing the underlying physics of both the charge measurement and resolution amplification schemes are established and validated. Due to the high quality factor of the resonator, the theoretical limit for the charge-input referred noise of the electrometer induced by thermomechanical noise is estimated in the order of ${10}^{-4}\mathrm{e}$/√Hz. This study also provides insights of resonant sensing applied to the next generation of electrometers and associated instrumentation systems.

Switching of the Dyakonov-Perel spin relaxation owing to the Rashba spin-orbit interaction (SOI) is theoretically studied in a semiconductor heterostructure with three quantum wells. The action of the present spin-relaxation switching is based on the gate-voltage induced electron transfer from the central well with vanishing Rashba SOI to the left or right well with large Rashba SOI. The spin-relaxation rate is calculated by extending the Dyakonov-Perel theory of the spin relaxation in a single subband to that in more than one subband in order to take into account the contribution from electrons in excited subbands at higher temperatures. It is shown that the on:off ratio of the spin-relaxation rate at room temperature reaches ${10}^6$ by choosing widths and compositions of well and barrier layers so as to reduce electron population in excited subbands, which gives an undesirable spin relaxation. The present spin-relaxation switching is expected to improve the on:off ratio of the current in the spin-lifetime field effect transistor.

We show theoretically that neural networks based on disordered exciton-polariton systems allow the realization of Toffoli gates. Noise in input signals is self-corrected by the networks, such that the obtained Toffoli gates are in principle cascadable, where their universality would allow for arbitrary circuits without the need of additional error-correcting codes. We further find that the exciton-polariton reservoir computers can directly simulate composite circuits, such that they are a highly efficient platform allowing circuits to operate in a single step, minimizing the delay of signal transport between elements and error-correction overhead.

Embedded eigenstates are nonradiative modes of an open structure with momentum compatible with radiation, yet characterized by unboundedly large Q factors. Traditionally, these states originate from total destructive interference of radiation from two or more nonorthogonal modes in periodic structures. In this work, we demonstrate a class of embedded eigenstates based on Berreman modes in epsilon-near-zero layered materials and propose realistic silicon carbide structures that support high-Q ($-{10}^3$) resonances based on these principles. The proposed structures demonstrate strong absorption in a narrow spectral and angular range, giving rise to quasicoherent and highly directive thermal emission.

Detectors of high-frequency radiation based on high-electron-mobility transistors benefit from low noise, room-temperature operation, and the possibility to perform radiation spectroscopy using gate-tunable plasmon resonance. Despite successful proof-of-concept demonstrations, the responsivity of transistor-based detectors of terahertz radiation, at present, remains fairly poor. To resolve this problem, we propose a class of devices supporting singular plasmon modes, i.e., modes with strong electric fields near keen electrodes. A large plasmon-enhanced electric field results in amplified nonlinearities, and thus efficient ac-to-dc conversion. We analyse subterahertz detectors based on a two-dimensional electron system in the Corbino geometry as a prototypical and exactly solvable model, and show that the responsivity scales as $1/r_0^2$ with the radius of the inner contact $r_0$. This enables responsivities exceeding 10 kV/W at subterahertz frequencies for nanometer-scale contacts readily accessible by modern nanofabrication techniques.

In this paper, the physical mechanisms responsible for optical gain in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{Pb}\mathrm{I}}_3$ (MAPI) polycrystalline thin films are investigated experimentally and theoretically. Waveguide structures composed by a MAPI film embedded in between PMMA and silica layers are used as an efficient geometry to confine emitted light in MAPI films and minimize the energy threshold for amplified spontaneous emission (ASE). We show that photogenerated exciton density at the ASE threshold is as low as $(2.4-12)\times {10}^{16}{\mathrm{cm}}^{-3}$, which is below the Mott transition density reported for this material and the threshold transparency condition deduced with the free-carrier model. Such a low threshold indicates that the formation of excitons plays an important role in the generation of optical gain in MAPI films. The rate-equation model including gain is incorporated into a beam-propagation algorithm to describe waveguided spontaneous emission and ASE in MAPI films, while using the optical parameters experimentally determined in this work. This model is a useful tool to design active photonic devices based on MAPI and other metal-halide semiconductors.

The interaction between high-irradiance light and molten metal is the complex multiphysics phenomenon that underpins industrial processes such as laser-based additive manufacturing, welding, and cutting. One aspect that requires careful attention is the formation and evolution of vapor depressions, or keyholes, within the molten metal. The dynamic behavior of these depressions can dramatically change the number of laser-beam reflections and is therefore intrinsically linked to the instantaneous energy coupled into the system. Despite its importance, there is a severe lack of direct in situ, experimental evidence of this relationship, which creates challenges for those who aim to model or control laser-based manufacturing processes. In this work, we combine two simultaneous state-of-the-art real-time measurement techniques (inline coherent imaging and integrating-sphere radiometry) to confirm and explore the definite positive correlation between the highly dynamic vapor-depression geometry and laser energy absorptance. For irradiances resulting in vapor-depression formation ($\ge 0.49{\mathrm{MW}/\mathrm{cm}}^2$), we observe excellent correlation (0.86) between the instantaneous depth (down to 800 $\mu \mathrm{m}$) and the absorptance (up to 0.92), directly demonstrating their interdependence. In the transition mode, an important regime for additive manufacturing, we observe temporary vapor-depression formation with concomitant changes in absorptance from 0.34 to 0.53. At higher irradiances, we detect stepwise increases in the absorbed laser power with a smoothly increasing keyhole depth, which is a real-time experimental observation of the effect of multiple reflections during laser-metal processing. The value of simultaneous depth and absorption measurements for predictive model validation is presented using ray-tracing simulations, which also confirm the absorption enhancement via incremental increases in the reflection count. This work provides insight into the underlying physics of laser-based metal manufacturing that is useful toward deterministic modeling and real-time process control.

We demonstrate that the orbital angular momentum of radiating waves can be used to drive a mechanical oscillator, an option that has remained elusive to date on experimental grounds whatever the nature of the waves. This is done using an amplitude-modulated ultrasonic wave interacting with a centimeter-size torsional pendulum. Achieved resonant quantitative measurements of the acoustic radiation torque and material properties set the basis for orbital-angular-momentum-based metrology applications and possibly cooling of the rotational degree of freedom of macroscopic objects.

Controlling the polarization state of light at a fast speed is important in both classical and quantum regimes. Here, we present an easily integrated compact tunable linear polarization-state generator of single photons in the telecom wavelength band on a periodically poled lithium niobate on insulator (PPLNOI) chip. The linear polarization of single photons can be continuously rotated by applying a transverse electric field utilizing the electro-optic (EO) effect. With strong light confinement and the large EO coefficient in the PPLNOI ridge waveguide, dramatically reduced driving voltage and fast-speed modulation is realized. Moreover, quantum entanglement is well maintained during the switching of polarization states, as proved in the time-energy entanglement measurement. The scheme holds promise for realizing integrated quantum optics.

Metagratings are a recently proposed class of metasurfaces for efficient manipulation of an impinging wavefront within a subwavelength layer. They avoid the requirement for fine discretization of gradient metasurfaces, and overcome their inherent limitations in efficiency. Here, we demonstrate experimentally the functioning principle of a reconfigurable acoustic metagrating for anomalous reflection with high efficiency, using coarse geometric design features. It is formed by a periodic array of C-shaped meta-atoms, which exhibit large Willis coupling, resulting in a controlled level of asymmetry in their scattering pattern. Our results reveal that the proposed acoustic metagrating can reroute an incident wave towards a large angle, beyond the limitations of gradient-phase approaches with nearly unitary reflection efficiency. The proposed designs offer a highly efficient tunable platform to control steering angle and operating frequency. In our experiments, an acoustic wave is successfully steered to the desired reflection direction by finite metagratings, demonstrating reconfigurability in angle and operating frequency.

We investigate the transport properties of a modified interface by intentionally inserting a nanometer-scale undoped ${\mathrm{Ba}\mathrm{Sn}\mathrm{O}}_3$ spacer layer at the ${\mathrm{La}\mathrm{In}\mathrm{O}}_3$/${\mathrm{Ba}}_{1\text{−}x}{\mathrm{La}}_x{\mathrm{Sn}\mathrm{O}}_3$ interface, thereby creating remotely doped heterostructures. Both the carrier density $(n_s)$ and the Hall mobility $({\mu }_H)$ continuously decrease as the thickness of the ${\mathrm{Ba}\mathrm{Sn}\mathrm{O}}_3$ spacer layer at the interface increases, indicating a changing electron-density profile as a function of the spacer thickness. We find the behavior is consistent with the recently proposed “interface-polarization” model by self-consistent one-dimensional Poisson-Schrödinger calculations. The decrease of $n_s$ makes it difficult to see the effect of the spacer layer on the mobility in the remotely doped structures due to the simultaneous decrease of $\mu$ caused by the ineffective screening of the remote Coulomb scattering from ionized donors in addition to the threading dislocation scattering. Hence, we control the band bending continuously via the field effect with a fixed spacer-layer thickness, leading to observation of enhanced mobility $({\mu }_{\mathrm{FE}})$ in the remotely doped 2DEG heterostructures in spite of high-density threading dislocations acting as the background charged impurities.

Electrically controlled switching in an antiferromagnet (AFM), utilizing a currentless mechanism, is theoretically examined at finite temperatures. The structure consists of a metallic AFM with biaxial magnetic anisotropy sandwiched between a ferromagnetic spin filter and a semiconductor Schottky junction in a two-terminal pillar configuration. The calculations show that the torque necessary for the desired ${90}^{\circ }$ rotation of the Néel vector between two easy axes can be provided efficiently by pumping spin-polarized electrons into and out of the AFM through the metallic ferromagnetic layer. Consideration of thermal fluctuations illustrates the stochastic nature of the switching, whose probability distribution can be tailored by the electrical signal pulse as well as by the device dimensions. Detection of the Néel-vector state following this rotation may also be achieved straightforwardly via the large anisotropic magnetoresistance of the biaxial antiferromagnetic material. These properties, along with an ultrafast switching speed and a low energy requirement, are expected to be well suited for applications in nonvolatile memory and probabilistic computing.