We report both experimentally and numerically that a broadband acoustic lens with asymmetric focusing is realized by dual-layer metasurfaces in air. The acoustic waves can be focused on a side with 0.4 wavelength resolution focusing characteristic, but cannot transmit through the lens in a large range of incident angles on the other side. This exotic phenomenon exists in the range of 2910–3610 Hz, which arises from the asymmetric phase manipulation induced from the dual-layer metasurfaces. More importantly, without changing the structure of both metasurfaces, the horizontal and vertical positions of the focus can be systematically manipulated by mechanically changing the distance between both metasurfaces and the incident angle, respectively. The proposed lens has the advantages of broadband multifunctionality, subwavelength focusing, planar structure, and tunable properties, showing promising applications in focused ultrasound therapy and medical ultrasound imaging.

We report on the observation of the giant photoconductance of a quantum point contact (QPC) in the tunneling regime excited by terahertz radiation. Studied QPCs are formed in a $\mathrm{Ga}\mathrm{As}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure with a high-electron-mobility two-dimensional electron gas. We demonstrate that irradiation of strongly negatively biased QPCs by laser radiation with frequency $f=0.69\mathrm{THz}$ and intensity $50\mathrm{mW}/{\mathrm{cm}}^2$ results in two orders of magnitude enhancement of the QPC conductance. The effect increases with the dark conductivity decrease. It is also characterized by a strong polarization dependence and a drastic reduction of the signal by increasing the radiation frequency to 1.63 THz. We demonstrate that all experimental findings can be well explained by the photon-assisted tunneling through the QPC. Corresponding calculations are in good agreement with the experiment.

Light manipulation through spin-orbit coupling opens new perspectives in photonics and particularly in integrated optics. The reverse spin Hall effect, where the guided wave direction affects the polarization properties of the light scattered by a nanostructure, is a key effect for the development of new functionalities at the connection between integrated structures and free space, and could find application in chiral quantum optics, modulation, and multiplexing. We show that metasurfaces represent a promising platform for the reverse spin Hall effect. Using a periodic array of Λ-shaped metallic nanoantenna, we control the polarization of the extracted light with the guided wave-propagation direction, but also the number of output directions and the polarization of each one. These results and the versatility of metasurfaces for the spin Hall effect could be extended to various frequencies and materials, such as silicon photonics at telecom wavelength.

We study theoretically the dynamics of a driven-dissipative qubit-resonator system. Specifically, a transmon qubit is coupled to a transmission-line resonator; this system is considered to be probed via a resonator, by means of either continuous or pulsed measurements. Analytic results obtained in the semiclassical approximation are compared with calculations in the semiquantum theory as well as with the previous experiments. We demonstrate that the temperature dependence of the resonator frequency shift can be used for the system thermometry and that the dynamics, which display a pinched-hysteretic curve, can be useful for realization of the memory devices termed quantum memcapacitors.

Optical frequency combs enable state-of-the-art applications including frequency metrology, optical clocks, astronomical measurements, and sensing. Recent demonstrations of microresonator-based Kerr frequency combs or microcombs pave the way to scalable and stable comb sources on a photonic chip. Generating microcombs in the short-wavelength range, however, has been limited by large material dispersion and optical loss. Here we demonstrate a scheme for efficiently generating microcombs near the edge of the visible spectrum in a high-$Q$ aluminum nitride microring resonator. The enhanced Pockels effect strongly couples infrared and near-visible modes into hybrid mode pairs, which participate in the Kerr microcomb generation process and lead to strong Cherenkov-like radiation in the near-visible band an octave apart. A surprisingly high on-chip conversion efficiency of 22% is achieved from a pulsed pump laser to the near-visible comb. As a result of pulse pumping, the generated microcombs are in the chaotic state. We further demonstrate a robust frequency tuning of the near-visible comb by more than one free spectral range and apply it to the absorption spectroscopy of a water-based dye molecule solution. Our work is a step towards high-efficiency visible microcomb generation and its utilization, and it also provides insights into the significance of the Pockels effect and its strong coupling with Kerr nonlinearity in a single microcavity device.

The polarization modulation effect of the gate dielectric on the performance of metal-oxide-semiconductor field-effect transistors has been investigated for more than a decade. However, there are no comparable studies in the area of organic field-effect transistors (FETs) using polymer ferroelectric dielectrics, where the effect of polarization rotation by ${90}^{\circ }$ is examined on the FET characteristics. We demonstrate the effect of polarization rotation in a relaxor ferroelectric dielectric, poly(vinylidene fluoride trifluorethylene) (PVDF-TrFE), on the performance of small-molecule-based organic FETs. The subthreshold swing and other transistor parameters in organic FETs can be controlled in a reversible fashion by switching the polarization direction in the PVDF-TrFE layer. X-ray diffraction and electron microscopy images from PVDF-TrFE reveal changes in the ferroelectric phase and domain size, respectively, upon rotating the external electric field by ${90}^{\circ }$. The structural changes corroborate density-functional-theoretical studies of an oligomer of the ferroelectric molecule in the presence of an applied electric field. The strategies enumerated here for polarization orientation of the polymer ferroelectric dielectric are applicable for a wide range of polymeric and organic transistors.

We propose an experimental scheme and present detailed theoretical description of the optical chopper in which functionality is based on the balance between the Casimir and light pressures. The proposed device consists of two atomically thin metallic mirrors forming the Fabry-Perot microfilter. One of the mirrors is deposited on a solid cube and another on a thinner wall subjected to bending under the influence of attractive Casimir force and repulsive force due to the pressure of light from a continuous laser amplified in the resonator of a microfilter. The separation distance between the mirrors should only slightly exceed the half wavelength of the laser light. It is shown that in this case the resonance condition in the microfilter alternatively obeys and breaks down resulting in the periodic pulses of the transmitted light. The Casimir pressure is calculated taking into account an anisotropy of the dielectric permittivity of a metal at several first Matsubara frequencies. The reflectivity properties of atomically thin metallic mirrors in the optical spectral range are found using the experimentally consistent phenomenological approach developed earlier in the literature. The specific values of all parameters, found for the microfilter made of quartz glass with $\mathrm{Ag}$ mirrors, demonstrate its workability. The proposed optical chopper may find prospective applications in the emerging field of nanotechnology exploiting the effects of quantum fluctuations.

Soft and highly-strain-hardening metals such as iron, aluminum, and tantalum, often called “gummy,” are notoriously difficult to cut. This is due to their tendency to exhibit redundant, unsteady plastic flow with large-amplitude folding, which results also in macroscale defects on the cut surface and large energy dissipation. In this work, we demonstrate that this difficulty can be overcome by merely coating the initial metal surface with common adhesive chemical media such as glues and inks. Using high-speed in situ imaging, we show that the media act by coupling unsteady surface-plastic-flow modes with interface energetics—a mechanochemical action—thereby effecting a ductile-to-brittle transition, locally. Consequently, the unsteady plastic flow with folding transitions to a periodic segmentation-type flow in the presence of the surface media, with near absence of defects on the cut surface and significantly lower energy dissipation (a reduction of up to 80%). This mechanochemical effect is controllable and not material specific, with the chemical media demonstrating comparable efficacy across different metal systems. This makes it quite distinct from other well-known mechanochemical effects, such as liquid-metal embrittlement and stress-corrosion cracking, that are both highly material specific and catastrophic. An analytic model incorporating local flow dynamics, stability of dislocation emission, and surface-media energetics is found to correctly predict the onset of the plastic-flow transition. The benign nature and simplicity of the media suggest wide-ranging opportunities for improving the performance of cutting and deformation processes for metals and alloys in practical settings.

The magnetization reversal of each individual layer in magnetic trilayers ($\mathrm{permalloy}/\mathrm{Nd}\mathrm{Co}/\mathrm{Gd}\mathrm{Co}$) is investigated in detail with x-ray microscopy and micromagnetic calculations. Two sequential inversion mechanisms are identified. First, magnetic vortex-antivortex pairs move along the field direction while inverting the magnetization of magnetic stripes until they are pinned by defects. The vortex-antivortex displacements are reversible within a field interval which allows their controlled motion. Second, as the reversed magnetic field increases, cycloidal domains appear in the permalloy layer as a consequence of the dissociation of vortex-antivortex pairs due to pinning. The field range where magnetic vortices and antivortices are effectively guided by the stripe pattern is of the order of tens of mT for the $\mathrm{Ni}\mathrm{Fe}$ layer, as estimated from the stability of cycloid domains in the sample.

Spin-momentum locking in Weyl semimetals correlates the orbital motion of electrons with background magnetic textures. We show here that the dynamics of a magnetic texture in a magnetic Weyl semimetal induces a pumped electric current that is free from Joule heating. This pumped current can be regarded as a Hall current induced by axial electromagnetic fields equivalent to the magnetic texture. Taking a magnetic domain wall as a test case, we demonstrate that a moving domain wall generates a pumping current corresponding to the localized charge.

We study optomechanically induced transparency in a microresonator coupled with nanoparticles. By tuning the relative angle of the nanoparticles, exceptional points (EPs) emerge periodically in this system and thus strongly modify both the transmission rate and the group delay of the signal. As a result, controllable slow-to-fast light switching can be achieved by manipulating external nanoparticles. This provides a new way to engineer EP-assisted optomechanical devices for applications in optical communications and signal processing.

Silicon nitride ($\mathrm{Si}\mathrm{N}$) is one of the emerging semiconductor materials that are used in both linear and nonlinear all-optical integrated devices. Its excellent dielectric properties, high material stability, and dispersion controllability are attractive to on-chip optical communications, optical signal processing, and even imaging devices. However, a large-aperture $\mathrm{Si}\mathrm{N}$ metalens with high numerical aperture (NA) is limited by the low refractive index and nanofabrication technologies, particular in the visible spectrum. Here, we experimentally realize the visible-spectrum $\mathrm{Si}\mathrm{N}$ divergent metalenses by fabricating the 695-nm-thick hexagonal arrays with a minimum space of 42 nm between adjacent nanopillars. A micro-size divergent metalens with $\mathrm{NA}\sim 0.98$ and subwavelength resolution enables objects to be shrunk as small as a single-mode fiber core. Another centimeter-size $\mathrm{Si}\mathrm{N}$ divergent metalens with over half a billion nanopillars, made by using the mature CMOS-compatible fabrication process, exhibits high-quality wide-angle imaging. Our findings may open a new door for the miniaturization of optical lenses in the fields of optical fibers, microendoscopes, and smart phones, as well as the applications in all-sky telescopes, large-angle beam shaping, and near-eye imaging.

The characteristics of pure spin current from the longitudinal spin Seebeck effect are investigated at room temperature in $\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ deposited on the ${({\mathrm{Pb}\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3)}_{0.7}{({\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3)}_{0.3}$ substrate, through which electrostrain can be controllably applied. The applied voltage enables the modification of magnetic anisotropy in the magnetic insulating ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ layer, and thus the sensitivity of the spin Seebeck effect at small magnetic fields, without suppressing the spin Seebeck effect and the inverse spin Hall effect. This is due to the robust intrinsic physical parameters in $\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Our results pave the way for the application of voltage-controlled pure spin-current devices.

We theoretically investigate a scheme for the generation of single hard-x-ray pulses of controllable duration in the range of 1–100 ns from a radioactive Mössbauer source. The scheme uses a magnetically perturbed ${}^{57}\mathrm{Fe}\mathrm{B}{\mathrm{O}}_3$ crystal illuminated with recoilless 14.4-keV photons from a ${}^{57}\mathrm{Co}$ radioisotope nuclide. Such a compact x-ray source is useful for the extension of quantum optics to the 10-keV energy scale, which has been in the spotlight in recent years. So far, experimental achievements have mostly been attained in synchrotron radiation facilities. However, the use of tabletop and portable hard-x-ray sources for time-resolved measurements and for implementing coherent control over nuclear quantum-optics systems has still been limited. The availability of compact hard-x-ray sources may become the engine for the application of quantum-information schemes down to the subatomic scale. We demonstrate that the present method is versatile and provides an economic solution, utilizing a Mössbauer source, to perform time-resolved nuclear scattering, to produce suitable pulses for photon storage, and to flexibly generate x-ray single-photon entanglement.

We describe an optically pumped Cs magnetometer containing a $1.5\text{−}\mathrm{mm}$-thick microfabricated vapor cell with nitrogen buffer gas operating in a free-induction-decay configuration. This allows us to monitor the free Larmor precession of the spin-coherent Cs atoms by separating the pump and probe phases in the time domain. A single light pulse can sufficiently polarize the atomic sample; however, synchronous modulation of the light field actively drives the precession and maximizes the induced spin coherence. Both amplitude and frequency modulation have been implemented with noise floors of 3 and $16\mathrm{pT}/\sqrt{\mathrm{Hz}}$, respectively, within the Nyquist-limited bandwidth of $500\mathrm{Hz}$.

Resonant metamaterials and phononic crystals are widely studied, with the aim of controlling and altering elastic wave propagation in solids. Beams with varying stiffness along the length are an archetypal example of a phononic crystal for bending waves, since the resulting nontrivial dispersion relation can be modeled analytically. In this work, the complex dispersion curve over the complete audible frequency range is measured using the inhomogeneous wave-correlation (IWC) technique, and compared to a unit-cell model and an explicit model of the finite beam. Multiple band gaps, each with a different attenuation coefficient, are predicted and validated. An iterative implementation of IWC allows the identification of a spatial spectrum of wave numbers, responsible for the intricate beam displacement shape. The energy carried by the fundamental wave number and its Bloch (sub)harmonics are determined, showing that the main dispersion branch is monotonically increasing. It is shown that only a small fraction of the wave energy travels along branches with a negative slope in the first Brillouin zone. Additionally, in band gaps affecting only bending waves, energy is converted to other wave types, such as longitudinal waves. This approach offers some important insights into the physics of the wave propagation, which cannot be gathered from unit-cell modeling alone.

Zinc tin nitride ($\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$) is attracting growing interest as a nontoxic and earth-abundant photoabsorber for thin-film photovoltaics. Carrier transport in $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ and consequently cell performance are strongly affected by point defects with deep levels acting as carrier recombination centers. In this study, the point defects in $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ are revisited by careful first-principles modeling based on recent experimental and theoretical results. It is shown that $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ does not have low-energy defects with deep levels, in contrast to previously reported results. Therefore, $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ is more promising as a photoabsorber material than formerly considered.

Decoherence remains a major challenge in quantum computing hardware, and a variety of physical-layer controls provide opportunities to mitigate the impact of this phenomenon through feedback and feed-forward control. In this work, we compare a variety of machine-learning algorithms derived from diverse fields for the task of state estimation (retrodiction) and forward prediction of future qubit-state evolution for a single qubit subject to classical, non-Markovian dephasing. Our approaches involve the construction of a dynamical model capturing qubit dynamics via autoregressive or Fourier-type protocols using only a historical record of projective measurements. A detailed comparison of achievable prediction horizons, model robustness, and measurement-noise-filtering capabilities for Kalman filters (KFs) and Gaussian process regression (GPR) algorithms is provided. We demonstrate superior performance from the autoregressive KF relative to Fourier-based KF approaches and focus on the role of filter optimization in achieving suitable performance. Finally, we examine several realizations of GPR using different kernels and discover that these approaches are generally not suitable for forward prediction. We highlight the linkages between predictive performance and kernel structure, and we identify ways in which forward predictions are susceptible to numerical artifacts.

We provide detailed insights into a link between structural and optical disorder of resonant optical structures, in particular, distributed Bragg reflectors (DBRs) and resonant microcavities ($\mu \mathrm{Cs}$). The standard (targeted) DBR structures have periodic square-wave–like refractive-index profiles, and their optical performance is determined by the refractive-index ratio of the two applied materials ($n_{12}=n_1/n_2$, $n_1>n_2$) and the number of DBR periods ($N$). It is well known that its structural disorder strongly affects its optical properties, but, despite that, this influence has not been quantitatively addressed in the literature. We propose a precise quantitative definition for a structural disorder of a single DBR unit cell (disorder factor $D_F$), completing the set of DBR fundamental parameters ($n_{12}$, $N$, $D_F$). Then we expose the basis for the effective refractive-index approximation (ERIA), showing that, as long as DBR optical properties are concerned, the influence of increasing structural disorder ($D_F\uparrow$) is virtually identical to the influence of decreasing refractive-index ratio ($n_{12}\downarrow$), with the latter influence being easily quantified. Making use of the ERIA method, simple analytical formulas, which enable rapid insights into the reflectivity and stop-band width of DBRs with different types of transient layers at the heterointerfaces, are derived and the results validated, via both transfer-matrix simulations and direct experimental measurements of imperfect DBRs. The insights of the ERIA method are then further applied on resonant $\mu \mathrm{Cs}$, providing a comprehensive link between their structural disorder and subsequent deterioration of their quality ($Q$) factor.

Electrical manipulation of antiferromagnets with specific symmetries offers the prospect of creating antiferromagnetic spintronic devices. Such devices aim to make use of the insensitivity to external magnetic fields and the ultrafast dynamics at the picosecond timescale intrinsic to antiferromagnets. The possibility to electrically switch antiferromagnets was first predicted for ${\mathrm{Mn}}_2\mathrm{Au}$ and then experimentally observed in tetragonal CuMnAs. Here, we report on the electrical switching and detection of the Néel order in epitaxial films of ${\mathrm{Mn}}_2\mathrm{Au}$. The exponential dependences of the switching amplitude on the current density and the temperature are explained by a macroscopic thermal-activation model taking into account the effect of the Joule heating in Hall cross devices, and we observe that the thermal activation plays a key role in the reorientation process of the Néel order. Our model analysis shows that the electrically set Néel state is long-term stable at room temperature, paving the way for practical applications in memory devices.