Metamaterials with properly engineered linear and nonlinear optical response are of great interest for many advanced applications in nanophotonics and quantum optics. In the present work, we perform a detailed spectral investigation of the third-order nonlinear optical properties (nonlinear refractive index and nonlinear absorption coefficient) of $ϵ$-near-zero ${\mathrm{Au/Al}}_2{\mathrm{O}}_3$ multilayer metamaterials in a broad range of the visible spectrum across their $ϵ$-near-zero (ENZ) wavelength, at different incidence angles with TE- and TM-polarized light. Multilayers with different gold filling fractions (16 and 33%) are produced by magnetron sputtering to tune the spectral position of the $ϵ$-near-zero wavelength. The results demonstrate that a continuous modulation of the linear and nonlinear optical parameters of these metamaterials can be obtained as a function of the angle of incidence, with a peak of the nonlinear optical coefficients close to the ENZ wavelength. A model is proposed to describe the nonlinear optical response of the metamaterials, and an optimal agreement between experimental and simulated results is obtained in all the configurations explored. This model represents a useful tool to design multilayer metamaterials with tailored nonlinear optical properties, to be used in different experimental configurations.

Measurement-based quantum computing (MBQC) promises an alternative approach to quantum computation that has natural compatibility with error-correction codes at the cost of a polynomial increase in physical qubits. MBQC implementations have previously focused on photonic systems where two-qubit gates are difficult. On the other hand, semiconductor spin qubit systems offer fast two-qubit gates via the exchange interaction. To explore the benefits of MBQC on spin systems, two hybrid measurement-exchange schemes for full qubit control with semiconductor double-quantum-dot spin qubits are considered. Protocol 1 fully realizes the cluster-state approach to MBQC but requires singlet-triplet qubits in a magnetic field gradient. Protocol 2 implements a direct measurement-based replacement for more traditional gate-based encoded operations, without need for magnetic field gradients. Both protocols demonstrate full single- and two-qubit control through a combination of interqubit and intraqubit exchange and measurement onto the singlet-triplet basis. We show that both schemes suppress individual qubit spin-state leakage errors and offer fast gate times, up to known phase and Pauli corrections.

Without using intrinsic chiral metamaterials with complex structures, here we use achiral metasurfaces to realize giant extrinsic chirality empowered by quasi-bound states in the continuum (quasi-BICs). The giant extrinsic chirality in the proposed achiral metasurface is mainly attributed to the breaking of the mirror symmetry of the whole system, combining the metasurface with incident waves. The transmittance of two different polarization states and the signs of circular dichroism (CD) can be arbitrarily controlled by tilting the plane of the metasurface relative to the incoming waves. Remarkably, we simultaneously select two polarization states at different bands for a certain achiral metasurface based on the symmetry-protected dual bound states in the continuum (BICs). The measured maximum CDs approach ±0.85. In addition, an application of the polarization-multiplexed-field image display assisted by chiral quasi-BICs is demonstrated. This work provides a general method to control extrinsic chirality in achiral metasurfaces and offers possibilities to design high-performance multiband spin-selective transmission, biodetection, chemical analysis, and on-chip chiral manipulation.

We investigate electron transport in asymmetric double-barrier ($\mathrm{Al},\mathrm{Ga})\mathrm{As}/\mathrm{Ga}\mathrm{As}$ thermionic cooling heterostructures. Measurements of temperature-dependent current-voltage characteristics confirm that the dominant electron transport is a sequential process of resonant tunneling injection into and thermionic emission from the quantum-well (QW) cooling layer. The thermal activation energy of the current is found to be strongly dependent on the bias voltage. Furthermore, instead of showing a simple thermal activation behavior, the current exhibits rather complicated temperature and voltage dependence, particularly when the thermionic emission barrier is low. To establish a quantitative understanding, we develop an intuitive analytical model for sequential electron transport that explicitly takes into account scattering effects in the thermionic emission process from the two-dimensional QW states to the three-dimensional above-barrier states. The observed temperature-dependent sequential current is well explained by the present theory.

Coiling up space is a metamaterial technique that utilizes a subwavelength labyrinthine structure to achieve a high refractive index. Despite the various advances in acoustics, the coiling-up-space technique cannot be used in the elastic case owing to its tensor-based physics. In this study, we show that an elastic coiling up space is possible if the “fluidlike connection” and “isocurvature path” conditions are satisfied. These conditions overcome the tensorial nature of elastic waves and allow the change of displacement fields according to the geometry, as in the acoustic case with scalar fields. The proposed elastic coiling-up-space technique is numerically and experimentally demonstrated.

We present a mode of operation for Duffing-type nonlinear microelectromechanical system devices whereby a self-sustained multifrequency output is generated. This self-sustained librator regime creates a limit cycle around a dynamical fixed point, i.e., around fixed points within the rotating frame, whereas a traditional oscillator generates a limit cycle around a static fixed point. The librator limit cycles thus created do not change the global topology of the rotating frame phase space, but are constrained by it. Because of the Duffing nonlinearity, different types of limit cycles could be generated within the same phase space, with each type possessing distinct dynamical features. Transitioning between these limit cycles requires crossing homoclinic bifurcations, which is done without generating chaos as the phase space dynamics are constrained in two dimensions. This work provides an alternative experimental tool for the study of microelectromechanical and nanoelectromechanical system dynamics and their application.

Acoustic metalenses have been pursued over the past decades due to their pivotal role in a wide variety of applications. Recent research efforts have demonstrated that, at ultrasonic regimes, acoustic levitation can be realized with standing waves, which are created by the interference between incoming and reflected focused waves. However, the conventional gradient-metasurface approach to focus ultrasonic waves is complex, leading to poor scalability. In this work, we propose a design principle for ultrasonic metalenses, based on metagratings—arrays of discrete scatters with coarser features than gradient metasurfaces. We achieve beam focusing by locally controlling the excitation of a single diffraction order with the use of metagratings, with geometry adiabatically varying over the lens aperture. We show that our metalens can effectively focus impinging ultrasonic waves to a focal point with a full width at half maximum of 0.364 of the wavelength. The focusing performance of the metalens is demonstrated experimentally, validating our proposed approach. This metagrating approach to focusing can be adopted for different operating frequencies by scaling the size of the structure, which has coarse features suitable for high-frequency designs, with potential applications ranging from biomedical science to nondestructive testing.

We present a study of full-wave tailoring with an elastic metasurface between two different media. Unlike previous research on elastic metasurface, the current study has a crucial difference that the metasurface is placed between two different media. Interestingly, we find that previous studies have a theoretical limitation that negates the simultaneous achievement of various phase shifts and full transmission between inhomogeneous media. To overcome this, we propose a double-unit elastic metasurface composed of two units, phase modulator and impedance matcher, to avoid the constraint restricting various phase shifts with full transmission. Numerical and experimental investigations are performed to validate the operation of the double-unit elastic metasurface. The proposed metasurface is expected to be applied in various ultrasonic applications.

This paper describes an acoustic system enabling us to achieve asymmetric transmission over a broad frequency range. The basic elements of the system are a moving coil loudspeaker placed along the axis of a duct, and a microphone flush mounted next to the front side of the loudspeaker. The loudspeaker acts both as a mass-spring oscillator through which incident waves are reflected and transmitted, and as a sound re-emitter controlled by the microphone through a feedback loop. It is shown that the reciprocity of the resulting two port is broken, and that it can be used to design a broadband asymmetric wave transmitter. The experimental results show that there exists a frequency band ranging from about 300 Hz up to 1500 Hz, where an almost perfect transmission in the forward direction is observed whereas a weak (i.e., of about 10%) transmission is achieved in the reverse direction. Owing to the broadbandness of this efficient and compact nonreciprocal scatterer, it is also shown through its response to incident pulses that it can be used as a sound trap.

Topological lattices have recently generated a great deal of interest based on the unique mechanical properties rooted in their topological polarization, including the ability to support localized modes at certain floppy edges. The study of these systems has been predominantly restricted to the realm of in-plane mechanics, to which many topological effects are germane. In this study, we stretch this paradigm by exploring the possibility to export certain topological attributes to the flexural wave behavior of thin lattice sheets. To couple the topological modes to the out-of-plane response, we assemble a bilayer lattice by stacking a thick topological kagome layer onto a thin twisted kagome lattice. The band diagram reveals the existence of modes whose out-of-plane character is controlled by the edge modes of the topological layer, a behavior elucidated via simulations and confirmed via laser vibrometer experiments on a bilayer prototype specimen. These results open an alternative direction for topological mechanics whereby flexural waves are controlled by the in-plane topology, leading to potential applications for flexural wave devices with engineered polarized response.

We report the controllable buckling of free-standing thin-film semiconductor metamaterials using a thin-walled structure spanning $\mathrm{Au}$ grid supports. The buckling is developed by compressing the $\mathrm{Au}$ grids, increasing the internal air pressure during the bonding of $\mathrm{Au}$ pads on the semiconductor to the grids, resulting in upward deformation. The stiffness of the free-standing semiconductor beam is controlled by the grid line spacing, as verified for a wide range of out-of-plane deformations, ranging from nearly flat to microns for grid line spacings from 20 to 150 µm, respectively. We also observe telephone-cord and domelike buckling as the grid line spacings and layer thicknesses are varied. Finite element analysis quantitatively predicts the shape and magnitude of the film deflection. We propose strategies to apply the distorted free-standing metamaterials for thermal energy harvesting, optical reflectors, and pressure sensors.

Switching induced by spin-orbit torque (SOT) is being vigorously explored, as it allows the control of magnetization using an in-plane current, which enables a three-terminal magnetic-tunnel-junction geometry with isolated read and write paths. This significantly improves the device endurance and the read stability, and allows reliable subnanosecond switching. Tungsten in the $\beta$ phase, $\beta$-$\mathrm{W}$, has the largest reported antidamping SOT charge-to-spin conversion ratio $({\theta }_{\mathrm{AD}}\approx -60\mathrm{\%})$ for heavy metals. However, $\beta$-$\mathrm{W}$ has a limitation when one is aiming for reliable technology integration: the $\beta$ phase is limited to a thickness of a few nanometers and enters the $\alpha$ phase above 4 nm in our samples when industry-relevant deposition tools are used. Here, we report our approach to extending the range of $\beta$-$\mathrm{W}$, while simultaneously improving the SOT efficiency by introducing $\mathrm{N}$ and $\mathrm{O}$ doping of $\mathrm{W}$. Resistivity and XRD measurements confirm the extension of the $\beta$ phase from 4 nm to more than 10 nm, and transport characterization shows an effective SOT efficiency larger than $-44.4\mathrm{\%}$ (reaching approximately $-60\mathrm{\%}$ for the bulk contribution). In addition, we demonstrate the possibility of controlling and enhancing the perpendicular magnetic anisotropy of a storage layer ($\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$). Further, we integrate the optimized $\mathrm{W}(\mathrm{O},\mathrm{N})$ into SOT magnetic random-access memory (SOT-MRAM) devices and project that, for the same thickness of SOT material, the switching current decreases by 25% in optimized $\mathrm{W}(\mathrm{O},\mathrm{N})$ compared with our standard $\mathrm{W}$. Our results open the path to using and further optimizing $\mathrm{W}$ for integration of SOT-MRAM technology.

The ongoing rapid progress towards quantum technologies relies on new hybrid platforms optimized for specific quantum computation and communication tasks, and researchers are striving to achieve such platforms. We study theoretically a spin qubit exchange-coupled to an anisotropic ferromagnet that hosts magnons with a controllable degree of intrinsic squeezing. We find this system to physically realize the quantum Rabi model from the isotropic to the Jaynes-Cummings limit with coupling strengths that can reach the deep-strong regime. We demonstrate that the composite nature of the squeezed magnon enables concurrent excitation of three spin qubits coupled to the same magnet. Thus, three-qubit Greenberger-Horne-Zeilinger and related states needed for implementing Shor’s quantum error-correction code can be robustly generated. Our analysis highlights some unique advantages offered by this hybrid platform, and we hope that it will motivate corresponding experimental efforts.

We analyze the magnetic mode structure of axially magnetized finite-length nanoscopic cylinders in a regime where the exchange interaction dominates, along with simulations of the mode frequencies of the ferrimagnet yttrium iron garnet. For the bulk modes, we find that the frequencies can be represented by an expression given by Herring and Kittel by using wavevector components obtained by fitting the mode patterns emerging from these simulations. In addition to the axial, radial, and azimuthal modes that are present in an infinite cylinder, we find localized “cap modes” that are “trapped” at the top and bottom cylinder faces by the inhomogeneous dipole field emerging from the ends. Semiquantitative explanations are given for some of the modes, in terms of a one-dimensional Schrodinger equation, which is valid in the exchange-dominant case. The assignment of the azimuthal-mode number is carefully discussed, and the frequency splitting of a few pairs of nearly degenerate modes is determined through the beat pattern emerging from them.

Deep learning is having a tremendous impact in many areas of computer science and engineering. Motivated by this success, deep neural networks are attracting increasing attention in many other disciplines, including the physical sciences. In this work, we show that artificial neural networks can be successfully used in the theoretical modeling and analysis of a variety of radiative-heat-transfer phenomena and devices. By using a set of custom-designed numerical methods able to efficiently generate the required training data sets, we demonstrate this approach in the context of three very different problems, namely (i) near-field radiative heat transfer between multilayer systems that form hyperbolic metamaterials, (ii) passive radiate cooling in photonic crystal slab structures, and (iii) thermal emission of subwavelength objects. Despite their fundamental differences in nature, in all three cases we show that simple neural-network architectures trained with data sets of moderate size can be used as fast and accurate surrogates for doing numerical simulations, as well as engines for solving inverse design and optimization in the context of radiative heat transfer. Overall, our work shows that deep learning and artificial neural networks provide a valuable and versatile toolkit for advancing the field of thermal radiation.

Active-scattering-cancellation techniques provide a means of achieving electromagnetic cloaking without dealing with passivity-based design and performance limitations. However, a major drawback is that the cloak must be adjusted to accommodate a specific set of illumination conditions. Until recently, configuration has been performed with these conditions known a priori. Although adequate for initial validation, this presents a major obstacle to the development of a practical device that must contend with an unknown and dynamic electromagnetic environment. As a solution, this paper presents the development of an estimation algorithm that uses field measurements, sampled within the vicinity of the object to be cloaked, to deduce incident-wave properties. This information can then be used to adaptively reconfigure the cloak without any prior knowledge of its surroundings. In addition, the use of iterative solvers is avoided in estimator development, sidestepping convergence issues that afflict previous designs while reducing run time. After presenting the development process and operation principles, performance evaluation is conducted by using the algorithm to estimate the incident angle, signal magnitude, and phase offset of an incident wave impinging on either a circular or polygonal quasi-two-dimensional conductive target. Input data sets are initially provided via full-field simulation models and later supplanted by real-world field measurements.

The conversion and interaction between quantum signals at the single-photon level are essential for scalable quantum photonic information technology. Using a fully optimized periodically poled lithium niobate microring, we demonstrate ultraefficient sum-frequency generation on a chip. The external quantum efficiency reaches $(65\pm 3)\mathrm{\%}$ with only $(104\pm 4)$-$\mu \mathrm{W}$ pump power. At peak conversion, $3\times {10}^{-5}$-noise photon is created during the cavity lifetime, which meets the requirement of quantum applications using single-photon pulses. Using a pump and signal in single-photon coherent states, we directly measure the conversion probability produced by a single pump photon to be ${10}^{-5}$, which is a significant improvement from the state of the art, and the photon-photon coupling strength to be 9.1 MHz. Our results mark steady progress toward quantum nonlinear optics at the ultimate single-photon limit, with potential applications in highly integrated photonics and quantum optical computing.

Geometric phases accompanying adiabatic quantum evolutions can be used to construct robust quantum control for quantum information processing due to their noise-resilient feature. A significant development along this line is to construct geometric gates using nonadiabatic quantum evolutions to reduce errors due to decoherence. However, it has been shown that nonadiabatic geometric gates are not necessarily more robust than dynamical ones, in contrast to an intuitive expectation. Here we experimentally investigate this issue for the case of nonadiabatic holonomic quantum computation (NHQC) and show that conventional NHQC schemes cannot guarantee the expected robustness due to a cross-coupling to the states outside the computational space. We implement a different set of constraints for gate construction in order to suppress such cross-coupling to achieve an enhanced robustness. Using a superconducting quantum circuit, we demonstrate high-fidelity holonomic gates whose infidelity against quasistatic transverse errors can be suppressed up to the fourth order, instead of the second order in conventional NHQC and dynamical gates. In addition, we explicitly measure the accumulated dynamical phase due to the abovementioned cross-coupling and verify that it is indeed much reduced in our NHQC scheme. We further demonstrate a protocol for constructing two-qubit NHQC gates also with an enhanced robustness.

Although the origin of sub-band-gap electroluminescence has been extensively studied for organic light-emitting diodes (OLEDs) with a planar heterojunction of rubrene-fullerene (${\mathrm{C}}_{60}$), the dynamic behaviors of exciplexes formed at the $\mathrm{rubrene}/{\mathrm{C}}_{60}$ interface are still vague. Herein, employing magnetoconductance (MC) and magnetoelectroluminescence (MEL) detection techniques, we find an unreported reverse intersystem crossing (RISC, ${\mathrm{EX}}^3$ → ${\mathrm{EX}}^1$) from triplet to singlet exciplexes dominated at the $\mathrm{rubrene}/{\mathrm{C}}_{60}$ interface. Besides this RISC evolution channel, the triplet exciplexes also participate in another two processes: one is the scattering channel of triplet-charge annihilation (TCASC, ${\mathrm{EX}}^3$ + $\overleftarrow{e}$ → $S_0$+ ↓e) with excessive charge carriers, which is disadvantageous for RISC to occur, and the other is Dexter-energy transfer to produce a triplet exciton (${\mathrm{EX}}^3$ → $T_1$) in rubrene followed by triplet-triplet annihilation (TTA, $T_1$+ $T_1$ → $S_1$+ $S_0$ → hν↑ + 2$S_0$). This TTA is the physical origin of sub-band-gap EL because TTA needs no respective injection of electrons and holes into the LUMO and HOMO energy levels and requires no direct recombination of LUMO electrons with HOMO holes in rubrene. Moreover, both the bias-current dependences of MC and MEL traces reveal that the RISC process is more obvious in relatively balanced devices due to weak TCASC processes, and they show a monotonic decrease and nonmonotonic variation in unbalanced and relatively balanced $\mathrm{rubrene}/{\mathrm{C}}_{60}$-based OLEDs, respectively, because of the combined effects of the applied electrical field and the TCASC process. Theoretically, a stronger RISC process should be observed in devices by inserting a thin bathocuproine (BCP) interlayer to modify the $\mathrm{rubrene}/{\mathrm{C}}_{60}$ interface due to the increased separation distance between electron and hole in exciplex states, but the opposite experimental results are obtained because of strong TCASC channels simultaneously occurred with the insertion of a BCP interlayer. The RISC process decreases monotonically upon lowering the operational temperature due to its endothermic property. This work deepens our comprehensive understanding of the dynamic behaviors of exciplexes and the physical origin of sub-gap EL features in $\mathrm{rubrene}/{\mathrm{C}}_{60}$-based OLEDs.

The emissivity and absorptivity of nonreciprocal thermal emitters are not constrained by the well-known Kirchhoff law of thermal radiation, which usually serves as the theoretical basis to characterize thermal properties. When thermal emitters are nondiffracting, which is the case in previous studies of nonreciprocal thermal emitters, the angular distribution of emissivity and absorptivity is mirror symmetric with respect to the normal direction, and the nonreciprocal effect is confined to a relatively narrow angular range. In this work, we consider nonreciprocal thermal emitters that can simultaneously couple to multiple diffraction channels. We show that the symmetry relation between absorptivity and emissivity can be broken in such multichannel emitters. The angular range in which the nonreciprocal effect is strong can also be significantly broadened. Such multichannel emitters can operate as one-way energy splitters. Our work significantly broadens the flexibility of thermal-radiation control in nonreciprocal thermal-emitter design.