We report on the observation of a complex nonlinear intensity dependence of the circular and linear photogalvanic currents induced by infrared radiation in compensated (${\mathrm{Bi}}_{0.3}{\mathrm{Sb}}_{0.7}{)}_2$(${\mathrm{Te}}_{0.1}{\mathrm{Se}}_{0.9}{)}_3$ three-dimensional topological insulators. The photocurrents are induced by direct optical transitions between topological surface and bulk states. We show that an increase in the radiation intensity results first in a highly superlinear rise in the amplitude of both types of photocurrents, whereas at higher intensities the photocurrent saturates. Our analysis of the observed nonlinearities shows that the superlinear behavior of the photocurrents is caused by heating of the electron gas, while the saturation is induced by slow relaxation of the photoexcited carriers, resulting in absorbance bleaching. The observed nonlinearities give access to the Fermi-level position with respect to the Dirac point and the energy relaxation times of Dirac fermions, providing an experimental room-temperature probe of topological surface states.

Scholte modes that are localized between a submerged axisymmetric structured elastic plate and surrounding fluid can undergo mode conversion via Umklapp diffraction into radiative modes; this radiative response is verified by experiments that show focusing of underwater sound across a broad range of frequencies. The diffracted beams, that form a cone, are engineered to exist at a desired spatial position, associated with an abrupt change in the patterning of the plate. These structures take the form of grooves present only on one side of the plate, yet the focusing phenomena is achieved on both sides, even as viewed from the flat surface.

We present an analysis of gated $(\mathrm{In},\mathrm{Ga})\mathrm{As}$/$(\mathrm{In},\mathrm{Al})\mathrm{As}$ heterostructures, a device platform to realize spin-orbitronic functionalities in semiconductors. The phenomenological model deduced from our magnetotransport experiments allows us to correlate the gate response of the two-dimensional electron systems with the design parameters of the heterostructure, in particular the indium concentration. We explain the occurrence of metastable electrostatic configurations showing reduced capacitive coupling and provide gate-operation strategies to reach classical field-effect control in such heterostructures. Our study highlights the role of the intrinsic $(\mathrm{In},\mathrm{Al})\mathrm{As}$ deep donor defects, as they govern the dynamics of the electrostatic response to gate-voltage variations through charge trapping and unintentional tunneling.

A coupled magnetostructural transition (MST) from a high-temperature low-magnetic ${\mathrm{Ni}}_2\mathrm{In}$-type hexagonal phase to a low-temperature high-magnetic $\mathrm{Ti}\mathrm{Ni}\mathrm{Si}$-type orthorhombic phase, and the associated large magnetocaloric effect, are explored for the $({\mathrm{Mn}}_{0.6}{\mathrm{Fe}}_{0.4})({\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x)\mathrm{Si}$ system. Room-temperature structural analysis is carried out to investigate the orthorhombic distortion of the hexagonal phase initiated by introducing $\mathrm{Co}$ at the $\mathrm{Ni}$ site. The indirect isothermal magnetic measurement technique is employed to estimate the magnetocaloric effect across the coupled phase transition. Fascinatingly, a relatively large value of isothermal magnetic entropy change (Δ$S_{M}$) of (−12.02 ± 0.34) J ${\rm{kg}}^{-1}\;{\rm{K}}^{-1}$ at (332.5 ± 0.2) K and relative cooling power (RCP) of (119.6 ± 3.5) J ${\rm{kg}}^{-1}$ is achieved for the composition $({\mathrm{Mn}}_{0.6}{\mathrm{Fe}}_{0.4})({\mathrm{Ni}}_{0.6}{\mathrm{Co}}_{0.4})\mathrm{Si}$, for a field change of 30 kOe. The complex hysteresis behavior around the phase transition determines the efficiency of the material, which is revealed by both temperature- and field-sweep-rate dependence magnetization studies. Superheating (or supercooling) of the parent phase observed at higher temperature and field-sweep rates leads to broad thermal- and field-induced hysteresis. Lower sweep rates of temperature and magnetic field remove the quenching phenomenon across the MST and enhance the working efficiency. A large magnetocaloric effect and negligible variation of hysteresis with the temperature- and magnetic-field-sweep rates compared with other alloys makes this nominal low-cost material a promising candidate for practical solid-state cooling applications.

The goal of quantum metrology is the precise estimation of parameters using quantum properties such as entanglement. This estimation usually consists of three steps: state preparation, time evolution during which information of the parameters is encoded in the state, and readout of the state. Decoherence during the time evolution typically degrades the performance of quantum metrology and is considered to be one of the major obstacles to realizing entanglement-enhanced sensing. We show, however, that under suitable conditions, this decoherence can be exploited to improve the sensitivity. Assume that we have two axes, and our aim is to estimate the relative angle between them. Our results reveal that the use of Markovian collective dephasing to estimate the relative angle between the two directions affords Heisenberg-limited sensitivity. Moreover, our protocol based on Markovian collective dephasing is robust against environmental noise: it is possible to achieve the Heisenberg limit by applying the collective dephasing even under the effect of independent dephasing. Our counterintuitive proposal with the decoherence leads to alternative applications in quantum metrology.

For field emission (FE), it is widely expected that its emitting current density $J$ will become space-charge-limited current (SCLC) within a gap spacing $D$ biased at sufficiently large voltage $V$. In this paper, we reveal a peculiar finding in which this expected two-stage transition (from FE to SCLC) is no longer valid for FE not obeying the traditional Fowler-Nordheim (FN) law. By employing a generalized FN scaling of $\ln (J/E_s^k)\propto -1/E_s$, where $E_s$ is the surface electric field at the cathode, we show the existence of a critical exponent $k_c\equiv 3/2$ where unusual behaviors occur for $k<k_c$: (a) only FE at small $D$ (no transition to SCLC possible) and (b) three-stage transition from FE first to SCLC then back to FE at large $D$. For $k>k_c$, the conventional two-stage transition from FE to SCLC will always occur for all $D$, including the classical case of $k$ = 2. Using various unconventional FE models with $k\ne 2$, we specifically demonstrate these peculiar transitions. Under a normalized model, our findings uncover the rich interplay between the source-limited FE and bulk-limited SCLC over a wide range of operating conditions.

The emergence of a photonic band gap in $\mathrm{Ge}$-on-$\mathrm{Si}$ micropillars ordered in a two-dimensional square lattice is demonstrated by the finite-element method. Candidate architectures are fabricated through epitaxy and the opening of the photonic band gap experimentally proved by photoluminescence spectroscopy. When the direct-gap emission of $\mathrm{Ge}$ is resonantly driven into the photonic gap, light propagation in the lattice plane is inhibited. Emission is eventually funneled out of plane, yielding a giant increase, i.e., about one order of magnitude, in the observed intensity. The demonstration of light routing in microcrystals’ lattices opens interesting possibilities for $\mathrm{Si}$ photonics. The epitaxial self-assembled microstructures introduced here can be monotonically integrated on $\mathrm{Si}$ to improve the performances of group-IV lasers or engineered to optimize the working wavelength of future quantum photonic circuits.

We analyze the complex impact of the local magnetic spin texture on the transverse Hall-type voltage in device structures utilized to measure magnetoresistance effects. We find a highly localized and asymmetric magnetic sensitivity in the eight-terminal geometries that are frequently used in current-induced switching experiments, for instance, to probe antiferromagnetic materials. Using current-induced switching of antiferromagnetic $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$ as an example, we estimate the change in the spin Hall magnetoresistance signal associated with switching events based on the domain-switching patterns observed via direct imaging. This estimate correlates with the actual electrical data after subtraction of a nonmagnetic contribution. Here, the consistency of the correlation across three measurement geometries with fundamentally different switching patterns strongly indicates a magnetic origin of the measured and analyzed electrical signals.

Hyperbolic metamaterials (HMMs) support propagating waves with arbitrarily large wavevectors over broad spectral ranges, and are uniquely valuable for engineering radiative thermal transport in the near field. Here, by employing a rational-design approach based on the electromagnetic local density of states, we demonstrate the ability of HMMs to substantially rectify radiative heat flow. Our idea is to establish a forward-biased scenario where the two HMM-based terminals of a thermal diode feature overlapped hyperbolic bands which result in a large heat current, and suppress the reverse heat flow by creating spectrally mismatched density of states as the temperature bias is flipped. As an example, we present a few high-performance thermal diodes by pairing HMMs made of polar dielectrics and metal-to-insulator transition materials in the form of periodic nanowire arrays, and considering three representative kinds of substrates. Upon optimization, we theoretically achieve a rectification ratio of 324 at a 100-nm gap, which remains greater than 148 for larger gap sizes up to 1 µm over a wide temperature range. The maximum rectification represents an almost 1000-fold increase compared with a bulk diode using the same materials, and is twice that of state-of-the-art designs. Our work highlights the potential of HMMs for rectifying radiative heat flow, and may find applications in advanced thermal-management and energy-conversion systems.

Optical high-finesse cavities are a well-known means to enhance light-matter interactions. Despite much progress in the realization of strongly coupled light-matter systems, the controlled positioning of single solid emitters in cavity modes remains a challenge. We pursue the idea of using nanofibers with subwavelength diameter as a substrate for such emitters. This paper addresses the question of how strongly optical nanofibers influence the cavity modes. We analyze the influence of the fiber position for various fiber diameters on the finesse of the cavity and on the shape of the modes.

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.