Anisotropy in electronic structures may ignite intriguing anisotropic optical responses, as has been well demonstrated in various systems including superconductors, semiconductors, and even topological Weyl semimetals. Meanwhile, it is well established in metal optics that the metal reflectance declines from one to zero when the photon frequency is above the plasma frequency ${\omega }_p$, behaving as a plasma mirror. However, the exploration of anisotropic plasma mirrors and corresponding applications remains elusive, especially at room temperature. Here, we discover a pronounced anisotropic plasma reflectance edge in the type-II Weyl semimetal ${\mathrm{WP}}_2$, with an anisotropy ratio of ${\omega }_p$ up to 1.5. Such anisotropic plasma mirror behavior and its robustness against temperature promise optical device applications over a wide temperature range. For example, the high sensitivity of polarization-resolved plasma reflectance edge renders ${\mathrm{WP}}_2$ an inherent polarization detector. We further achieve a room-temperature ${\mathrm{WP}}_2$-based optical switch, effectively controlled by simply tuning the light polarization. These findings extend the frontiers of metal optics as a discipline and promise the design of multifunctional devices combining both topological and optical features.

How to measure the optical conductivity of atomically thin crystals is an important but challenging issue due to the weak light-matter interaction at the atomic scale. The photonic spin Hall effect, as a fundamental physical effect in light-matter interaction, is extremely sensitive to the optical conductivity of atomically thin crystals. Here we report a precision measurement of the optical conductivity of graphene, where the photonic spin Hall effect acts as a measurement pointer. By use of the weak-value-amplification technique, the optical conductivity of monolayer graphene taken as a universal constant of $(0.993\pm 0.005){\sigma }_0$ is detected, and a high measurement resolution of $1.5\times {10}^{-8}{\mathrm{\Omega }}^{-1}$ is obtained. For few-layer graphene without a twist, we find that the conductivities increase linearly with the number of layers. Our idea could provide an important measurement technique for probing other parameters of atomically thin crystals, such as the magneto-optical constant, circular dichroism, and optical nonlinear coefficient.

We experimentally and numerically demonstrate nonlinear flexural wave propagation using an on-chip nanoelectromechanical waveguide. More specifically, third-order nonlinear phenomena of self-phase modulation, cross-phase modulation, and four-wave mixing caused by the mechanical Kerr effect are realized. Our experimental observations are correctly described by the nonlinear Schrödinger equation. The nanomechanical and integratable platform enables on-chip manipulation of mechanical wave propagation, thus offering the potential to develop novel functional devices and study nonlinear mechanical phenomena.

It is well known that surface codes are favorable candidates toward realizing large-scale fault-tolerant quantum computation. While methods to reduce errors by surface codes have been well explored, it is still challenging to achieve small-enough errors as stated by the threshold theorem in practical experimental settings. This prompts the integration of surface codes and protected quantum operations to further mitigate the negative effect of errors, creating the challenge of executing protected surface codes. In this work, we propose one scheme to perform holonomic quantum operations in a physical system by use of auxiliary qubits. The system Hamiltonian describes a quantum spin model with adjustable coupling strength. Various noncommutable single-qubit and nontrivial two-qubit holonomic gates are applicable on the basis of the interaction between the auxiliary qubits and physical qubits. By considering a two-dimensional lattice of qubits, we explore the execution of surface codes. The utilization rate of physical qubits as data qubits is about $24.7\mathrm{\%}$ in executing the protected surface codes. Our scheme is based on an experimentally achievable Hamiltonian and thus it brings us closer to realizing protected quantum operations on quantum-error-correction codes with a modest resource.

The discovery of the disorder effect in traditional metamaterials has opened up possibilities in the search for disordered metasurfaces. Photonic, dielectric, and elastic metamaterials that exhibit added value of the disorder effect on wave-propagation physics are reported. Despite this extensive attention and progress in disordered metamaterials, elastic metasurfaces, however, involving disorder have not yet been reported. Here, we introduce the concept of disordered elastic metasurface composed of identical pillared resonators with a random arrangement in the subwavelength range. Based on theoretical formalism and direct acoustic measurement, we observe anomalous deflection and focusing effects of flexural waves in a plate and elucidate, in detail, the related physics. This research extends the disorder effect to metasurfaces and may lead to innovative acoustoelastic devices.

As the counterpart of $\mathcal{P}\mathcal{T}$ symmetry, for anti-$\mathcal{P}\mathcal{T}$-symmetry abundant phenomena and potential applications have been predicted or demonstrated theoretically. However, experimental realization of the coupling required in anti-$\mathcal{P}\mathcal{T}$ symmetry is difficult. By our coupling two yttrium iron garnet spheres commonly to a microwave cavity, the large cavity dissipation rate makes the magnon-magnon coupling dissipative and purely imaginary. Thereby, the hybrid magnon-cavity system obeys a two-dimensional anti-$\mathcal{P}\mathcal{T}$ Hamiltonian. In terms of the magnon-readout method, the method adopted here, we demonstrate the validity of our method in constructing an anti-$\mathcal{P}\mathcal{T}$ system and present the counterintuitive level-attraction process. Our work provides a platform to explore the anti-$\mathcal{P}\mathcal{T}$-symmetry properties and paves the way to study dynamical evolution and topological properties around exceptional points in multimagnon-cavity systems.

Organic-inorganic hybrid halide perovskites have ignited tremendous interest for photovoltaic applications. However, their nonlinear optical response has not been studied although many of these structures lack the centrosymmetry and exhibit ferroelectricity. In this work, we employ our developed large-scale parallel, first-principles simulation tool (ArchNLO) to explore second-order nonlinear optical properties of a typical family of organic-inorganic hybrid halide perovskites, ${\mathrm{CH}}_3{\mathrm{NH}}_{3}M{X}_3$ (M = $\mathrm{Ge}$, $\mathrm{Sn}$, $\mathrm{Pb}$; X = $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{I}$). We find that these hybrid perovskites exhibit second harmonic generation and a linear electro-optic effect. The nonlinear optical effects are strongly influenced by the types and positions of cations and anions, and the corresponding band gaps. In particular, the distorted cubic phase, which is essentially triclinic, of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{Sn}\mathrm{I}}_3$ shows significant second harmonic generation and an electro-optic effect, which are comparable with more widely used materials, such as $\mathrm{Ga}\mathrm{As}$. These second-order optical properties of organic-inorganic hybrid halide perovskites and their low-temperature, solution-based fabrication pave the way for achieving and implementing nonlinear optical devices with low cost.

The coupling between a superconducting qubit and a control line inevitably results in radiative decay of the qubit into the line. We propose a Josephson quantum filter (JQF), which protects the data qubit (DQ) from radiative decay through the control line without reducing the gate speed on the DQ. The JQF consists of a qubit strongly coupled to the control line of the DQ and its working principle is a subradiance effect that is characteristic of waveguide quantum electrodynamics setups. The JQF is a passive circuit element and is therefore suitable for integration in a scalable superconducting qubit system.

We experimentally study the transition to large oscillatory motions of a thin elastic plate buckled into a second mode sinusoidal deformation. This deformation, intended to promote the onset of structural instabilities, is created by applying a permanent external net compressive force to the plate through a series of equispaced tension lines. These lines are attached to the two lateral edges of the plate, which are oriented parallel to the mean flow direction. We subject the deformed plate to an incompressible flow and find an early onset of the flutter instability (lower critical flow velocity, $U_c$) and increased range of periodic large oscillatory motions relative to previous studies of plates mounted in flag or inverted-flag configurations. No upstream vortex shedding or additional transversal rigidity is provided to achieve this lower $U_c$, which are methods used by previous researchers to initiate instability. The reduced $U_c$ is instead achieved by the combination of three mechanisms: a deformed plate that exhibits bistability, the destabilizing action of the net compressive force on the plate, and the pressure drag on the plate existent prior to large plate oscillations. Subjected to a mean flow, $U_o$, the deformed plate evinces three states of motion: (a) a buckled plate with small perturbations for $U_o<U_c$ (perturbations are of the order of 0.1% of the plate’s length); (b) a plate with large oscillatory motions appearing as the superposition of traveling and standing waves for $U_o>U_c$, showing fundamental oscillatory frequencies $f_o$, which scale with $U_o$, along with higher harmonics; and (c) a plate with chaotic oscillatory motions for $U_o>U_t$, a transitional flow velocity where $U_t>U_c$, characterized by a broad frequency spectrum still with a characteristic $f_o$, and slightly reduced relationship between $U_o$ and $f_o$.

We investigate the dynamics of a microelectromechanical self-sustained oscillator supporting multiple resonating and interacting modes. In particular, the interaction of the first four flexural modes along with the first torsional mode are studied, whereby 1:2, 1:3, and 2:1 internal resonances occur. Even and odd modes are induced to couple by breaking the longitudinal symmetry of the structure. Self-oscillations are induced in the second flexural mode via a gain-feedback loop; thereafter its frequency is pulled into a commensurate frequency ratio with the other modes, enabling the oscillator to act as a driver or pump for four modes simultaneously.

A simple scheme for a broadband polarization rotator with tunable rotation angle is proposed and verified experimentally. The rotator consists of only three wave plates, one of which is a full wave plate. The robust approach inspired by the composite pulses analogy allows us to compensate for the wave plate dispersion to a large extent.

There have been considerable efforts devoted to the study of topological phases in certain non-Hermitian systems that possess real eigenfrequencies in the presence of gain and loss. However, it is challenging to experimentally realize such non-Hermitian topological insulators in either quantum or photonic systems, due to difficulties in introducing controlled gain and loss. On the other hand, the wide choices of active circuit components provide us with unprecedented convenience and flexibility in engineering non-Hermitian topological insulators in electrical circuits. Here, we report on the experimental realization of a one-dimensional non-Hermitian topological circuit that exhibits topologically protected edge states purely induced by gain and loss. We show that, by tuning the value of the positive and negative resistors in the circuit, our system can switch between different topological phase regions. The topological edge states and interface states are observed at the circuit edge and at the interface between a trivial and nontrivial circuit; these states are manifested by a prominent impedance peak at the midgap frequency, which is topologically robust to variations of circuit parameters. Our work opens up an alternative gateway towards actively controllable topological systems.

By harnessing the rich dynamics associated with nonhomogeneous magnetization configurations in confined nanostructures, the free layer of a magnetic tunnel junction (MTJ) can be forced, via a localized magnetic field, to transition between two magnetic states: the quasi-uniform and vortex states. Here, we demonstrate that such transitions can be driven back and forth to a dynamic equilibrium by exciting a MTJ with ac magnetic fields close to the resonant modes of the magnetic vortex. The frequency dependence of the dynamic-state transitions leads to a proposal for an alternative type of broadband analogue frequency-to-resistance converter (FRC). In this report, two quasilinear bands of the FRC are demonstrated: a relatively lower frequency converter operating between 200 and 350 MHz with 250-kHz resolution (∼500 channels) and a higher frequency converter operating between 2.5 and 6 GHz with a 2.5-MHz resolution (∼800 channels). The fact that a single CMOS-compatible nanoscale device can perform such a complex function over a multitude of frequency bands, with high-frequency resolution relative to that of conventional CMOS circuits, can enable the next generation of wireless communication circuit topologies.

We study the influence of inverse biases applied before and after a write pulse on the write-error rate (WER) of voltage-driven magnetization switching in magnetic tunnel junctions. It is demonstrated that the inverse bias applied after the write pulse is effective not only for improving the minimum value of the WER but also for broadening the operating pulse-width window. In contrast, an inverse bias applied before the write pulse hardly improves the WER or even narrows the operating window. Numerical simulations based on the macrospin approximation reproduce these experimental results well and a detailed analysis of the magnetization trajectory reveals that the observed write-error increase is due to a change in the magnetization angle before writing. These experimental results, as well as the numerical analysis, should facilitate the development of writing schemes in voltage-driven magnetic random-access memories.

The excitation efficiency and external luminescence quantum efficiency of trivalent ${\mathrm{Eu}}^{3+}$ ions doped into gallium nitride ($\mathrm{Ga}\mathrm{N}$) is studied under optical and electrical excitation. For small pump fluences, the excitation of ${\mathrm{Eu}}^{3+}$ ions is limited by an efficient carrier trap that competes in energy transfer from the host material. For large pump fluences, the limited number of high-efficiency ${\mathrm{Eu}}^{3+}$ centers and the small excitation cross section of the majority ${\mathrm{Eu}}^{3+}$ centers limit the quantum efficiency. At low temperatures, under optimal excitation conditions, the external luminescence quantum efficiency reaches a value of 46%. These results show the high potential for this material as an efficient light emitter and demonstrate the importance of excitation conditions on the light-output efficiency.

Superconductor-ferromagnet (S/F) hybrid systems show interesting magnetotransport behaviors that result from the transfer of properties between both constituents. For instance, magnetic memory can be transferred from the F into the S through the pinning of superconducting vortices by the ferromagnetic textures. The ability to tailor this type of induced behavior is important to broaden its range of application. Here we show that engineering the F magnetization reversal allows the tuning of the strength of the vortex pinning (and memory) effects, as well as the field range in which they appear. This is done by using magnetic multilayers in which Co thin films are combined with different heavy metals ($\mathrm{Ru}$, $\mathrm{Ir}$, $\mathrm{Pt}$). By choosing the materials, thicknesses, and stacking order of the layers, we can design the characteristic domain size and morphology, from out-of-plane magnetized stripe domains to much smaller magnetic skyrmions. These changes strongly affect the magnetotransport properties. The underlying mechanisms are identified by comparing the experimental results to a magnetic pinning model.

Compared with traditional bulk materials, two-dimensional materials can exhibit exotic optoelectronic properties and especially provide large photoreactive contact areas, making them more attractive for designing alternative optoelectronic devices. In this work, we use first-principles methods based on density-functional theory to study the electronic and optical properties of few-layer $\beta$-$\mathrm{Sn}\mathrm{Se}$. It is found that single-layer, double-layer, and triple-layer $\beta$-$\mathrm{Sn}\mathrm{Se}$ are semiconducting with direct band gaps of 1.38, 1.20, and 1.05 eV, respectively, which fall within the optimum band gap for solar cells. For triple-layer $\beta$-$\mathrm{Sn}\mathrm{Se}$, the optical absorbance reaches 56% and the upper limit of the energy-conversion efficiency is 15.4%, which is comparable to the current efficiency record. Furthermore, few-layer $\beta$-$\mathrm{Sn}\mathrm{Se}$ has very high carrier mobility, reaching ${10}^7{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ for triple-layer $\beta$-$\mathrm{Sn}\mathrm{Se}$. The strong visible-light absorption and high carrier mobility of few-layer $\beta$-$\mathrm{Sn}\mathrm{Se}$ provide promising opportunities for applications in solar cells.

We present recent achievements in the application of optical diffraction radiation (ODR) to the measurement of the transverse beam size of a 1.3 GeV micrometer-size electron beam performed with an instrument installed in the extended extraction line of the KEK Accelerator Test Facility. ODR is a recent technique for the measurement of the transverse size and emittance of highly relativistic particle beams. ODR has the advantage of being a noninvasive and relatively inexpensive technique and is a candidate for the operation of linear particle accelerators, where no simple alternatives (e.g., synchrotron radiation) are available. In an effort to improve the resolution and performance, we establish an alternative target microfabrication technology, adopt solutions to improve the signal-to-noise ratio, and perform an experiment in the UV spectrum at 250 nm. With such a configuration, a transverse beam size as low as $3\mu \mathrm{m}$ is measured, drastically improving on past measurements reported in the literature. In light of these results, ODR, when combined with a high-resolution optical-transition-radiation monitor, represents a credible diagnostics solution for low-emittance high-intensity particle beams.

Two-dimensional (2D) transition-metal dichalcogenide (TMD) monolayers, which reveal remarkable semiconductor properties, are the subject of active experimental research. It should be noted that, unlike bulk TMDs, which are indirect-band semiconductors, 2D TMD monolayers have the extreme points of the conduction and valence bands at the same K and K′ points of the Brillouin zone. Therefore, they are direct-band semiconductors and can claim to be widely used in optoelectronics devices. Recently, it has been shown experimentally that quantum yield in ${\mathrm{Mo}\mathrm{S}}_2$ and ${\mathrm{W}\mathrm{Se}}_2$ monoatomic layers can reach values close to unity when electrostatic doping makes them intrinsic semiconductors. However, the available theoretical description does not provide an understanding of the physical mechanisms underlying the gate voltage control of quantum yield. This work is an attempt to propose a consistent semi-phenomenological theory of photoinduced charge-carrier relaxation in 2D TMDs, which allows an analytical dependence of the quantum yield on the voltage applied to the field effect transistor gate to be obtained. We consider a standard experimental situation, when the 2D TMD monolayer and the metal gate are plates of a flat capacitor, and the capacitor charge is proportional to the gate voltage. Owing to very strong electron-hole interaction in TMD monolayers, quantum yield on the gate voltage and the carrier generation rate have been calculated for the cases of the prevailing recombination of excitons (radiative and Auger recombination). Analytical expressions are derived for the dependence of quantum yield on the gate voltage and photoinduced carriers generation rate at a fixed gate voltage. Quantitative agreement with experiment allows us to draw conclusions about the relevance of the proposed theoretical model for the description of carrier photogeneration and recombination in 2D TMD monolayers. The obtained results demonstrate the possibilities of 2D TMD quantum yield control by the gate voltage and indicate that 2D TMDs are promising candidates for modern optoelectronics devices.

We study the electrodynamic impedance of percolating conductors with a predefined network topology using a scanning microwave impedance microscope at gigahertz frequencies. For a given percolation number we observe strong spatial variations across a sample that correlate with the connected regions (clusters) in the network when the resistivity is low such as in aluminum. For the more-resistive material ($\mathrm{Nb}$,$\mathrm{Ti}$)$\mathrm{N}$, the impedance becomes dominated by the local structure of the percolating network (connectivity). The results can be qualitatively understood and reproduced with a network current-spreading model based on the pseudoinverse Laplacian of the underlying network graph.