The enhancement of stimulated Raman scattering (SRS) with a $\mathrm{Ga}\mathrm{As}$/$\mathrm{Al}\mathrm{As}$ intermixed superlattice that works as a ${\chi }^{(3)}$-quasi-phase-matched structure is studied, where such Kerr-induced effects as four-wave mixing (FWM), self-phase-modulation (SPM), cross-phase-modulation (XPM), and two-photon absorption (TPA) are included. In particular, the efficiency of anti-Stokes generation is enhanced here; anti-Stokes generation inherently has an extremely small efficiency due to a phase mismatch in the interaction of the pump, Stokes, and anti-Stokes waves (while the efficiency of Stokes generation is sufficiently large because of no such phase mismatch). The superlattice enhances the anti-Stokes efficiency up to the order of ${10}^3$ when compared with that without the superlattice, particularly at a small pump intensity. In this enhancement, it is seen that there is an efficiency boost via simultaneous FWM. In this situation, it is shown how much SPM and XPM degrade the efficiency enhancement. Furthermore, an optimal superlattice length is identified that provides the highest efficiency. The degradation of the efficiency at the optimized length due to TPA is also analyzed. Finally, to gain more anti-Stokes efficiency (or control the sizes of the Stokes and anti-Stokes efficiencies), a photonic-band-gap cavity structure is proposed.

The Rayleigh law describes the linear dependence of the permittivity of a ferroelectric on the applied ac electric field amplitude due to irreversible motions of domain walls. We show that this gives rise to a new equivalent-circuit element predestined to fit the impedance spectra of ferroelectrics based on an accepted physical model. Such impedance spectroscopy is a powerful tool to obtain a dielectric and resistive representation of the entire sample structure. The superiority of the Rayleigh analysis based on impedance spectroscopy compared to the common single-frequency approach is demonstrated for a ferroelectric $\mathrm{Si}:{\mathrm{Hf}\mathrm{O}}_2$ thin film.

We investigate the magnetization dynamics of a pair of ferromagnetic insulators (FMIs) deposited on the surface of a topological insulator (TI). Because of the nonlinear nature of the underlying physics and intrinsic dynamics, the FMIs can exhibit oscillatory behaviors even under a constant applied voltage. The motion of the surface electrons of the TI, which obeys relativistic quantum mechanics, provides a mechanism of direct coupling between the FMIs. In particular, the spin-polarized current of the TI surface electrons can affect the magnetization of the two FMIs, which in turn modulates the electron transport, giving rise to a hybrid relativistic quantum and classical nonlinear dynamical system. We find robust phase and antiphase locking between the magnetization dynamics. As driving the surface electrons of a TI requires only extremely low power, our finding suggests that nanoscale FMIs coupled by a spin-polarized current on the surface of a TI have the potential to serve as the fundamental building blocks of unconventional, low-power computing paradigms.

We propose to achieve wideband, high-efficiency polarization conversions of transmission waves with the synergy of the waveguide and spoof surface plasmon polariton (SSPP) modes. High-efficiency transmissions based on SSPP mode coupling and waveguide mode coupling are designed in two orthogonal directions, respectively. By designing the nonlinear dispersions of the SSPP and waveguide modes, the required phase differences for polarization conversion can be obtained in a wide frequency range. To verify this principle, two different polarization conversion metamaterials (PCMs) are designed and fabricated for linear-to-circular (LTC) and circular-to-circular (CTC) polarization conversions, respectively. It is found that the LTC polarization conversion transmittivity of the LTC PCM is greater than −0.5 dB from 7 to 12.6 GHz, and the cross-polarization transmittivity of the CTC PCM for a circularly polarized incidence wave is greater than −0.5 dB in the frequency range of 7–11 GHz, showing a wide band feature.

We consider the generalized Shockley-Queisser (GSQ) model, which is based on a single assumption that photocarriers and emitted photons are in chemical equilibrium and described by the Boltzmann distribution functions with the same chemical potential. The model takes into account the frequency-dependent absorption (emission), photon trapping and recycling, photocarrier multiplication, and nonradiative recombination processes. For the noninteracting photocarriers, we obtain an exact analytical solution of the GSQ model. We present the conversion efficiency and other photovoltaic (PV) characteristics in a convenient form via the Lambert $W$ function. Photocarrier multiplication and recombination via three-body Auger processes are also directly included in this formalism. We derive universal formulas for useful energy, thermal losses, and emission losses per absorbed photon. We show that the relation between the maximal conversion efficiency and the photo-induced chemical potential, obtained by Henry [J. Appl. Phys. 51, 4494 (1980)] for the ideal SQ limit, is also valid in the GSQ model. In the general case of interacting electrons, in particular for the Shockley-Read-Hall processes, the solution is presented in an iterative form. We analyze photocarrier kinetics and derive a general relation between the optimal photocarrier collection time and photocarrier lifetime with respect to all radiative and nonradiative processes. Finally, we analyze finite mobility limitations and show that PV devices with photon trapping and recycling provide the fast photocarrier collection required by the GSQ model.

The possibility of affecting the magnetic properties of a material by dielectric means, and vice versa, remains an attractive perspective for modern electronics and spintronics. Here, we report on epitaxial $\mathrm{Bi}({\mathrm{Fe}}_{0.5}{\mathrm{Mn}}_{0.5}){\mathrm{O}}_3$ thin films with exceptionally low Gilbert damping and magnetoelectric coupling above room temperature ($<400$ K). The ferromagnetic order, not observed in bulk, has been detected with a total magnetization of 0.44 ${\mu }_B/$formula units with low Gilbert damping parameter (0.0034), both at room temperature. Additionally, a previously overlooked check-board ordering of oxygen vacancies is observed, providing insights on the magnetic and dielectric origin of the multifunctional properties of the films. Finally, intrinsic magnetodielectric behavior is observed as revealed by the variation of dielectric permittivity well above room temperature. These findings show the possibility of electric-field-controlled magnetic properties, in low Gilbert-damping-based spintronic devices, using single-phase multiferroic materials.

Recent interest in topological quantum computing has driven research into topological nanowires, one-dimensional quantum wires that support topological modes, including Majorana fermions. Most topological nanowire designs rely on materials with strong spin-orbit coupling, such as $\mathrm{In}\mathrm{As}$ or $\mathrm{In}\mathrm{Sb}$, used in combination with superconductors. It would be advantageous to fabricate topological nanowires with $\mathrm{Si}$ owing to its mature technology. However, the intrinsic spin-orbit coupling in $\mathrm{Si}$ is weak. One approach that could circumvent this material deficiency is to rotate the electron spins with nanomagnets. Here we perform detailed simulations of realistic $\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$ systems with an artificial spin-orbit gap induced by a nanomagnet array. Most of our results are generalizable to other nanomagnet-based topological nanowire designs. By studying several concrete examples, we gain insight into the effects of nanomagnet arrays, leading to design rules and guidelines. In particular, we develop a recipe for eliminating unwanted gaps that result from realistic nanomagnet designs. Finally, we present an experimentally realizable design using magnets with a single polarization.

Phase-change materials are attracting much attention in the scientific and engineering communities owing to their applications and underlying basic phenomena. ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ is reversible-phase-change material (amorphous to crystalline and vice versa) that is used for optical data storage and phase-change random-access memory and has recently been explored for use as a reversible near-infrared (NIR) window [Singh et al., Appl. Phys. Lett. 111, 261102 (2017)]. For a reversible NIR window, large transmission contrast between two phases and low phase-transition temperature are required to reduce the power consumption. In the present work, phase transition in thermally deposited $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{100-x}{\mathrm{Ag}}_x$ ($x=0$, 1, 3, 5, and 10) thin films is achieved by vacuum thermal annealing. Transmission sharply decreases with phase transition in the NIR region. ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ shows large transmission contrast (more than $50$%) in the wavelength range from 1600 to 3200 nm with phase transition from an amorphous to a hexagonal-close-packed structure at $260{}^{\circ }\mathrm{C}$. In $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{90}{\mathrm{Ag}}_{10}$ thin films, a similar transmission contrast is achieved at a comparatively lower temperature ($160{}^{\circ }\mathrm{C}$) due to reduction of the rocksalt phase. Distortion of the host lattice with addition of 10% $\mathrm{Ag}$ is confirmed from the drastic change in the density of states in the valence band and the shift in core-level (3d) spectra of $\mathrm{Ag}$, $\mathrm{Sb}$, and $\mathrm{Te}$. This distortion enables a hexagonal-close-packed phase in $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{90}{\mathrm{Ag}}_{10}$ thin films to be obtained at $160{}^{\circ }\mathrm{C}$. $({\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5{)}_{90}{\mathrm{Ag}}_{10}$ could be a potential candidate for a reversible NIR window as it requires less power to achieve phase transition and high transmission contrast.

Routing electromagnetic energy at a scale smaller than the wavelength is a highly sought functionality in a variety of applications, including compact lightweight satellite communications, slow-waves sensors, all-optical information processing, and energy harvesting. Unfortunately, strong field confinement at this scale requires the use of coupled subwavelength resonators, implying a large sensitivity to geometrical imperfections and disorder-induced backscattering. We propose a very unconventional solution to this problem by exploiting the interface modes occurring at the boundary between two chiral metamaterials composed of resonant metamolecules with opposite chirality. Our numerical and experimental results demonstrate the inherent robustness of these interface states to disorder in both the position and resonance frequency of the metamaterial’s meta-atoms. By computing transmission averages over many realizations of disorder, we quantitatively demonstrate the superiority of this form of subwavelength routing over previously proposed designs, including frequency-defect lines, symmetry-based topological edge modes, and Valley-Hall interface states.

Quasiballistic heat conduction from nanoscale heat sources of size comparable to phonon mean free paths has recently become of intense interest both scientifically and for its applications. Prior work has established that, in the quasiballistic regime, the apparent thermal properties of materials depend both on intrinsic mechanisms and the characteristics of the applied thermal gradient. However, many aspects of this regime remain poorly understood. Here, we experimentally study the thermal response of crystals to large thermal gradients generated by optical heating of nanoline arrays. Our experiments reveal the key role of the spatial frequencies and Fourier series amplitudes of the heating profile for thermal transport in the quasiballistic regime, in contrast to the conventional picture that focuses on the geometric dimensions of the individual heaters. Our work provides the insight needed to rationally mitigate local hot spots in modern applications by manipulating the spatial frequencies of the heater patterns.

We provide an in-depth characterization of the dislocation distribution in partially relaxed ${\mathrm{Si}}_{0.92}{\mathrm{Ge}}_{0.08}/\mathrm{Si}(001)$ films. This is achieved by an innovative and general method, combining two state-of-the-art characterization techniques through suitable modeling. After having inferred the dislocation positions from transmission-electron-microscopy images, we theoretically reproduce scanning-x-ray-diffraction-microscopy tilt maps measured on the very same region of the sample. We obtain a nearly perfect match between model predictions and experimental data. As a result, we claim that it is possible to establish a local, direct correlation between the dislocations revealed by the transmission-electron-microscopy analysis and the measured lattice tilt distribution.

Previously reported acoustic metasurfaces consisting of fixed channels are untunable to meet broadband requirements and alterable functionalities. To overcome this limitation, we propose a screw-and-nut mechanism of tunability and design a type of continuously tunable acoustic metasurface with unit components of helical cylinders which are screwed into a plate. The spiral channel length can be tuned continuously by the screwed depth, and thus a metasurface with a continuously tunable acoustic phase of independent pixels is attained. Different distributions of the unit components can form an arbitrary metasurface profile. We also develop an approximate equivalent medium model to predict the tunability of the unit component. As an example, we present the design details of a circular tunable metasurface for three-dimensional (3D) adjustable broadband acoustic focusing of different airborne sound sources in a wide frequency range. A sample of the metasurface is manufactured by polylactic acid plastic with 3D-printed helical cylinders. Experiments of the adjustable acoustic focusing are performed to validate the equivalent medium model and the finite element simulation. It is shown that the results of the equivalent medium model, the finite element simulation, and the experiments are in good agreement.

Viral capsid proteins have the remarkable ability to self-assemble around a cargo core, whether their genome, a polyelectrolyte or an inorganic nanoparticle. Although the equilibrium properties of gold nanoparticles encapsulated in viral capsids have been broadly investigated, little is known about the kinetic pathways leading to their assembly. Through the combination of time-resolved absorbance spectroscopy and small-angle x-ray scattering, we elucidate a three-step mechanism with timescales ranging from hundred milliseconds to hours. By modeling the scattering intensities, we deduce the average structure and the time evolution of transient, amorphous aggregates held in a matrix of capsid proteins. The biocompatibility of metal nanoparticles induced by adsorbed proteins is of great importance for their use in biomedicine, and the reported results will help better understand the formation process.

Van der Waals heterojunctions (vdWHs) have gained extensive attention because they can integrate the excellent characteristics of the stacked materials and most vdWHs exhibit type-II band alignment. However, type-III vdWHs with broken gaps are still very rare, which limits the development and application of two-dimensional (2D) materials in the fields of tunnel FETs (TFETs). Here, we theoretically demonstrate that 2D phosphorene/${\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) vdWHs possess type-III (broken-gap) band alignment, and their I-V curves present negative differential resistance (NDR) effects. The BTBT transport mechanism and its applications in TFETs are analyzed. Interestingly, a positive electric field can enlarge the tunnelling window and a negative electric field can realize multiple-band-alignment transformation (type I, type II, and type III). Thus, this work presents the intrinsic physics mechanism and electric field tunable multiple-band alignments in 2D type-III vdWHs and related electronic devices.

The Meyer-Neldel (MN) compensation rule, implying an exponential increase in the prefactor with increasing activation energy in a thermally activated process, is naturally emerging in two-site transition rates as a result of multiphonon excitation processes. However, it has been recently demonstrated [Phys. Rev. B. 90, 245201 (2014)] that the experimentally observed compensation behavior for the temperature-activated charge transport in thin-film organic field-effect transistors (OFETs) is not a genuine phenomenon, but rather it is an apparent extrapolated effect that arises as a consequence of the partial filling of the Gaussian DOS distribution. To resolve the contradiction, we investigate the impact of different jump-rate models on macroscopic hopping-charge transport in a random organic system using an effective medium analytic approach. The principal result of this study is that the averaging over the individual jump rates in a conventional Gaussian disordered system erodes the genuine thermodynamically determined MN compensation effect, and therefore, the macroscopic transport no longer reflects the microscopic rates. The apparent compensation behavior observed for OFET mobilities upon varying the carrier concentrations can be reproduced regardless of the single-phonon or multiphonon character of activated transitions. Another remarkable finding is that the disorder formalism does predict a genuine MN compensation effect using multiphonon rates if a disordered semiconductor contains a significant concentration of deep traps, so that the cumulative DOS features a double-peak Gaussian. Thus, this study bridges the gap between Gaussian disorder and multi-excitation entropy (MEE) models concerning the MN effect, and has important implications for the interpretation of the isokinetic MN temperature in disordered organic semiconductors.

The prospect of computational hardware with quantum advantage relies critically on the quality of quantum-gate operations. Imperfect two-qubit gates are a major bottleneck for achieving scalable quantum-information processors. Here, we propose a generalizable and extensible scheme for a two-qubit tunable coupler that controls the qubit-qubit coupling by modulating the coupler frequency. Two-qubit gate operations can be implemented by operating the coupler in the dispersive regime, which is noninvasive to the qubit states. We investigate the performance of the scheme by simulating a universal two-qubit gate on a superconducting quantum circuit, and find that errors from known parasitic effects are strongly suppressed. The scheme is compatible with existing high-coherence hardware, thereby promising a higher gate fidelity with current technologies.

Ultrafast electron diffraction (UED) with high temporal and spatial resolutions is a powerful tool to observe transient structural changes in materials on an atomic scale. This technique is based on a pump-probe method using ultrashort laser and electron pulses. Therefore, UED requires that the measured transients be highly repeatable. Moreover, the relative time jitter between laser and electron pulses significantly affects the UED temporal resolution. To overcome the UED technical limitations, we propose a technique called compressed ultrafast electron diffraction imaging (CUEDI). In this technique, we encode time-evolving electron diffraction patterns with random codes on an electron encoder. Then, the encoded electron diffraction pattern is measured by a detector after a temporal shearing operation. Finally, the evolution process of the electron diffraction pattern is reconstructed using a compressed sensing algorithm. We confirm the feasibility of our proposed scheme by numerically simulating the polycrystalline gold melting process based on the experimental data measured with the pump-probe method. Because CUEDI employs a continuous or long electron pulse, the relative time jitter between laser and electron pulses can be eliminated. Additionally, CUEDI measures transients with a single shot, which allows irreversible processes to be directly observed.

Phononic crystals and acoustic metamaterials are architected lattices designed to control the propagation of acoustic or elastic waves. In these materials, the dispersion properties and the energy transfer are controlled by selecting the geometry of the lattices and their constitutive material properties. Most designs, however, only affect one mode of energy propagation, transmitted either as acoustic airborne sound or as elastic structural vibrations. Here, we present a design methodology to attenuate both acoustic and elastic waves simultaneously in all polarizations. We experimentally realize a three-dimensional load-bearing architected lattice, composed of a single material, that responds in a broadband frequency range in all directions and polarizations for airborne sound and elastic vibrations simultaneously.

Dynamical decoupling sequences with multiple pulses can be considered to exhibit filter functions for the time evolution of a qubit superposition state. They contribute to the increase of coherence time and qubit-phase accumulation due to a time-varying field and can thus be used to achieve high-frequency-resolution spectroscopy. Such behaviors find useful application in highly sensitive detection based on qubits for various external fields, such as a magnetic field. Hence, decoupling sequences are indispensable tools for quantum sensing. In this study, we experimentally and theoretically investigate the effects of finite-width pulses in the sequences on ac magnetometry using nitrogen-vacancy centers in an isotopically controlled diamond. We reveal that the finite pulse widths cause a deviation of the optimum time to acquire the largest phase accumulation due to the sensing field from that expected by filter functions ignoring the pulse widths, even if the widths are considerably shorter than the time period of the sensing field. Moreover, we experimentally demonstrate that the deviation can be corrected by an appropriate time-frequency conversion. Our results provide a guideline for the detection of an ac field with an accurate frequency and linewidth in quantum sensing with multiple-pulse sequences.

As the hyperentanglement of photon systems presents lots of unique opportunities in high-capacity quantum networking, the hyperentanglement purification protocol (hyper-EPP) becomes a vital project work and the quality of its accomplishment attracts considerable attention recently. Here we present the first theoretical scheme of faithful hyper-EPP for nonlocal two-photon systems in two degrees of freedom (DOFs) by constructing several fidelity-robust quantum circuits for hyper-encoded photons. With this faithful hyper-EPP, the bit-flip errors in both the polarization and spatial-mode DOFs can be efficiently corrected and the maximal hyperentanglement in two DOFs could in principle be achieved by performing the hyper-EPP multiple rounds. Moreover, the fidelity-robust quantum circuits, parity-check quantum nondemolition detectors, and swap gates make this hyper-EPP works faithfully as the errors coming from practical scattering, in these quantum circuits, are converted into a detectable failure rather than infidelity. Furthermore, this hyper-EPP can be directly extended to purify photon systems entangled in single polarization or spatial-mode DOF and that hyperentangled in polarization and multiple-spatial-mode DOFs.