Compared to nanoscale friction of translational motion, the mechanisms of rotational friction have received less attention. Such motion becomes an important issue for the miniaturization of mechanical machinery that often involves rotating gears. In this study, molecular-dynamics simulations are performed to explore rotational friction for solid-state gears rotating on top of different substrates. In each case, viscous damping of the rotational motion is observed and found to be induced by the pure van der Waals interaction between the gear and the substrate. The influence of different gear sizes and various substrate materials is investigated. Furthermore, the rigidities of the gear and the substrate are found to give rise to different dissipation channels. Finally, it is shown that the dominant contribution to the dissipation is related to the excitation of low-frequency surface phonons in the substrate.

Electromagnetic scattering on subwavelength structures is attracting much attention owing to the broad range of possible applications where this phenomenon can be used. Fundamental limits of the scattering cross section, which are well understood in spherical geometries, are overlooked in cases of low-symmetry resonators. Here we revise the notion of superscattering and link this property with symmetry groups of scattering potentials. We demonstrate pathways to spectrally overlap several eigenmodes of a resonator in a way such that they interfere constructively and increase the scattering cross section. As a particular example, we demonstrate spectral overlapping of several electric and magnetic modes in a subwavelength ceramic resonator. The optimized structures have a dipolar scattering cross-section limit exceeding that for a sphere by up to a factor of 4. The revealed rules, which link symmetry groups with fundamental scattering limits, allow assessment of the designs and performance of subwavelength superscatterers, which can be used in label-free imaging, compact antennas, long-range radio-frequency identification, and many other fields.

Compressed ultrafast photography (CUP) has the highest imaging speed and sequence depth in capturing ultrafast nonrepeatable or unstable dynamic events with snapshots. However, due to the Coulomb interaction of electrons in a streak camera, it is difficult for the imaging speed of CUP to break the speed limit of ${10}^{12}$ frames/s. Here, we propose a molecular-alignment-assisted CUP (MACUP) scheme by introducing a gas-phase temporal-spatial converter with all-optical deflection imaging into conventional CUP. Based on our simulation of a carbon dioxide molecular deflector, combined with point-spread restrictions in imaging, MACUP is able to achieve an imaging speed beyond 180 × ${10}^{12}$ frames/s and a sequence depth of about 300 frames in a single exposure. To demonstrate the feasibility of MACUP, we simulate the spatiotemporal intensity measurement of a chirped femtosecond laser pulse and study the image reconstruction accuracy in the intensity and wavelength evolutions. These results show that MACUP is a promising single-shot ultrafast optical imaging strategy to unravel unprecedented dynamics in ultrafast atomic and molecular optics.

In this paper, we discuss effects of space-charge fields on imaging performance in a single-shot time-resolved transmission electron microscope. Using a Green’s function perturbation method, we derive analytical estimates for the effects of space-charge nonlinearity on the image formation process and the associated aberration coefficients. The dependence of these coefficients on the initial beam phase space distribution is elucidated. The results are validated by particle tracking simulations and provide fundamental scaling laws for the trade-off between temporal and spatial resolutions in single-shot time-resolved transmission electron microscopy.

The broken mirror symmetry in subwavelength photonic systems has manifested many interesting chiroptical effects, such as optical rotation and circular dichroism. When such systems are placed periodically in a lattice form, in addition to intrinsic chirality, extrinsic chirality also takes part, and the overall effect depends not only on the basis and lattice but also the excitation configuration. Here, we study planar chiral nanohole arrays in square lattice that support Bloch-like surface plasmon polaritons (SPPs) and clarify how the system geometry and the excitation contribute to circular dichroism. By using temporal coupled mode theory, the dissymmetry factor and the scattering matrix of the arrays are analytically formulated. Remarkably, we find the dissymmetry factor depends only on the coupling polarization angle and the in-coupling phase difference between the p- and s-polarizations. Furthermore, the upper limit of the dissymmetry factor at ±2 can be reached simply by orienting the lattice of the arrays for properly exciting the Bloch-like SPPs and at the same time making the basis mimic two orthogonal and relatively displaced dipoles, demonstrating the interplay between extrinsic and intrinsic chirality. The models have been verified by numerical simulations and experiments, yielding dissymmetry factors of 1.82 and 1.55, respectively, from the proposed dual slot system.

A wide variety of imaging systems have been designed to measure phase variations, with applications from physics to biology and medicine. In this work, we theoretically compare the precision of phase estimations achievable with classical phase microscopy techniques, operated at the shot-noise limit. We show how the Cramér-Rao bound is calculated for any linear optical system, including phase-contrast microscopy, phase-shifting holography, spatial light interference microscopy, and local optimization of wavefronts for phase imaging. Through these examples, we demonstrate how this general framework can be applied for the design and optimization of classical phase microscopes. Our results show that wavefront shaping is required to design phase microscopes with optimal phase precision.

Properties of tunnel junctions (TJs) grown by plasma-assisted molecular beam epitaxy are investigated. Examined TJs consist of a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum well (QW) sandwiched between ${\mathrm{In}}_{0.02}{\mathrm{Ga}}_{0.98}\mathrm{N}$ barriers. The influence of piezoelectric polarization, doping, and width of a TJ QW on resistivity of TJs is studied. A decrease of TJ resistivity is observed for high doping and high ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ composition. Additionally, the crystal quality of TJs as a function of magnesium and silicon doping in a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ QW is discussed. Generation of defects in highly doped layers is observed in transmission electron microscopy and by defect selective etching. High doping leads to deterioration of surface morphology, which is corroborated by atomic force microscopy imaging. TJs with low resistance and high crystalline quality, without formation of structural defects, are demonstrated. A theoretical model for calculation of tunneling currents is proposed based on Kane tunneling theory, which considers a realistic electric field distribution in a TJ. A good agreement with the experimental data is obtained. The model is used to predict electrical parameters in a wide range of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ compositions (x = 0–0.40), doping ($n_{\mathrm{Mg},\mathrm{Si}}={10}^{17}-5\times {10}^{21}{\mathrm{cm}}^{-3}$), and TJ QW width (d = 2 – 12 nm).

By considering many-body effects explicitly, we study the quasiparticle electronic structure, exciton, and optical properties of two-dimensional (2D) material blue phosphorene and their evolution with biaxial strain. Although the pristine system is an indirect wide-band-gap semiconductor, it could evolve systematically toward an almost direct one when an intermediate tensile strain is applied. The absorption edge shows an obvious red shift with an increase in the lattice constant, enabling its access to a wide spectral range. Interestingly, when the band gap increases, the exciton binding energy decreases. This unusual relationship is found to be accompanied by an enhanced exciton effective mass and decreased dielectric screening with an increase in the lattice constant. Together with the evolution of the band edge, a funnel effect is revealed in the inhomogeneously strained 2D membrane of blue phosphorene. The gate-controlled carriers at the two sides could drift efficiently toward the center with maximum strain, where the relatively strong binding energy and short lifetime are beneficial for the realization of the emission of green and blue light.

Ventilation barriers allowing simultaneous sound blocking and free airflow passage are a great challenge but are necessary for particular scenarios calling for soundproofing ventilation. Previous studies using local resonance or Fano-like interference consider a narrow working range around the resonant or destructive-interference frequency. Efforts made with regard to broadband designs show a limited bandwidth typically smaller than half an octave. Here we conceptually propose an ultrabroadband ventilation barrier via hybridization of dissipation and interference. Confirmed by experiments, our hybrid-functional metasurface, empowered by its synergistic effect, significantly expands the range of the operating frequencies, enabling an effective blocking of more than $90\mathrm{\%}$ of incident energy in the range from $650$ to $2000\mathrm{Hz}$, while its structural thickness is only $53\mathrm{mm}$ (approximately $\lambda /10)$. Our design showcases the great flexibility of customizing the broadband and is capable of tackling sound coming from various directions, which has potential in air-permeable yet soundproofing applications.

Dynamic light scattering from three-dimensional intracellular motions in living tissues or biopsies creates dynamic speckle due to broadband Doppler shifts that are recorded as broadband spectra obtained from speckle fluctuation spectroscopy. Doppler frequency shifts from active intracellular motion extend across three orders of magnitude from 10 mHz to 10 Hz. Biodynamic imaging is a full-field optical coherence tomography technique performed with low-coherence digital holography. The holographic reconstruction allows phase recovery, but mechanical stability limitations favor homodyne detection over heterodyne detection because homodyne is relatively insensitive to phase drift. However, by obtaining phase-displacement distributions from consecutive frames, the phase excursions form a robust probability distribution. The phase-probability density function of the phase-displacement distribution captures average phase displacements over time and is used to reconstruct stable heterodyne spectra. Phase-sensitive biodynamic imaging is applied to detect Levy-stable distributions from living tissue that may reflect heavy-tailed anomalous transport within intracellular media.

We show how to implement stationary one-way quantum steering with strong entanglement in a cavity magnonic system that consists of two magnon modes and a microwave cavity. The cavity is driven by a squeezed vacuum field generated by a flux-driven Josephson parameter amplifier and coupled to two Kittel modes via magnetic dipole interaction. We find that the steering directivity only depends on the ratio of two coupling rates (i.e., the ratio of coherent information exchange frequencies) and is barely affected by the dissipation of the system. Meanwhile, the entanglement and steering can be significantly enhanced due to the squeezed vacuum field and thus are more robust against thermal noises. This provides an active method to manipulate the steering directivity instead of adding asymmetric losses or noises to subsystems at the cost of reducing steerability.

Spin-orbit torque provides an efficient pathway to manipulate the magnetic state and magnetization dynamics of magnetic materials, which is crucial for energy-efficient operation of a variety of spintronic devices, such as magnetic memory, logic, oscillator, and neuromorphic computing. Here, we describe and experimentally demonstrate a strategy for the realization of a spin torque gate magnetic sensor with an extremely simple structure by exploiting the longitudinal field dependence of the spin torque driven magnetization switching. Unlike most magnetoresistance sensors, which require a delicate magnetic bias to achieve a linear response to the external field, the spin torque gate sensor can achieve the same without any magnetic bias, which greatly simplifies the sensor structure. Furthermore, by driving the sensor using an ac current, the dc offset is automatically suppressed, which eliminates the need for a bridge or compensation circuit. We verify the concept using the ${\mathrm{W}\mathrm{Te}}_2/\mathrm{Ti}/(\mathrm{Co},\mathrm{Fe})\mathrm{B}$ trilayer and demonstrate that the sensor can work linearly in the range of ±3–10 Oe with negligible dc offset.

We present an imaging modality that enables detection of magnetic moments and their resulting stray magnetic fields. We use wide-field magnetic imaging that employs a diamond-based magnetometer and has combined magneto-optic detection (e.g., magneto-optic Kerr effect) capabilities. We employ such an instrument to image magnetic (stripe) domains in multilayered ferromagnetic structures.

A vector vortex beam presents local polarization states that are spatially modulated in the plane transverse to propagation. By employing a two-crystal-sandwich source allowing for nonlinear interactions between input fields of any polarization, we analyze experimentally the process of stimulated parametric down-conversion and observe unique effects when the pump and seed fields are vector vortex beams. We reconstruct the transverse polarization pattern of the generated idler beam by Stokes polarimetry. The experimental results obtained are in good qualitative agreement with theory and exhibit manifold polarization transverse distributions for different combinations of pump and seed vector vortex beams, which may find applications in both classical and quantum optics.

Shortcuts to adiabaticity are well-known methods for controlling the quantum dynamics beyond the adiabatic criteria, where counterdiabatic (CD) driving provides a promising means to speed up quantum many-body systems. In this work, we show the applicability of CD driving to enhance the digitized adiabatic quantum computing paradigm in terms of fidelity and total simulation time. We study the state evolution of an Ising spin chain using the digitized version of the standard CD driving and its variants derived from the variational approach. We apply this technique in the preparation of Bell and Greenberger-Horne-Zeilinger states with high fidelity using a very shallow quantum circuit. We implement this proposal on the IBM quantum computer, proving its usefulness for the speed up of adiabatic quantum computing in noisy intermediate-scale quantum devices.

In recent years there has been a number of proposals of thermal devices operating in the near-field regime that make use of phase-transition materials. Here, we present a theoretical study of near-field thermal diodes and transistors that combine superconducting materials with normal (nonsuperconducting) metals. To be precise, we show that a system formed by two parallel plates made of $\mathrm{Nb}$ and $\mathrm{Au}$ can exhibit unprecedented rectification ratios very close to unity at temperatures around the $\mathrm{Nb}$ superconducting critical temperature and for a wide range of gap size values within the near-field regime. Moreover, we also show that a superconducting $\mathrm{Nb}$ layer placed between $\mathrm{Au}$ plates can operate as a near-field thermal transistor where the amplification factor can be greatly tuned by varying different parameters such as the temperature and thickness of the $\mathrm{Nb}$ layer or the distance between the $\mathrm{Nb}$ layer and the $\mathrm{Au}$ plates. Overall, our work shows the potential of the use of superconductors for the realization of near-field thermal devices.

Anion-cation antisite defects usually have low density in the group III-V (e.g., $\mathrm{Ga}\mathrm{N}$) and ${\mathrm{II}-\mathrm{IV}-\mathrm{V}}_2$ (${\mathrm{Zn}\mathrm{Ge}\mathrm{N}}_2$, ${\mathrm{Zn}\mathrm{Sn}\mathrm{P}}_2$) semiconductors, and thus, have not drawn enough attention in defect studies of ${\mathrm{Zn}\mathrm{Ge}\mathrm{P}}_2$ since 1976. However, our first-principles calculations based on a hybrid functional show that the anion-cation antisite defects (${\mathrm{Ge}}_{\mathrm{P}}$ and ${\mathrm{P}}_{\mathrm{Ge}}$) can have very high density (${10}^{17}-{10}^{18}{\mathrm{cm}}^{-3}$), making them the dominant defects in ${\mathrm{Zn}\mathrm{Ge}\mathrm{P}}_2$. Their calculated photoluminescence (PL) spectra agree well with the 1.4 and 1.6 eV PL peaks observed experimentally, indicating that they may be the origin of defects, which challenges previous assumptions that the $\mathrm{P}$ vacancy ($V_{\rm{P}}$) defect is responsible for the two PL peaks. Although the anion-cation antisites (${\mathrm{Ge}}_{\mathrm{P}}$ and ${\mathrm{P}}_{\mathrm{Ge}}$) and cation-cation antisites (${\mathrm{Ge}}_{\mathrm{Zn}}$ and ${\mathrm{Zn}}_{\mathrm{Ge}}$) both have densities as high as ${10}^{17}{\mathrm{cm}}^{-3}$, ${\mathrm{Zn}\mathrm{Ge}\mathrm{P}}_2$ suffers from serious donor-acceptor compensation, which results in a low carrier density (below ${10}^{10}{\mathrm{cm}}^{-3}$), and thus, poor electrical conductivity. These results explain the mysterious observation that ${\mathrm{Zn}\mathrm{Ge}\mathrm{P}}_2$ crystals grown using different methods have a high defect density, but low carrier density and high resistivity, and also indicate that it is challenging to suppress the defect-induced optical absorption in the development of high-power ${\mathrm{Zn}\mathrm{Ge}\mathrm{P}}_2$-based optical devices.

We show that long-range and robust acoustic pulling can be achieved by using a pair of one-way chiral surface waves supported on the interface between two phononic crystals composed of spinning cylinders with equal but opposite spinning velocities embedded in water. When the chiral surface mode with a relatively small Bloch wave vector is excited, the particle located in the interface waveguide will scatter the excited surface mode to another chiral surface mode with a greater Bloch wave vector, resulting in an acoustic pulling force, irrespective of the size and material of the particle. Thanks to the backscattering immunity of the chiral surface waves against local disorders, the particle can be pulled, following a flexible trajectory, as determined by the shape of the interface. As such, this acoustic pulling scheme overcomes some of the limitations of the traditional acoustic pulling using structured beams, such as short pulling distances, straight-line type pulling, and a strong dependence on the scattering properties of the particle. Our work may also inspire the application of topological acoustics to acoustic manipulations.