We report on an ultrastable optical magnetometer based on nonlinear magneto-optical rotation in ${}^{87}\mathrm{Rb}$ vapor. The atomic vapor is both optically pumped and probed on the $F=2\to F^{\prime }=1$ hyperfine transition of the $D_1$ manifold. A measurement over 26 h quantifies the magnetometer’s performance across 8 orders of magnitude in the Fourier-frequency domain, allowing us to measure the magnetic response into the microhertz domain. We demonstrate a room-temperature sensitivity floor of 15 ${\mathrm{fT}}_{\mathrm{rms}}/\sqrt{\mathrm{Hz}}$ at a magnetic-field strength of $2.5\mu \mathrm{T}$, which corresponds to a record $9\mathrm{ppb}/\sqrt{\mathrm{Hz}}$ fractional sensitivity. The magnetometer’s performance is photon shot-noise limited from 40 Hz to 10 kHz, while below 40 Hz it slowly degrades as approximately $f^{-1/4}$. At 1 mHz the performance is still better than 1 ${\mathrm{pT}}_{\mathrm{rms}}/\sqrt{\mathrm{Hz}}$. We show that this worsening performance is not a characteristic of the sensor, but instead associated with minute fluctuations of the magnetic field at the sensor from various identified sources.

We study the effects of inversion asymmetry and tunable band gap on the transmission of particles in bilayer graphene junctions to propose a model for a valley-filter and valley-valve device. The device is made up of a series of two $n\text{−}p\text{−}n$ (or $p\text{−}n\text{−}p$) junctions produced by electric gate potentials: one of them, being applied by an out-of-layer magnetic field, functions as a valley filter and the other as a valley valve. The valley filtering is based on the valley-dependent Zeeman interaction between the applied magnetic field and the orbital magnetic moment that can exist in a system with broken inversion symmetry. The valley valve is performed by varying the size of band gap via the gate potential that controls the flow of valley-polarized particles. We demonstrate that the device can achieve nearly perfect valley polarization and very high efficiency of valley-valve effect. By fixing magnetic field, the device can be operated by electric gate potentials only, and the tunable band gap makes a continuous control of valley-separate switching on and off possible.

We study the electrocaloric effect (ECE) in a typical relaxor ferroelectric ceramic of ${\mathrm{Pb}}_{0.91}{\mathrm{La}}_{0.06}{\mathrm{Zr}}_{0.8}{\mathrm{Ti}}_{0.2}{\mathrm{O}}_3$ by analyzing the respective contributions of long-range-ordered ferroelectric macrodomains and short-range-ordered polar nanoregions (PNRs) by both direct and indirect characterization. The ECE exhibits two peaks in its dependence on temperature, the former responds to the transition from long-range ferroelectric order to short-range-ordered PNRs and the latter likely to the transition between two kinds of PNRs with different phase structures or motion states. The contributions of different mechanisms are clarified by resolving the overlapping peaks in direct heat flow experimental results. The first peak has a $\mathrm{\Delta }{T}_{\max }=0.7\mathrm{K}$ at 80 °C (approximately $T_d$) and the second a $\mathrm{\Delta }{T}_{\max }=0.98\mathrm{K}$ at 140 °C (approximately $T_m$) under 40 kV/cm. We conclude that the thermodynamic method based on the Maxwell relation is not applicable if PNRs are involved as their signatures are only picked up by direct measurements, but is valid for the case of ferroelectric domain rotation or phase transition where it provides a reasonably accurate ECE result. Besides the impact on aspects related to metrology, our work suggests that the region between the peaks in ΔT-T curves may be used to produce a large electrocaloric ΔT over a broad temperature range, which is highly desirable in cooling applications.

In microscale fluids, fields of physical force and streaming play central roles in manipulating and tweezing objects, but it is difficult to disentangle and obtain accurate pictures of them. We develop a multiradius microparticle image velocimetry ($\mathrm{MR}\mu \mathrm{PIV}$) protocol to solve this problem in miniaturized spaces. By using several monodisperse suspensions of spherical particles, each with its own specific particle radius, two-dimensional (2D) mapping separating the fields of radiation force and streaming is demonstrated in a microfluidic chamber driven by standing or focused surface acoustic waves, while motorized scanning is unnecessary and no special assumptions need to be made about the driving field. The results also allow the extraction of other physical parameters such as the acoustic pressure amplitude. The principle of $\mathrm{MR}\mu \mathrm{PIV}$ relies on a quasiequilibrium assumption for the particle motion and a linear dependence of the field force on particle volume. Therefore, it is also applicable to tweezing techonologies using optical, dielectrophoretic, and magnetic forces, constituting an extension of the PIV technique impactful for microscale physics in general.

We demonstrate the viability of using high-order modes in localized optical resonances combined with gain media to dramatically reduce the linewidths of absorption spectra. Our theoretical study provides a rational design route for small-footprint absorption with a high quality factor, which is typically a trade-off for plasmonic light absorbers. Specifically, we design and numerically investigate a metal-insulator-metal absorber and a graphene Salisbury screen. Additionally, we study the potential application of these structures in sensing in order to achieve higher performance compared with conventional sensors based on the fundamental mode. Our approach, which can readily operate in a multiplexed fashion, has the potential for sensing minute amounts of analytes with a high figure of merit.

Floquet topological insulators have inspired analogues in photonics, optics, and acoustics, in which nonreciprocal wave propagation in time-modulated materials is achieved due to the breaking of time-reversal symmetry. We investigate a mechanical wave analogue of Thouless pumping and the quantum Hall effect (QHE) in one- (1D) and two-dimensional (2D) periodically time-modulated materials, respectively. In one dimension, wave propagation in the time-modulated system is characterized based on the adiabatic theorem, and topologically protected one-way edge modes are numerically demonstrated by the principle of bulk-edge correspondence. In two dimensions, a time-modulation scheme of a hexagonal lattice is suggested and polarized edge states characteristics of QHE are put into evidence by the plane wave expansion (PWE) method. The transition from the trivial state to the topological one is captured and interpreted by an invariant Chern number. We numerically demonstrate the existence of topologically protected one-way edge states immune to scattering by sharp corners, defects, randomly disordered modulation phases, and dissipation effects.

Finite-difference time domain (FDTD) calculations reveal that resonant plasmonic nanoantennas are capable of generating intense magnetic field pulses in the midinfrared frequency region. The magnetic field pulse generated by nanoantennas is spatially concentrated within a nanoscale region, with its intensity increased by more than two orders of magnitude compared to that of the incident light. Given the highly localized confinement of the intense magnetic field, the nanoantenna can be used as a nanoscale source of magnetic field pulse at optical frequencies, which can locally manipulate the magnetism within a very short time scale. Micromagnetic numerical results demonstrate the excitation of the magnetization oscillation in a ferromagnetic nanoelement, which is coherently coupled to the magnetic field. After the termination of magnetization oscillation, propagating spin waves emerge from the excitation magnetic field region and travel perpendicular to the static magnetization direction. The result demonstrates the potential of resonant plasmonic nanoantennas in optically triggering magnetization dynamics and subsequently generating spin waves in the GHz frequency domain. The result opens up an interesting perspective for applying plasmonic nanoantennas in the ultrafast optical manipulation of magnetism on the nanometer scale.

A model of nonlinear optical interaction in the external cavity of a semiconductor disk laser is derived and numerically studied. The model is formalized in a system of differential equations with time delay (TD). The steady-state operation point, its stability, and transient dynamics of intracavity singly resonant optical parametric oscillators (ICSROs) are analyzed in the framework of this model. Also, the same quantities are studied with the help of the normal-mode expansion (NME) method. Both models result in the same steady-state solution, while the stability and dynamic behavior appear to be quite different. The characteristic equation defining the steady-state stability of the TD dynamic system is shown to have an infinite number of roots that may be grouped into two sets of complex-conjugate pairs. For the TD model, the Hopf bifurcation occurs when the primary pump exceeds the bifurcation value. The characteristic equation corresponding to the NME model in the single-mode approximation has only three roots with the real parts not exceeding zero; i.e., the steady-state operation point maintains stability for all values of the primary pump used in the simulation.

We present and analyze a hybrid quantum system that interfaces a Majorana-hosted semiconductor nanowire with a single nitrogen-vacancy (N-V) center via a magnetized torsional cantilever. We show that the torsional mode of the mechanical resonator can strongly couple to the Majorana qubit and the spin qubit simultaneously, which allows us to interface them for quantum-state conversion through a dark-state protocol. This work provides a promising interface between the topological qubit and the conventional spin qubit. This interface may have interesting applications in quantum information and computation.

We report on the experimental demonstration of chirality-selective mechanical separation of randomly distributed assemblies of right-handed and left-handed chiral microparticles by optical means. Chiral-resolution experiments are made using two-dimensional emulsions of chiral-liquid-crystal droplets under the action of circularly polarized laser beams and do not require information on the initial location of the particles. Also, we numerically identify that the cooperative effects of hydrodynamic interactions mediated by the viscous fluid surrounding the particles can speed up the enantioseparation process.

Electromagnetic metasurfaces can be characterized as intelligent if they are able to perform multiple tunable functions, with the desired response being controlled by a computer influencing the individual electromagnetic properties of each metasurface inclusion. In this paper, we present an example of an intelligent metasurface that operates in the reflection mode in the microwave frequency range. We numerically show that, without changing the main body of the metasurface, we can achieve tunable perfect absorption and tunable anomalous reflection. The tunability features can be implemented using mixed-signal integrated circuits (ICs), which can independently vary both the resistance and reactance, offering complete local control over the complex surface impedance. The ICs are embedded in the unit cells by connecting two metal patches over a thin grounded substrate and the reflection property of the intelligent metasurface can be readily controlled by a computer. Our intelligent metasurface can have a significant influence on future space-time modulated metasurfaces and a multitude of applications, such as beam steering, energy harvesting, and communications.

Dynamical decoupling can enforce a symmetry on the dynamics of an open quantum system. Here we develop an efficient dynamical-decoupling-based strategy to create the decoherence-free subspaces for a set of qubits by optimally transferring the states of these qubits. Our method is to transfer the state of each qubit to all of the other qubits in an optimal and efficient manner, so as to produce an effective collective-type noise for all qubits. This collective pseudonoise is essentially derived from arbitrarily independent real baths and needs only nearest-neighbor state transfers for its implementation. Moreover, our scheme requires only $N$ steps to produce the effective collective-type noise for a system of $N$ qubits. It provides an experimentally feasible and efficient approach to achieving the decoherence-free subspaces for encoding the protected logical qubits.

Scalable spin-to-photon interfaces require quantum emitters with strong optical-transition dipole moment and low coupling to phonons and stray electric fields. It is known that particularly for coupling to stray electric fields, these conditions can be simultaneously satisfied for emitters that show inversion symmetry. Here, we show that inversion symmetry is not a prerequisite criterion for a spectrally stable quantum emitter. We find that identical electron density in ground and excited states can eliminate the coupling to the stray electric fields. Further, a strong optical-transition dipole moment is achieved in systems with altering sign of the ground and excited wave functions. We use density-functional perturbation theory to investigate an optical center that lacks inversion symmetry. Our results show that this system close to ideally satisfies the criteria for an ideal quantum emitter. Our study opens an additional rationale in seeking promising materials and point defects towards the realization of robust spin-to-photon interfaces.

Thermal metamaterials have served as a practical way to manipulate heat flow thanks to their judiciously designed structures, which achieve various functionalities. However, the connection and interplay between various possible structural configurations and their functions’ switchability have long been neglected and this is usually perceived as “one configuration, one function.” Here, we propose a doublet thermal metadevice in which we utilize the local configuration of two unequal conductivity phases to manipulate global temperature distribution. Based on effective medium theory, we apply the phase interchange identity to show that the functionalities of transparency, cloaking, concentrating, and rotating can be unified. The doublet thermal metadevice is capable of realizing tunable multifunctions by simply adjusting the angular displacement of specific sublayers of the assembly. The functional diversity of the device can be further extended in two ways. One way is to use a secondary transformation and the other way is to alter the shape parameter of the unit phase. This work provides a different framework to understand and design thermal metamaterials, which might be extended to other disciplines such as optics, electromagnetics, and acoustics.

The effect of a drift electric field on the spin transport in a heavily doped $\mathrm{Si}$ channel is investigated using nonlocal devices. A simple method to accurately quantify spin drift is established, using the ratio of the nonlocal spin-valve signal for two measurement configurations that have a different electric field pattern in the channel. The spin-transport length is obtained as a function of the electric field, and it is found that in heavily doped $\mathrm{Si}$, drift electric fields of ±400 V/cm modify the spin-transport length (either up or down) by about a factor of two, relative to the length scale for purely diffusive transport. Although the trends are in agreement with the theory for spin drift, a quantitative comparison reveals that the theory significantly overestimates the effect of spin drift (by a factor of three). This highlights that an accurate experimental quantification of spin drift, as provided here, is crucial for a correct assessment of the role and importance of spin drift in practical spin-transport devices.

Self-propulsion of magneto-elastic composite microswimmers is demonstrated under a uniaxial field at both the air-water and the water-substrate interfaces. The microswimmers are made of elastically linked magnetically hard $\mathrm{Co}\text{−}\mathrm{Ni}\text{−}\mathrm{P}$ and soft $\mathrm{Co}$ ferromagnets, fabricated using standard photolithography and electrodeposition. Swimming speed and direction are dependent on the field frequency and amplitude, reaching a maximum of 95.1 µm/s on the substrate surface. Fastest motion occurs at low frequencies via a spinning (air-water interface) or tumbling (water-substrate interface) mode that induces transient inertial motion. Higher frequencies result in low Reynolds number propagation at both interfaces via a rocking mode. Therefore, the same microswimmer can be operated as either a high or a low Reynolds number swimmer. Swimmer pairs agglomerate to form a faster superstructure that propels via spinning and rocking modes analogous to those seen in isolated swimmers. Microswimmer propulsion is driven by a combination of dipolar interactions between the $\mathrm{Co}$ and $\mathrm{Co}\text{−}\mathrm{Ni}\text{−}\mathrm{P}$ magnets and rotational torque due to the applied field, combined with elastic deformation and hydrodynamic interactions between different parts of the swimmer, in agreement with previous models.

We generalize normal mode expansion of Green’s tensor $\overline{\overline{G}}(\mathit{r},{\mathit{r}}^{\prime })$ to resonators in open systems, resolving a long-standing open challenge. We obtain a simple yet robust formulation whereby radiation of energy to infinity is captured by a complete, discrete set of modes rather than a continuum. This enables rapid simulations by providing the spatial variation of $\overline{\overline{G}}(\mathit{r},{\mathit{r}}^{\prime })$ over both $\mathit{r}$ and ${\mathit{r}}^{\prime }$ in one simulation. Systems with or without material losses can be treated. Few eigenmodes are often necessary for nanostructures, facilitating both analytic calculations and unified insight into computationally intensive phenomena such as Purcell enhancement, radiative heat transfer, van der Waals forces, and Förster resonance energy transfer. We bypass all implementation and completeness issues associated with the alternative quasinormal eigenmode methods by defining modes with permittivity rather than frequency as the eigenvalue. We obtain true stationary modes that decay rather than diverge at infinity, and are trivially normalized. Completeness is achieved both for sources located within the inclusion and the background through use of the Lippmann-Schwinger equation. Modes are defined by a linear eigenvalue problem, readily implemented with any numerical method. We demonstrate its simple implementation with comsol multiphysics using the default inbuilt tools. The results are validated against direct scattering simulations, including analytic Mie theory, attaining arbitrarily accurate agreement regardless of source location or detuning from resonance.

Frequency combs based on nonlinear optical phenomena in integrated photonics are a versatile light source that can explore new applications, including frequency metrology, optical communications, and sensing. We demonstrate robust frequency-control strategies for near-infrared, octave-bandwidth soliton frequency combs created with nanofabricated silicon nitride ring resonators. Group-velocity-dispersion engineering allows operation with a 1064-nm pump laser and generation of dual-dispersive-wave frequency combs linking wavelengths approximately between 767 and 1556 nm. To tune the mode frequencies of the comb, which are spaced by 1 THz, we design a photonic chip containing 75 ring resonators with systematically varied dimensions and we use a thermo-optic tuning range of $50{}^{\circ }\mathrm{C}$. This single-chip frequency-comb source provides access to every wavelength, including those critical for near-infrared atomic spectroscopy of rubidium, potassium, and cesium. To make this possible, solitons are generated consistently from device to device across a single chip with use of rapid pump-frequency sweeps that are provided by an optical modulator.

Broadband ferromagnetic resonance (FMR) and high-temperature (T) FMR measurements are carried out to study interlayer exchange coupling (IEC) in a magnetic hard-soft bilayered system where the hard layer is a $\mathrm{Fe}\mathrm{Pt}$ thin film with strong perpendicular anisotropy and the soft layer is a thin film made of $\mathrm{Fe}$, $\mathrm{Co}$, or their alloys. The data indicate that the effective exchange field ($H_{\mathrm{ex}}$) produced by the IEC on the soft layer increases with a decrease in the thickness or saturation induction (4π$M_s$) of the soft layer. With an increase in T, $H_{\mathrm{ex}}$ drops by an amount larger than 4π$M_s$. The effective damping constant of the soft layer increases with $H_{\mathrm{ex}}$ and can vary by two orders of magnitude. In samples with $H_{\mathrm{ex}}$ > 4π$M_s$, the damping constant is insensitive to the choice of material of the soft layer. In samples with $H_{\mathrm{ex}}$ < 4π$M_s$, the damping constant strongly depends on the choice of material. When T is increased from room temperature to the Curie temperature of the hard layer, the FMR linewidth drops significantly in samples where $H_{\mathrm{ex}}$ is relatively large, but remains constant or even increases slightly at high T in samples where $H_{\mathrm{ex}}$ is very small. The effects of $H_{\mathrm{ex}}$ on the damping and linewidth can be understood by considering two distinct components in the overall damping, an intrinsic component mainly due to spin-flip magnon-electron scattering and an extrinsic component due to IEC-associated spin pumping at the interface.

Hypersonic boundary-layer transition induced by the Mack second mode is a fundamental issue in fluid mechanics and hypersonic vehicle design, whose physics are not yet fully understood. Nevertheless, given the acoustic nature of the Mack second mode, ultrasonic absorptive coatings have been proposed to dissipate the wave energy and thus stabilize the hypersonic boundary-layer flow. We, however, show that even with little damping, the Mack second mode can be greatly suppressed by introducing an artificial boundary of near-zero surface acoustic impedance. This phenomenon can be attributed to the minimized acoustic pressure perturbation at the antinode of the Mack second mode, which prevents the surface-wavelike mode from being effectively excited. As a practical realization, we present a grooved acoustic metasurface and numerically verify its feasibility. Results reveal that the out-of-phase behavior between the incident and reflected waves at the resonant frequency minimizes the near-surface acoustic pressure, largely inhibiting the growth of the Mack second mode. Our study sheds light on the physical mechanism of the Mack second mode and opens up alternative possibilities toward full control of hypersonic boundary-layer transition with acoustic metasurfaces.