We characterize spin-wave propagation and its modification by an electrical current in permalloy($\mathrm{Py}$)/$\mathrm{Pt}$ bilayers with $\mathrm{Py}$ thickness between 4 and 20 nm. First, we analyze the frequency nonreciprocity of surface spin waves and extract from it the interfacial Dzyaloshinskii-Moriya interaction constant $D_s$ accounting for an additional contribution due to asymmetric surface anisotropies. Second, we measure the spin-wave relaxation rate and deduce from it the $\mathrm{Py}$/$\mathrm{Pt}$ spin-mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow }$. Last, applying a dc electrical current, we extract the spin Hall conductivity ${\sigma }_{\mathrm{SH}}$ from the change of spin-wave relaxation rate due to the spin Hall spin-transfer torque. We obtain a consistent picture of the spin-wave propagation data for different film thicknesses using a single set of parameters $D_s=0.25$ pJ/m, $g_{\mathrm{eff}}^{\uparrow \downarrow }=3.2\times {10}^{19}{\mathrm{m}}^{-2}$ and ${\sigma }_{\mathrm{SH}}=4\times {10}^5$ S/m.

Droplets with diameters in the micrometer range find numerous applications, for example, as biological reaction compartments. In that context, adding and subtracting the smallest amounts of liquid in a well-controlled manner is a key step. Here, we demonstrate a principle that allows the transfer of tiny amounts of dissolved species to an aqueous femtoliter droplet reciprocating between two aqueous reservoirs under the influence of a dc electric field. Mass transfer is size selective and adaptive, for example, via tuning the viscosity of the surrounding oil phase or the electric-field strength. We provide a map of the dynamic regimes, indicating under which conditions the reciprocating droplet motion occurs. We also formulate a model based on diffusive mass transfer that describes the amount of species taken up and transferred by the droplet. Overall, our results significantly help to facilitate the handling and manipulation of femtoliter droplets.

Acoustic communication is crucial in underwater exploration, where sound is the dominant information carrier, with significantly less loss and scattering than that of electromagnetic waves. However, the capacity of acoustic communication channels is limited due to the intrinsically low speed of sound relative to that of electromagnetic waves and because the attenuation of acoustic waves underwater increases with frequency. Recently, orbital angular momentum (OAM) has emerged as an alternative multiplexing degree of freedom to encode data onto vortex beams for increasing the capacity of acoustic communication. For information retrieval from the multiplexed acoustic vortices, an active scanning method and a passive resonant method are explored. Time-consuming scanning and complex postprocessing significantly restrict the data-transmission speed, while the large amount of resonant cascaded devices in the passive technique intrinsically results in a low efficiency and bulky volume of the system. Here, we propose and experimentally demonstrate a passive and nonresonant approach with the ability to separate different OAM states of multiplexed acoustic vortex beams in parallel using a parabolic-phased metasurface. The metasurface converts the spiral-phase patterns of vortex beams carrying various angular momenta into plane waves with different in-plane linear momenta. Our approach is compatible with multiplexing technologies, significantly enhancing the speed in acoustic communication.

We demonstrate the enhancement and optimization of a cold strontium atomic beam from a two-dimensional magneto-optical trap (2D MOT) transversely loaded from a collimated atomic beam by adding a sideband frequency to the cooling laser. The parameters of the cooling and sideband beams are scanned to achieve the maximum atomic beam flux and are compared with Monte Carlo simulations. We obtain a 2.3 times larger total atomic flux and a brightness increase of a factor 4 compared with a conventional, single-frequency 2D MOT for a given total power of 200 mW. We show that the sideband-enhanced 2D MOT can reach the loading-rate performances of space-demanding Zeeman-slower-based systems, while it can overcome systematic effects due to thermal-beam collisions and hot-black-body-radiation shift, making it suitable for both transportable and accurate optical lattice clocks. Finally, we numerically study possible extensions of the sideband-enhanced 2D MOT to other alkaline-earth species.

Metastable inorganic materials with unique properties are important in many practical applications, but their synthesis is often challenging. In physics, epitaxial stabilization, also known as pseudomorphic growth, is used to synthesize metastable polymorphs, but usually only as very thin films and on expensive single-crystal substrates. In chemistry, the templated growth of inorganic solid-state materials on self-assembled monolayers of organic molecules is reported. Bridging these two fields, here, we show that the synthesis of metastable polymorphs is possible up to large film thickness on amorphous substrates covered with thin inorganic seed layers that serve as templates. The stabilization of a 500-nm-thick metastable wurtzite (WZ) $\mathrm{Mn}\mathrm{Te}$ film by a 5-nm-thick $\mathrm{Zn}\mathrm{Te}$ seed layer sputtered on amorphous glass substrates is experimentally demonstrated. Theoretical calculations explain this experimental observation by the small WZ polymorph energy relative to that of the ground-state nickeline (NC) structure of $\mathrm{Mn}\mathrm{Te}$ and a large lattice constant difference of the two. The resulting metastable WZ-$\mathrm{Mn}\mathrm{Te}$ polymorph exhibits a wide band gap of 2.7 eV and a low hole density of ${10}^{12}{\mathrm{cm}}^{-3}$, which is relevant to optoelectronic applications. These properties are in sharp contrast to those of the narrow-band-gap highly doped NC-$\mathrm{Mn}\mathrm{Te}$ with 1.3 eV band gap and ${10}^{19}{\mathrm{cm}}^{-3}$ hole density. The difference in hole density is due to the calculated difference in the formation energy of manganese vacancy acceptor defects. Overall, these results suggest that templated growth on amorphous substrates with seed layers can be used to synthesize metastable polymorphs of other materials, without the need for expensive single-crystal substrates.

In the areas of micro- and nanoresonant sensing, energy harvesting, and signal processing, the ability to provide resonance over a broad frequency bandwidth is a persistent goal. One strategy for a broadband resonator that researchers focus on is to exploit the relatively large-amplitude response of micro- and nanoresonators operating in the geometrically nonlinear dynamic regime. Geometric nonlinearity is well known to have a hardening effect on the resonance curve, thereby generating a broadband resonance, the bandwidth of which is limited by the linearized resonant frequency (lower bound) and the drop-down bifurcation frequency (upper bound). With everything else kept constant, increasing the drop-down bifurcation frequency in the frequency response of a nonlinear resonator enhances the broadband resonance. Here, the ultrabroadband resonance of microresonators with geometrically nonlinear stiffness is investigated and validated experimentally. Specifically, a microresonator with a cubic stiffness nonlinearity is excited at its base and, as the excitation amplitude approaches a critical level, a sudden and significant increase in the resonant bandwidth of the fundamental bending mode is observed. The significant implications of this nonlinear phenomenon in sensing, signal processing, and other applications at the microscale are discussed.

A thorough understanding of the electron-phonon, thermo-optic, and acoustic properties of materials is of paramount importance for many applications in materials science and advanced applications adopting laser-induced sound waves. Even though metals are usually opaque to light, optical methods for materials characterization can still be developed, especially in the high-frequency regime, where phonon dynamics governs the thermal, acoustic, and even optical properties of metals. Ultrafast laser pulses incident on metals can lead to the generation of coherent phonon wave packets with frequencies in the gigahertz to terahertz range, providing a means to study the material properties in this otherwise inaccessible frequency range. While this principle has been known, the complex interplay of light and matter in both the generation and detection of such ultrafast hypersound pulses has limited its use mainly to geometrical effects. Here, we demonstrate the quantitative characterization of a range of different material properties using laser-driven hypersound. We use all-optical generation and detection of hypersound pulses to sensitively probe the bulk properties of various metals. We introduce an advanced two-dimensional numerical model that captures the generation, propagation, and detection of these hypersound waves in full detail. The combination of experiment and simulation allows us to unravel and elucidate various physical effects that appear over a wide range of different time scales. Through least-squares fitting of the data to the simulation results, we extract quantitative information about the electron-phonon, thermo-optic, and acoustic properties of metal films, establishing the ability to use light as a sensitive probe for the study of opaque materials.

We present the design and experimental demonstration of an impedance-matched circular-polarization-selective surface that also offers spin-selective phase modulations at microwave frequencies. We achieve this by leveraging the theory of Pancharatnam-Berry phase shift and cascading four tensor impedance layers, each comprising an array of crossed meander lines. These meander lines are precisely tuned and rotated to implement particular tensor surface impedance values to satisfy the impedance-matching condition for the transmitted right-handed circularly polarized field while inducing Pancharatnam-Berry phase shift for only the reflected left-handed circularly polarized field. We present a detailed numerical synthesis technique to obtain the required impedance values to satisfy the impedance-matching condition, and demonstrate spin-selective phase modulations based on Pancharatnam-Berry phase shifts. To verify the proposed idea, we experimentally demonstrate nearly reflectionless transmission of right-handed circular polarization at broadside and reflection of left-handed circular polarization at ${33}^{\circ }$ off broadside at 12 GHz. For this purpose, a free-space quasioptical setup and a near-field measurement system are used to measure the transmitted and reflected circularly polarized fields, respectively.

In this paper, a momentum-space geometrical structure in helical evanescent electromagnetic waves is revealed. It is shown that for every helical evanescent wave on a helicity-dependent half tangent line in momentum space, the orientation of each of its field, spin, and Poynting vectors is the same. This geometric structure is revealed in a remarkable relation between the far-field and near-field components of the angular spectrum. Any general evanescent wave vector is linked to two points on the $k_{\rho }=k_0$ circle of propagating wave vectors via two helicity-dependent tangent lines. Knowing the field of a general dipolar source on the $k_{\rho }=k_0$ circle is sufficient to determine its entire evanescent angular spectrum. By applying this concept, we gain insights into near-field directionality by showing that every zero in the angular spectrum is a helicity singularity where two half tangent lines of opposite helicity intersect. A powerful method for the synthetic design of near-field directional sources is also devised, using structured helical illumination to gain full control of the near-field directionality. The results provide fundamental insight into helical evanescent waves and have implications in areas where chiral light-matter interaction plays a central role.

Using numerical simulations, we investigate the effects of deep traps and deep acceptor levels in magnesium-doped $\mathrm{Ga}\mathrm{N}$ on interface charges in the semiconductor-oxide interface. Specifically, in this work we address two open issues observed in experimental studies on $\mathrm{Ga}\mathrm{N}$ trench metal-oxide-semiconductor field-effect transistors. (i) We investigate the observed clockwise hysteresis in the transfer characteristics and elucidate the underlying physical mechanism causing it. By employing appropriate models for substitutional carbon at nitrogen sites ($C_N$) and nitrogen vacancies ($V_N$), we calculate the hysteresis dependence on the trap concentrations and the measurement sweep duration $T_s$. We show that $C_N$ acceptor traps in $p$-$\mathrm{Ga}\mathrm{N}$ are likely responsible for this phenomenon and the largest hysteresis is predicted for a sweep duration of $T_s\approx 30\mathrm{s}$. (ii) We also address the apparent inconsistency between the experimental and theoretically predicted magnesium-ionization levels and the variations of the measured transfer characteristics, specifically the threshold voltage. We show that the bands bending in the channel area creates a layer in which magnesium is completely ionized. As a result, the magnesium partial ionization does not have an effect and, while the threshold voltage decreases, nor does the breakdown voltage, as observed experimentally. The measured threshold voltage, which is lower than the theoretically predicted value, is caused by fixed and trapped charges at the interface, in agreement with values reported in the literature.

With advances in nanotechnology, ferroelectric switching by individual domain walls (DWs) has become a subject of broad interest. Conventional models consider DW motion in a fixed homogeneous or inhomogeneous electric field. However, it is clear that the electric field commonly evolves in time due to the redistribution of bound charges and screening free charges on the ferroelectric surface, particularly due to surface conductance. Taking this effect into account remains a serious challenge. Here we propose a simple concept to describe simultaneously the evolution of the electric field and the DW motion in a ferroelectric sample. The approach is based on a full analogy between charge transport during ferroelectric switching and heat transport in a moving melting front: the classical Stefan problem. The analogy helps in the establishment of control of DW motion in thin films. Experimentally, DWs are displaced by voltage pulses under electrodes with a limited conductivity and detected using piezoresponse force microscopy. In the frame of a combined theoretical and experimental approach, DW-friction mechanisms and the impact of the DW energy on its shape are discussed. The study facilitates the development of nanoelectronic devices.

The strongly localized electric field achieved at the Mie resonances of a silicon nanoparticle enables the generation of a large carrier density, which offers us the opportunity to manipulate the linear and nonlinear optical properties of silicon nanoparticles by optically injecting a dense electron-hole plasma. Here, we show that the dense electron-hole plasma created in a silicon nanoparticle significantly modifies the complex dielectric constant of silicon, which in turn leads to wavelength shift and amplitude change in the magnetic dipole resonance. We demonstrate that the maximum wavelength shift of the magnetic dipole resonance can be revealed by exploiting the hot-electron luminescence emitted by the silicon nanoparticle, which acts as a built-in light source with a broad bandwidth and a short lifetime. We demonstrate that the quantum efficiency of the hot-electron luminescence of silicon nanoparticles can be enhanced a factor of more than 5 through the injection of a dense electron-hole plasma. More interestingly, an acceleration of the radiative recombination process is found at high carrier densities. Our findings are helpful for understanding the modification of Mie resonances in silicon nanoparticles induced by the dense electron-hole plasmas and useful for designing silicon-based photonic devices.

Floating liquid marbles, small nonwetting liquid droplets encapsulated by fine hydrophobic powder located on a free liquid surface, can serve as a unique microbioreactor platform. Trapping and positioning of these liquid marbles are essential for efficient sample handling. We investigate the dielectrophoretic trapping process of floating liquid marbles that are moving on a free water surface. The dynamic behavior of the trapping process is analyzed experimentally and theoretically. More importantly, a governing equation describing successful trapping is generalized from the energy balance. This study facilitates insights into dielectrophoretic manipulation of liquid marbles and fills the gap of knowledge in handling small soft systems.

High-performance imaging is essential for widespread applications of photoacoustic tomography (PAT) in biomedicine. So far, no comprehensive studies are reported on the impact of system factors on imaging performance, in spite of their importance. Based on a prototype PAT scanner, we study eight factors associated with the acoustic reception process in PAT, namely, detector view angle, element number, center frequency, bandwidth, aperture size, focusing, orientation error, and scan step angle error, and investigated how they impact on the image quality. Simulations and experiments are both presented to support the findings in this study. This work is expected to provide a practical guide for advanced PAT scanner design with enhanced imaging performance.