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.

We demonstrate that a high kinetic inductance disordered-superconductor nanowire can realize a circuit element – known as a superinductor – with a characteristic impedance greater than the quantum resistance ($R_Q=h/4e^2\simeq 6.5\mathrm{k}\mathrm{\Omega }$) and a quality factor of 25 000 at single-photon excitation. By examining loss rates, we demonstrate that the microwave dissipation can be fully understood in the framework of two-level-system dielectric loss. Superinductors can suppress the quantum fluctuations of charge in a circuit, which has applications, for example, in devices for quantum computing, photon detection, and other sensors based on mesoscopic circuits.

Spiral phase contrast (SPC) is an important and commonly used image processing technology in edge detection applications, and realization of a broader field-of-view (FOV) is a long-pursued aim in imaging systems to allow more regions of the illuminated objects to be observed. Using the image processing technologies described, we first acquire both an enhanced FOV and SPC images using second-order nonlinear frequency up-conversion from the near-infrared (NIR) spectrum to the visible spectrum in two different configurations. In addition, SPC operation and sum-frequency generation using two different wavelengths is performed simultaneously within a periodically poled potassium titanyl phosphate (PPKTP) crystal by applying a vortex filter to the pump beam. When compared with the NIR spectrum, the up-converted image in the visible spectrum benefits from the advantages of higher detection sensitivity and lower potential speckle. By optimizing the temperature of the PPKTP crystal, controllable spatial patterns of up-converted images with observation cone angle enhancement of the FOV by more than 2.1 times is realized in both configurations. Additionally, we perform numerical simulations of the above phenomenon and the results agree well with those of the experimental observations. Our results provide a very promising image processing method that may be widely used in biomedicine and up-conversion monitoring applications.

Photovoltaic substrates have attracted significant attention for neural photostimulation. The control of the Faradaic and capacitive (non-Faradaic) charge transfer mechanisms by these substrates are critical for safe and effective neural photostimulation. We demonstrate that the intermediate layer can directly control the strength of the capacitive and Faradaic processes under physiological conditions. To resolve the Faradaic and capacitive stimulations, we enhance photogenerated charge density levels by incorporating PbS quantum dots into a poly(3-hexylthiophene-2,5-diyl):([6,6]-Phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend. This enhancement stems from the simultaneous increase of absorption, well matched band alignment of PbS quantum dots with P3HT:PCBM, and smaller intermixed phase-separated domains with better homogeneity and roughness of the blend. These improvements lead to the photostimulation of neurons at a low light intensity level of 1 mW ${\mathrm{cm}}^{-2}$, which is within the retinal irradiance level. These findings open up an alternative approach toward superior neural prosthesis.

We investigate how the voltage control of the exciton lateral dipole moment induces a transition from singly to doubly connected topology in type-II $\mathrm{In}\mathrm{As}$/${\mathrm{Ga}\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ quantum dots. The latter causes visible Aharonov-Bohm oscillations and a change of the exciton $g$ factor, which are modulated by the applied bias. The results are explained in the frame of realistic $\overrightarrow{k}\cdot \overrightarrow{p}$ and effective Hamiltonian models and could open a venue for new spin quantum memories beyond the $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ realm.

A giant, anomalous piezoresponse of fully depleted silicon-on-insulator devices under mechanical stress is demonstrated by impedance spectroscopy. This piezoresponse strongly depends on the measurement frequency, $\omega$, and consists of both a piezoresistance (PZR) and a piezocapacitance, whose maximum values are $-1100\times {10}^{-11}$ and $-900\times {10}^{-11}{\mathrm{Pa}}^{-1}$, respectively. These values should be compared with the usual bulk PZR in $p$-type silicon, $70\times {10}^{-11}{\mathrm{Pa}}^{-1}$. The observations are well described by models of space-charge-limited hole currents in the presence of fast electronic traps having stress-dependent capture rates (${\omega }_c$) and emission rates. Under steady-state conditions (i.e., when $\omega \ll {\omega }_c$), where the impedance-spectroscopy measurements yield results that are directly comparable with those of previously published reports of PZR in depleted, silicon nano-objects, the overall piezoresponse is just the usual, bulk silicon PZR. Anomalous PZR is observed only under non-steady-state conditions when $\omega \approx {\omega }_c$, with a symmetry suggesting that the electromechanically active fast traps are native ${\mathrm{Pb}}_0$ interface defects. The observations suggest new functionalities for fully depleted silicon-on-insulator, and shed light on the debate over the PZR of carrier-depleted nanosilicon.

Energy transport is of central importance in understanding a wide variety of transitions of physical states in nature. Recently, the coherence and noise have been identified for their existence and key roles in energy-transport processes, for instance, in a photosynthesis complex, DNA, and odor sensing etc., of which one may have to reveal the inner mechanics in the quantum regime. Here we present an analog of Newton’s cradle by manipulating a boundary-controlled chain on a photonic chip. Long-range interactions can be mediated by a long chain composed of $21$ strongly coupled sites, where single-photon excitations are transferred between two remote sites via simultaneous control of intersite weak and strong couplings. We observe a high retrieval efficiency in both uniform and defect-doped chain structures. Our results may offer a flexible approach to Hamiltonian engineering beyond geometric limitation, enabling the design and construction of quantum simulators on demand.

Dynamic helicity switching by utilizing metasurfaces is challenging because it requires deep modulation of polarization states. To realize such helicity switching, this paper proposes a dynamic metasurface functioning as a switchable quarter-wave plate, the fast axis of which can be dynamically rotated by ${90}^{\circ }$. The device is based on the critical transition of an anisotropic metallic checkerboard, which realizes the deep modulation and simultaneous design of the switchable states. After verifying the functionality of the ideally designed device in a simulation, we tune its structural parameters to realize practical experiments in the terahertz frequency range. By evaluating the fabricated sample with vanadium dioxide, the conductivity of which can be controlled by temperature, its dynamic helicity switching function is demonstrated.

The photovoltaic effect in noncentrosymmetric ferroelectrics is gaining much research interest due to its remarkable above band-gap photovoltage. Among the available ferroelectric systems, the 0.5$\mathrm{Ba}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_3$-$0.5({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{Ti}\mathrm{O}}_3$ with its high piezocoefficient is considered as an attractive lead-free multifunctional ferroelectric material. The photovoltaic studies on this compound reveal the discovery of a large photogenerated electric field of 368 V cm, one of the highest values reported in polycrystalline ferroelectric systems. It is demonstrated that the large photovoltaic response in this ferroelectric system is due to the symmetry breaking structural transformation from the tetragonal to rhombohedral and orthorhombic phases. Density functional calculations show that this symmetry breaking transformation leads to the reduction of electron and hole effective masses and delocalization and reorientation of the conduction charge cloud along the polarization direction, which, in turn, gives a high photovoltaic response.

High-resolution microwave leaky-wave holograms excited by a center-fed cylindrical-surface-wave launcher show a null in the object-wave direction, which is an undesired effect for electromagnetic beamforming. Also, planar leaky-wave metasurfaces generating a tilted beam suffer greatly from the destructive effect of nonforward surface leaky waves at frequencies other than the design frequency and they are operable at almost only a single frequency. Here we propose a two-dimensional modified-hologram configuration using a parabolic surface reflector to collimate the nonforward leaky modes into forward leaky modes. The modified hologram exhibits a null-free radiation pattern and greatly increased operating-frequency bandwidth. The consequent frequency bandwidth provides the scannability property by frequency variation. The forward-mode-dominant surface-wave excitation of the hologram allows the metasurface to generate the object beam more precisely; therefore, high directivity over the operating bandwidth is obtained. The concept is verified by fabrication and experimentally tested, confirming beam maintenance over a reasonable frequency range and scannability property. The measurement results are in excellent agreement with full-wave simulation, prove applicable wideband leaky-wave holograms.

The light ideality factor determined by measuring the open-circuit voltage (V) as a function of light intensity is often used to identify the dominant recombination mechanism in solar cells. Applying this “Suns-V” technique to perovskite cells is problematic since the V evolves with time in a way that depends on the previously applied bias (V), bias light intensity, device architecture and processing route. Here, we show that the dominant recombination mechanism in two structurally similar ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ devices containing either mesoporous ${\mathrm{Al}}_2{\mathrm{O}}_3$ or ${\mathrm{Ti}\mathrm{O}}_2$ layers can be identified from the signature of the transient ideality factor following application of a forward bias, V, to the device in the dark. The transient ideality factor is measured by monitoring the evolution of V as a function of time at different light intensities. The initial values of ideality found using this technique are consistent with estimates of the ideality factor obtained from measurements of photoluminescence vs light intensity and electroluminescence vs current density. Time-dependent simulations of the measurement on modeled devices, which include the effects of mobile ionic charge, reveal that this initial value can be correlated to an existing zero-dimensional model while steady-state values must be analyzed taking into account the homogeneity of carrier populations throughout the absorber layer. The analysis shows that Shockley-Read-Hall (SRH) recombination through deep traps at the charge-collection interfaces is dominant in both architectures of measured device. Using transient photovoltage measurements directly following illumination on bifacial devices, we further show that the perovskite–electron-transport-layer interface extends throughout the mesoporous ${\mathrm{Ti}\mathrm{O}}_2$ layer, consistent with a transient ideality signature corresponding to SRH recombination in the bulk of the film. This method will be useful for identifying performance bottlenecks in alternative variants of perovskite and other mixed ionic-electronic conducting absorber-based solar cells.

We report on a detailed investigation of terahertz-emission properties related to resonant cavity modes. We discuss data for an underdoped and an optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}\mathrm{Cu}}_2{\mathrm{O}}_{8+x}$ (BSCCO) intrinsic junction stack having the same geometry. At high bias, in the presence of a hot spot, the emission frequency seems to be continuously tunable by changing the bias current and the bath temperature. By contrast, at low bias the emission frequencies $f_e$ are remarkably discrete and temperature independent for both stacks. The values of $f_e$ point to the formation of $(0,m)$ cavity modes with $m=3$ to 6. The total voltage $V$ across the stack varies much stronger than $f_e$, and there seems to be an excess voltage indicating groups of junctions that are unlocked. For the case of the underdoped stack we perform intensive numerical simulations based on coupled sine Gordon equations combined with heat-diffusion equations. Many overall features can be reproduced well and point to an unexpected large value of the in-plane resistivity. However, unlike in experiment, in simulations the different resonant modes strongly overlap. The reason for this discrepancy is presently unclear.

Superconducting radio-frequency (SRF) cavities, cooled by superfluid helium-4 ($\mathrm{He}$ II), are key components in modern particle accelerators. Quenches in SRF cavities caused by Joule heating from local surface defects can severely limit the maximum achievable accelerating field. Existing methods for quench-spot detection include temperature mapping and second-sound triangulation. These methods are useful but also have known limitations. Here we describe an alternative method for surface quench-spot detection by visualizing the heat transfer in $\mathrm{He}$ II via tracking ${\mathrm{He}}_2^{\ast }$ molecular tracer lines. A proof-of-concept experiment is conducted, in which a miniature heater mounted on a plate is pulsed on to simulate a surface quench spot. A ${\mathrm{He}}_2^{\ast }$ tracer line created nearby the heater deforms due to the counterflow heat transfer in $\mathrm{He}$ II. By analyzing the tracer-line deformation, we well reproduce the heater location within a few hundred microns, which clearly demonstrates the feasibility of this alternative technology. Our analysis also reveals that the heat content transported in $\mathrm{He}$ II is only a small fraction of the total input heat energy. We show that the remaining energy is essentially consumed in the formation of a cavitation zone near the heater. By estimating the size of this cavitation zone, we discuss how the existence of the cavitation zone may explain a decades-long puzzle observed in many past second-sound triangulation experiments.

Ferroelectric materials are potential candidates for solid-state thermal switches since their polarizations, interatomic force constants, and microstructures can be tuned by applying an electric field. However, recent experiments on lead zirconate titanate [Ihlefeld et al., Nano Lett. 15, 1791–1795 (2015)] and bismuth ferrite [Ning et al., Phys. Rev. Appl. 8, 054049 (2017)] thin films only observed weak dependence of thermal conductivity k on an electric field. In order to explain the experimental results and theoretically explore the electric field-controlled thermal switch, we solve the linearized Boltzmann transport equation with first-principles force constants to calculate k of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ at different electric fields. For a single-domain structure, the k variation under an electric field is caused by the intrinsic structural response, whose main effects are manifested through the phonon-phonon scattering. The k accumulation function indicates that in order to observe the intrinsic electric field (E) effects, the domain size d should be larger than the critical domain size (CDS), around 30 nm at 300 K and 150 nm at 80 K. Introducing domain wall (DW) scattering creates an additional, extrinsic mechanism for k control: the effects of an E on d can either increase or decrease the k variation, and the k(T) function generally exhibits a peak at some temperature T < 300 K due to the competition between phonon-phonon scattering and DW scattering. This study identifies three strategies to maximize the k variation in response to an E: using samples with d larger than the CDS, maximizing the sensitivity of d to an E, and decreasing T to an optimum.

We theoretically investigate the temperature and thickness dependence of the effective Gilbert damping constant $(\alpha )$ in the $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$/$\mathrm{Mg}\mathrm{O}$ system using atomistic spin dynamics. We consider a high damping constant at the interface layer and a low damping constant for the bulklike layers due to large interfacial spin-orbit coupling. We find a strong dependence of the effective Gilbert damping with the film thickness, in quantitative agreement with experimental data. The temperature dependence of the effective damping arising from thermal-spin fluctuations up to temperatures of 400 K is weak, with no apparent change over the studied temperature range. Interestingly, we find that the temperature produces a different effect: a statistical fluctuation of the Gilbert damping parameter for a given relaxation induced solely from the finite size of the system. This statistical variation of the Gilbert damping is an intrinsic effect and is important for spintronic devices operating at gigahertz frequencies, where the dynamic response must be carefully controlled.

We theoretically study a graphene-based plasmonic waveguide composed of graphene, hexagonal boron nitride and active layer heterostructure, which can gate the transmission of a surface plasmon polariton (SPP) localized near the interface between the active layer and graphene. When a gate voltage is applied above a certain critical value, the charge-density modulation in the quasi-2D electron gas formed in the inversion layer can induce a local plasmon resonance. Since the local plasmon resonance is strongly coupled to the SPP, it can suppress the transmission of the SPP. The main advantage of our device lies in the sharpness of the switching line shape, since the operation manifests the underlying resonance phenomenon. By taking the active layer to be the $p$-type $\mathrm{Si}$(100) layer, we calculate the propagation length of the SPP with varying gate voltage. We show that the wavelength of the SPP is reduced to approximately $1/100$ of that of an incident light and the propagation length of the SPP is significantly reduced by a factor of approximately $20$ upon switching. This ensures that our plasmonic waveguide can operate effectively as a plasmonic switch for the SPP.

We present a light-scattering setup coupled to a commercial rheometer operated in the plate-plate geometry. The apparatus allows the microscopic dynamics to be measured, discriminating between the contribution due to laminar flow or affine deformation and additional mechanisms, such as plasticity. Light backscattered by the sample is collected with an imaging optical layout, thereby allowing the average flow velocity and the microscopic dynamics to be probed with both spatial and temporal resolution. We successfully test the setup by measuring the Brownian diffusion and flow velocity of dilute colloidal suspensions, both at rest and under shear. The potentialities of the apparatus are explored in the start-up shear of a biogel. For small shear deformations, $\gamma \le 2\mathrm{\%}$, the rheological response of the gel is linear. However, striking deviations from affine deformation are seen from the very onset of shear, due to temporally and spatially heterogeneous rearrangements having intriguing similarities with a stick-slip process.