Driven by the technological importance of the recently discovered ferroelectric ${\mathrm{Hf}\mathrm{O}}_2$, we explore a magnetoelectric effect at the ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectric-ferromagnetic interface. Using density-functional-theory calculations of the $\mathrm{Ni}/{\mathrm{Hf}\mathrm{O}}_2/\mathrm{Ni}$ (001) heterostructure as a model system, we predict a stable and sizable ferroelectric polarization in a few-nm-thick ${\mathrm{Hf}\mathrm{O}}_2$ layer. For the $\mathrm{Ni}/{\mathrm{Hf}\mathrm{O}}_2$ interface with opposite polarization directions (pointing to or away from the interface), we find a sizable difference in the interfacial $\mathrm{Ni}$—$\mathrm{O}$ bonding, resulting in dissimilar degrees of depletion of the electron density around the interface. The latter affects the relative population of the exchange-split majority and minority spin bands at the interface and thus the interfacial magnetic moments. The sizable change in the interface magnetization with ferroelectric polarization reversal of ${\mathrm{Hf}\mathrm{O}}_2$ manifests a significant ferroelectrically induced magnetoelectric effect at the $\mathrm{Ni}/{\mathrm{Hf}\mathrm{O}}_2$ interface. Our results reveal promising prospects of ferroelectric-ferromagnetic composite multiferroics based on ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectric materials.

We report a gradient metasurface design at microwave bands as an elegant approach to realize the goal of “rainbow trapping” for the storage of waves involving wave localization and absorption phenomena. A longitudinally placed coplanar waveguide is loaded with gradient metasurfaces on both sides, where split-ring resonators (SRRs) are the basic cell. The same SRRs are arranged along the transverse direction to establish magnetoinductive channels. Waves of different frequencies are coupled to corresponding SRRs at different positions in metasurfaces. Resonant trapping with a long oscillation life time enhances the absorption caused by inherent losses of the materials, thereby suppressing reflections. Both simulations and measurements verify the existence of “rainbow trapping.” The proposed strategy enhances the interaction between waves and matter, opening an avenue for further component designs, including absorptive filters, multiplexers, and buffers.

Resonant magnetoelastic coupling (MEC) is demonstrated in an $\mathrm{Fe}$ thin film epitaxially grown on a piezoelectric $\mathrm{Ga}\mathrm{As}$ substrate with application of subgigahertz surface acoustic waves (SAWs). The frequency at which resonant MEC is achieved is reduced far below 1 GHz by the application of a small in-plane magnetic field. Moreover, the resonance, observable by attenuation and velocity changes of the SAW, can be switched on and off by a small (${0.1}^{\circ }$) angular rotation of this in-plane field. This angular sensitivity makes SAW-ferromagnet devices attractive for sensing applications, such as wireless, battery-free, and interrogable magnetic-field monitors. Using a simple magnetization dynamics model that takes into account the $\mathrm{Fe}$ magnetic anisotropy and the softening of the magnetic precession modes, we are able to describe the observed salient features.

To construct global quantum communication networks, reliable free-space quantum key distribution is essential, but there has appeared a bottleneck severely hindering practical implementation, i.e., background noise. Rather than questing for filters closer to the ultimate performance, we seek a preferable solution from continuous-variable techniques employing coherent detection. In this regard, we investigate the dependence of the performance of continuous-variable quantum key distribution on background noise; the background noise is quantified and experimentally verified. The results indicate effective resistance against background noise. This characteristic demonstrates the feasibility of all-day free-space quantum communication using coherent detection, thus offering an alternative candidate for building networks.

We report a three-dimensional instability-triggered mechanical metamaterial that simultaneously possesses negative stiffness, negative bulk modulus, and negative Poisson's ratio. This metamaterial is a periodic arrangement of binder-shell elements. Combining numerical simulation and micromechanics analysis, we show that the thin spherical shells dent inward under compression, which contributes to a negative Poisson's ratio. Snap-through instability is predicted by the Riks method-based simulation, revealing that the metamaterial exhibits negative incremental stiffness. Negative bulk modulus is an outcome of negative stiffness and negative Poisson's ratio. Interestingly, both incremental stiffness and incremental Poisson's ratio approach negative infinity under displacement-controlled loading when snap back takes place. We further show that a triply negative metamaterial can satisfy the strong ellipticity condition, providing a mechanism to prevent the formation of localized deformation bands even when material softening takes place. The proposed triply negative metamaterial can also serve as structural elements for applications such as extreme damping composites, extreme stiff composites, and mechanical actuators.

The Shockley and Queisser limit, a well-known efficiency limit for a solar cell, is based on unrealistic physical assumptions and its maximum limit is seriously overestimated. To understand the power loss mechanisms of record efficiency cells, a more rigorous approach is necessary. We establish a formalism that can accurately predict absolute performance limits of solar cells in conventional thin-film form. In particular, a formulation for a strict evaluation of the saturation current in a nonblackbody solar cell is developed by taking the incident angle, light polarization, and texture effects into account. Based on the established method, we estimate the maximum efficiencies of 13 well-studied solar cell materials [$\mathrm{Ga}\mathrm{As}$, $\mathrm{In}\mathrm{P}$, $\mathrm{Cd}\mathrm{Te}$, $a\text{−}\mathrm{Si}:\mathrm{H}$, ${\mathrm{Cu}\mathrm{In}\mathrm{Se}}_2$, ${\mathrm{Cu}\mathrm{Ga}\mathrm{Se}}_2$, ${\mathrm{Cu}\mathrm{In}\mathrm{Ga}\mathrm{Se}}_2$, ${\mathrm{Cu}}_2{\mathrm{Zn}\mathrm{Sn}\mathrm{Se}}_4$, ${\mathrm{Cu}}_2{\mathrm{Zn}\mathrm{Sn}\mathrm{S}}_4$, ${\mathrm{Cu}}_2\mathrm{Zn}\mathrm{Sn}(\mathrm{S},\mathrm{Se}{)}_4$, ${\mathrm{Cu}}_2{\mathrm{Zn}\mathrm{Ge}\mathrm{Se}}_4$, ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, $\mathrm{HC}({\mathrm{NH}}_2{)}_2{\mathrm{PbI}}_3$] in a 1-µm-thick physical limit. Our calculation shows that over 30% efficiencies can be achieved for absorber layers with sharp absorption edges ($\mathrm{Ga}\mathrm{As}$, $\mathrm{In}\mathrm{P}$, $\mathrm{Cd}\mathrm{Te}$, ${\mathrm{Cu}\mathrm{In}\mathrm{Ga}\mathrm{Se}}_2$, ${\mathrm{Cu}}_2{\mathrm{Zn}\mathrm{Ge}\mathrm{Se}}_4$). Nevertheless, many record efficiency polycrystalline solar cells, including hybrid perovskites, are limited by open-circuit voltage and fill-factor losses. We show that the maximum conversion efficiencies described here present alternative limits that can predict the power generation of real-world solar cells.

In this work, we aim to design a heterostructure-based nanoscale thermoelectric generator that can maximize the waste-heat conversion efficiency at a given output power. The primary objective to be achieved for this is to realize a boxcar-shaped (band-pass) electronic transmission function [Phys. Rev. Lett. 112, 130601 (2014)]. In order to achieve that, we propose the use of an electronic analog of an optical Fabry-Perot cavity over a central resonant-tunneling structure. We further explore the optimum design possibilities by varying the geometry of the cavity wall to ensure a nearly perfect band-pass energy filtering of electrons. Based on our findings, we propose a general design guideline to realize such transmission and demonstrate that such devices can be excellent thermoelectric generators as compared to the existing proposals in terms of boosting the output power without a cost in efficiency. It is theoretically demonstrated using the nonequilibrium Green’s function technique coupled with self-consistent charging effects that an enhancement in the maximum output power up to $116\mathrm{\%}$ can be achieved through this scheme at a $10\mathrm{\%}$ higher efficiency as compared to resonant-tunneling-based devices. Furthermore, an elaborate comparative study of the linear response parameters is also presented and explained in terms of the physical transport properties. This study suggests an optimal device design strategy for an improved thermoelectric generator and sets the stage for a class of thermoelectric generators facilitated via transmission line-shape engineering.

We report an in situ mass measurement of approximately-4.7-$\mu \mathrm{m}$-diameter, optically levitated microspheres with an electrostatic co-levitation technique. The mass of a trapped, charged microsphere is measured by holding its axial (vertical) position fixed with an optical feedback force, under the influence of a known electrostatic force. A mass measurement with $1.8\mathrm{\%}$ systematic uncertainty is obtained by extrapolating the electrostatic force required to support the microsphere against gravity in the absence of optical power. In three cases the microspheres are recovered from the trap on a polymer-coated silicon beam and imaged with an electron microscope to measure their radii. The simultaneous precision characterization of the mass and radius of individual microspheres implies a density of $1.55\pm 0.08\mathrm{g}/{\mathrm{cm}}^3$. The ability to recover individual microspheres from an optical trap opens the door to further diagnostics.

We demonstrate full control of acoustic and thermal periodic deformations at solid surfaces down to subnanosecond time scales and few-micrometer length scales via independent variation of the temporal and spatial phase of two optical transient grating (TG) excitations. For this purpose, we introduce an experimental setup that exerts control of the spatial phase of subsequent time-delayed TG excitations depending on their polarization state. Specific exemplary coherent control cases are discussed theoretically and corresponding experimental data are presented in which time-resolved x-ray reflectivity measures the spatiotemporal surface distortion of nanolayered heterostructures. Finally, we discuss examples where the application of our method may enable the control of functional material properties via tailored spatiotemporal strain fields.

Bias-polarity-dependent photocurrent spectra of a terahertz quantum-well photodetector with an asymmetric stepped-quantum-well absorption layer are explored both in theoretical and experimental aspects. The parameters of the stepped quantum well are designed to realize broadband detection that the photoresponse, corresponding to the electron transitions of ground subband $(E_0)$ to the first $(E_1)$ and second $(E_2)$ subbands, covers a broadband frequency range of 4.0–7.0 THz. Due to the asymmetric shape of the modulus squared wave functions of the $E_2$ subband, the escape probability of the photoexcited electrons in subband $E_2$ to continuum states is strongly bias-polarity dependent, which is responsible for the different shapes of photocurrent spectra with positive and negative bias voltages applied across the terahertz quantum-well photodetector. Our results show that not only the band structures, but also the wave functions play key roles in the analysis of the photoresponse of terahertz quantum-well photodetectors.

When a signal is injected in a parametric oscillator close enough to its resonance, the oscillator frequency and phase are locked to those of the injected signal. Here we demonstrate two frequency-locking schemes using a Josephson mixer in the parametric down-conversion regime, pumped beyond the parametric oscillation threshold. The circuit then emits radiation out of two spectrally and spatially separated resonators at frequencies determined by the locking schemes that we choose. When we inject the signal close to a resonance, it locks the oscillator emission to the signal frequency by injection locking. When we inject the signal close to the difference of resonances, it locks the oscillator emission by parametric locking. We compare both schemes and investigate the dependence of the parametric locking range on the pump and the injected-signal power. Our results can be interpreted using Adler’s theory for lasers, which provides a link between laser physics and superconducting circuits that could enable better understanding of pumped circuits for quantum-information applications such as error correction, circulators, and photon-number detectors.

We report the theoretical and experimental realization of four types of passive phased array prisms with various asymmetric acoustic manipulations (AAMs) in air. The designed prisms consist of 31 transmitted unit cells of phase manipulation, which are composed of a n layer symmetric Helmholtz resonator (SHR) unit cell. Based on asymmetric phase distributions created by the nonparallel characteristic of exit interfaces on both sides of the structures, we can design a passive prism with asymmetric acoustic transmission (AAT) in which its fractional bandwidth can reach about 0.26. More interestingly, by adjusting the parameter of the SHR unit cells in the prism, we realize three other types of AAMs with the same mechanism, such as bidirectional acoustic insulation, asymmetric acoustic focusing, and reversed AAT. The proposed prisms have the advantages of broad bandwidth, multifunctional AAM, and passive structure as well as being easy to fabricate, and have great potential applications in integrated sound devices.

Temporal fluctuations in the solar irradiance are important for renewable-energy harvesting, as well as for climate and ecological-systems analysis. Environmental studies usually focus on slow fluctuations of the solar irradiance, assuming that fast fluctuations are averaged out, while studies of solar power generation emphasize the role of faster fluctuations, due to their effect on grid stability. Several analyses have reported a power-law dependence of the generated-power spectrum for frequencies ranging from 1/min to 1/year. However, both the origin of this power law and its geographic dependence are not fully understood, an issue of significance when one considers smoothing of solar-photovoltaic-power fluctuations by combining geographically distributed generation sources. Here we show that the power law appears in the global and direct components of the solar radiation and not only in the generated power. We also show that the exponent of the power law has a clear latitudinal dependence. At most locations, the spectral power-law dependence of the intermediate ($1/\mathrm{day}<f<1/\mathrm{h}$) frequencies is different from that of the high ($f>1/\mathrm{h}$) frequencies. Using a simple model, we explain the origin of the power-law spectra and the latitudinal dependence. The implications of the power law for photovoltaic power generation and grid stability are discussed. We also suggest that the analysis of solar power generation should consider the power spectrum of the clear-sky irradiance at the location of interest in order to separate atmospheric effects from geographical ones.

We investigate the magnetic proximity effect in a van der Waals heterostructure formed by a monolayer semiconductor stacked on a two-dimensional (2D) ferromagnet, where the lattice mismatch and twisting between the layers typically lead to the formation of a moiré pattern. We find that the magnetic proximity effect arising from the spin-dependent interlayer coupling depends sensitively on the interlayer atomic registry. Consequently, in the moiré pattern, the spatial variation of the atomic registry leads to a lateral modulation of a magnetic proximity field. Such a moiré modulated magnetic proximity effect manifests as a miniband spin splitting that strongly depends on the moiré periodicity, which can be mechanically tuned by a relative twisting and/or strain between the layers. We also show, because of the moiré modulation on the interlayer distance, a perpendicular electric field can be used to control the miniband spin splitting. Our results suggest potential nanodevices where the moiré modulated magnetic proximity effect can lead to unique spin controllability.

Slow light enables spatiotemporal manipulation of electromagnetic waves at the nanoscale and allows access to a plethora of nonlinear optical phenomena. Although the guided waves in plasmonic waveguides are known to inherently possess a slow energy velocity, their ultimate light-trapping performance remains unknown as the effect of the waveguide's shape alteration has not been considered systematically so far. In this work, we theoretically demonstrate a free-form optimized metal-insulator-metal plasmonic waveguide for light trapping that exhibits a quality factor several times higher than that of the conventional linearly tapered structures. The quality factor of the optimized waveguide saturates to the theoretical limit at a surprisingly short device length, which shows a nontrivial inverse logarithmic dependence on the material loss. The demonstrated design has a quality-factor-to-footprint ratio comparable to that of state-of-the-art photonic cavities.

Room-temperature surface exciton polaritons (SEPs) were successfully demonstrated recently by replacement of the metal thin film in the Kretschmann-Raether configuration of surface plasmon polaritons (SPPs) with a film of the J-aggregate cyanine dye 5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)benzimidazol-2-ylidene]propenyl]-1-ethyl-3-(4-sulfobutyl)benzimidazolium hydroxide (TDBC). Compared with SPPs (i.e., photons coupled to highly mobile free electrons at the metal surface), the coupling of photons to less-mobile excitons supports stronger field confinement at the TDBC surface. This extraordinary field confinement of SEPs beyond SPPs motivated us to explore the feasibility of refractive-index (RI) sensing, in particular, surface RI sensing. We find that the bulk RI sensitivity of ${118}^{\circ }$ (${140}^{\circ }$) per refractive-index unit (RIU) of the proposed SEP sensor in the RI range from 1.0 to 1.001 (from 1.33 to 1.36) for a gaseous analyte (an aqueous solution) is higher than that of a conventional gold-based SPP sensor [${73}^{\circ }/$RIU (${124}^{\circ }/$RIU)]. For surface RI sensing, the SEP sensor has both higher sensitivity and a higher figure of merit than the SPP sensor. These numerical results undoubtedly reveal SEPs to be a promising mechanism for sensing.

The unprecedented capability of the metasurface to manipulate light propagation is extended to the nonlinear optical regime, which enables a plethora of applications for nonlinear optical security and encryption. One of the key issues in this area is how to arbitrarily control the vectorial polarization profile of nonlinear waves. Here, we propose and experimentally demonstrate a nonlinear photonic metasurface that can generate a vectorial polarization profile for nonlinear imaging. The high-resolution grayscale image can be read out through a second harmonic generation (SHG) beam from an ultrathin nonlinear metasurface. The polarization of SHG waves can be locally manipulated by simply changing the orientation angle of the plasmonic meta-atoms. The grayscale image is revealed using a linear polarizer based on the Malus law. We believe that the rich polarization information based on nonlinear metasurfaces has significant potential for applications in nonlinear optical image encryption, security, anti-counterfeiting, and so on.

We investigate theoretically and experimentally the efficient control of the light spectrum in a fiber-loop circuit with two incorporated phase modulators (PMs). By varying the modulation phases in the PMs, photonic gauge potentials can be introduced in the loop, enabling full control of spectrum shift and band width. The spectrum evolution also depends rigorously on the resonant modulation condition, which suggests that the time delay of light in one circulation equals integer multiples of the modulation period such that the gauge potentials remain constant in distinct circulations. As the two PMs possess the same gauge potentials under resonant conditions, a 50-GHz frequency shift and three-fold band expansion are experimentally realized. For the out-of-phase modulation under resonant conditions, we also achieve the frequency analogue of perfect imaging for arbitrarily input spectra. As the resonant condition is broken, both the spectrum shift and band width are dramatically suppressed. This study may find potential applications in the spectrum management for both optical communications and signal processing.

This paper studies the theory and applications of the diffraction of electromagnetic waves by space-time-periodic (STP) diffraction gratings. We show that, in contrast with conventional spatially periodic gratings, a STP diffraction grating produces spatial diffraction orders, each of which is formed by an infinite set of temporal diffraction orders. Such spatiotemporally periodic gratings are endowed with enhanced functionalities and exotic characteristics, such as an asymmetric diffraction pattern, nonreciprocal and asymmetric transmission and reflection, and an enhanced diffraction efficiency. The theory of the wave diffraction by STP gratings is formulated through satisfying the conservation of both momentum and energy, as well as rigorous Floquet mode analysis. Furthermore, the theoretical analysis of the structure is supported by time- and frequency-domain finite-difference time-domain (FDTD) numerical simulations for both transmissive and reflective STP diffraction gratings. Additionally, we provide the conditions for Bragg and Raman-Nath diffraction regimes for STP gratings. Finally, as a particular example of a practical application of the STP diffraction gratings to communication systems, we propose an original multiple-access communication system featuring full-duplex operation. STP diffraction gratings are expected to find exotic practical applications in communication systems, especially for the realization of enhanced-efficiency or full-duplex beam coders, nonreciprocal beam splitters, nonreciprocal and enhanced-resolution holograms, and illusion cloaks.

Double-stranded DNA (dsDNA) molecules possess a unique chirality, where chirality-induced spin selectivity offers the promise of organic-molecule-based spintronic devices, and they have become an interesting field of their own. When coupled to ferromagnetic leads, such systems, when driven by a temperature gradient, are expected to manifest new thermoelectric behaviors. Herein, we investigate the spin-related thermoelectricity of a chiral dsDNA molecular junction in terms of linear response theory and the nonequilibrium Green’s function formalism. Not only is a strong violation of the Wiedemann-Franz law found in the charge-Seebeck transport regime, but the spin figure of merit is also larger than unity. More interestingly, a finite spin-Seebeck coefficient with a charge thermopower of zero can be obtained, which may serve as a pure spin-current generator. The detailed dependence of the basic thermoelectric quantities on the system parameters is elaborated on, and the underlying physics is presented. Our results have substantial implications for the construction of spin caloritronic devices based on dsDNA or other chiral molecules.