We introduce chiral gradient metasurfaces that allow perfect transmission of the incident wave into a desired direction and simultaneous perfect rotation of the polarization of the refracted wave with respect to the incident one. In the lossless limit, transmission can reach 100% with a single metasurface layer. Besides using gradient polarization densities that provide bending of the refracted wave with respect to the incident one, the use of metasurface inclusions that are chiral allows the polarization of the refracted wave to be rotated. We suggest a possible realization of the proposed device by discretizing the required equivalent-surface polarization densities and synthesizing the chiral discrete polarizabilities with properly sized helical inclusions at each discretization point. By using only a single optically thin layer of chiral inclusions, we are able to deflect a normal incident plane wave to a refracted plane wave at ${45}^{\circ }$ with a $72\mathrm{\%}$ power efficiency that is accompanied by a ${90}^{\circ }$ polarization rotation. The proposed concepts and the design method may find practical applications in polarization-rotation devices in microwaves as well as in optics, especially when the incident power is required to be deflected.

We study numerically the interaction of surface acoustic waves propagating at the surface of a multilayer structure (the so-called Sezawa waves) with localized surface plasmons at the bottom of metallic pillars deposited on the substrate. The pillars are made of gold and the multilayer structure is constituted by a layer of gold on a silicon substrate and covered by a thin spacer of dielectric material. We are interested in localized plasmons, which are the analog of metal-dielectric-metal surface plasmons. The physical characteristics of these plasmons, such as their eigenvalues and absorption or reflection spectra, are modulated by surface acoustic waves due to deformation of the structure in the vicinity of the pillars. The surface waves are generated in the area in front of the pillars deposited on the substrate. Two types of acoustic modes are envisioned to interact with the localized plasmons, namely, confined modes associated with the local resonances of the pillars and the propagating Sezawa modes that deform the structure in the vicinity of the surface. We discuss the efficiency of phonon-plasmon coupling for both types of modes and select those that can be useful for an experimental realization based on this platform. Also, we show that, besides the plasmon frequency, the phonon-plasmon coupling magnitude strength is sensitive to the distance between the pillars, which reveals the role of interactions between the pillars on the coupling. The geometrical parameters are chosen such that the acoustic waves are in the sub-GHz range and the plasmons are around the telecommunication wavelength of 1.55 μm. This work may help with the design of optomechanical devices that can be controlled by fast coherent acoustics and facilitate applications involving enhanced sound-light interactions, such as light modulation.

The efficiency of single-junction solar cells is limited to about 30% (the Shockley-Queisser limit). Spectral mismatch losses (transparency to low-energy photons, thermalization of high-energy photons) strongly contribute to lowering the maximum efficiency. To reduce thermalization losses, photon splitting is proposed and observed for a variety of lanthanide-doped materials. For ${\mathrm{Er}}^{3+}$, even a one-to-three photon-splitting process has been reported, yielding three IR photons at around 1530 nm following absorption of one blue-green photon. This is especially beneficial for narrow band gap solar cells, such as crystalline $\mathrm{Ge}$. Here, we report on photon splitting for ${\mathrm{Er}}^{3+}$ in ${\mathrm{YVO}}_4$. Following absorption in the ${}^2{\mathrm{H}}_{11/2}$ and ${}^4{\mathrm{S}}_{3/2}$ levels (520–550 nm), efficient cross-relaxation (CR) yields two excited ${\mathrm{Er}}^{3+}$ ions: one in the ${}^4{\mathrm{I}}_{9/2}$ state and one in the ${}^4{\mathrm{I}}_{13/2}$ state (CR1). A second CR step from the ${}^4{\mathrm{I}}_{9/2}$ state, leaving both ${\mathrm{Er}}^{3+}$ ions in the ${}^4{\mathrm{I}}_{13/2}$ excited state (CR2), is crucial in realizing efficient three IR photon splitting. It is demonstrated here that the second step has a low efficiency, as a result of competing fast multiphonon relaxation, ${}^4{\mathrm{I}}_{9/2}{\to }^4{\mathrm{I}}_{11/2}$, and a large energy mismatch, which makes the CR2 step thermally activated. Based on experiments and theory, a maximum quantum efficiency of 170% is calculated for IR emission, following blue-green excitation in ${\mathrm{YVO}}_4:{\mathrm{Er}}^{3+}$. An outlook is presented for three-photon splitting in low-phonon-energy hosts, where nonradiative multiphonon relaxation is suppressed. The anti-Stokes nature of the second CR step makes three-photon splitting unlikely and prevents the realization of IR quantum yields above 200%.

The role of meniscus motion and ink viscosity in the formation of a secondary tail and its breakup are studied experimentally during the picoliter-droplet formation process of a MEMS piezoacoustic inkjet print head using laser-induced 8-ns single-flash stroboscopic imaging with a temporal resolution of 100 ns. It is found that the formation of the secondary tail is driven by meniscus motion and that the secondary tail forms reproducibly between the primary tail and the meniscus in the final microseconds before pinchoff. We demonstrate that the stability of the secondary tail can be controlled through the motion of the meniscus after the primary tail has formed. A 4 times increase in stretching rate results in a 2.2 times increase in the secondary-tail length and a 3 times higher number of femtoliter satellites. Furthermore, as expected for Rayleigh breakup, a 43% increase in ink viscosity is found to increase the secondary-tail length by 50%. Finally, it is found that, during inkjet printing, the secondary tail cascades into tertiary and quaternary tails. We show that the formation of higher-order tails is irreproducible and therefore driven by noise. The formation of thicker secondary and thinner higher-order tails results in a bimodal satellite size distribution, where the secondary satellites with a volume greater than or equal to $4\mathrm{fL}$ are located closer to the primary-tail droplet, while satellites with a volume less than $4\mathrm{fL}$ are located closer to the nozzle. The main findings of the present work, that the stability of the secondary tail decreases with a decrease in stretching rate and ink viscosity, can be employed in the inkjet-printing community for waveform design to minimize internal contamination of inkjet printers.

Recent years have seen a fascinating pollination of ideas from quantum theories to elastodynamics—a theory that phenomenologically describes the time-dependent macroscopic response of materials. Here, we open a route to transfer additional tools from non-Hermitian quantum mechanics. We begin by identifying the differences and similarities between the one-dimensional elastodynamics equation without body forces and the time-independent Schrödinger equation and finding the condition under which the two are equivalent. Subsequently, we demonstrate the application of the non-Hermitian perturbation theory to determine the response of elastic systems; the calculation of leaky modes and the energy decay rate in heterogenous solids with open boundaries using a quantum mechanics approach; and construction of degeneracies in the spectrum of these assemblies. The latter result may be of technological significance, as it introduces an approach to harness extraordinary wave phenomena associated with non-Hermitian degeneracies for practical devices, by designing them in simple elastic systems. As an example of such an application, we demonstrate how an assembly of elastic slabs that is designed with two degenerate shear states according to our scheme can be used for mass sensing with enhanced sensitivity by exploiting the unique topology near the exceptional point of degeneracy.

Arrays of spin-torque nano-oscillators are promising for broadband microwave signal detection and processing, as well as for neuromorphic computing. In many of these applications, the oscillators should be engineered to have equally spaced frequencies and equal sensitivity to microwave inputs. Here we design spin-torque nano-oscillator arrays with these rules and estimate their optimum size for a given sensitivity, as well as the frequency range that they cover. For this purpose, we explore analytically and numerically conditions to obtain vortex spin-torque nano-oscillators with equally spaced gyrotropic oscillation frequencies and having all similar synchronization bandwidths to input microwave signals. We show that arrays of hundreds of oscillators covering ranges of several hundred MHz can be built taking into account nanofabrication constraints.

We propose ultralow-power plasmonic tweezers with no external optical source. They consist of a one-dimensional array of graphene-based plasmonic units driven by the optical transitions within the underlying array of $(\mathrm{Al},\mathrm{In})\mathrm{As}/(\mathrm{Ga},\mathrm{In})\mathrm{As}/(\mathrm{Al},\mathrm{In})\mathrm{As}/(\mathrm{Ga},\mathrm{In})\mathrm{As}/(\mathrm{Al},\mathrm{In})\mathrm{As}$ quantum cascaded heterostructures (QCHs), electrically biased in series. Each QCH unit formed in a nanopillar can act as a built-in optical source required for exciting the localized surface plasmons (LSPs) at the surface of the overlying circular graphene nanodisk. The stimulated emission due to intersubband transition within each optical source evanesces through the top $(\mathrm{Al},\mathrm{In})\mathrm{As}$ cladding layer and interacts with the overlying graphene nanodisk, inducing the LSPs required for the formation of the plasmonic tweezers. Numerical simulations show, under 145–170 mV applied voltages, that the tweezers with graphene nanodisks of 16–30 nm in diameter and chemical potentials of 0.5–0.7 eV can trap polystyrene nanoparticles of 9 nm in diameter and larger, demonstrating acceptable sensitivities for variations in the nanoparticle diameter and refractive index. These lab-on-a-chip plasmonic tweezers, benefiting from their small footprints and ultralow power consumptions, which are capable of sensing and trapping nanoparticles without requiring expensive external optical sources, open up a different horizon for developing compact on-chip plasmonic tweezers.

When heat transfers through the interface between two different materials, it will encounter an interfacial thermal resistance (ITR) that makes the temperature discontinuous. This effect has been totally ignored so far in the research on a thermal cloak, in particular when the thermal cloak is built with multilayer structures. In this paper, we investigate the effect of ITR on the performance of a thermal cloak by both an analytical method and a numerical method. Our results show that the existence of ITR will distort the external field and thus destroy the cloak. Moreover, we find that the effect of ITR can be quantified by a parameter called the “characteristic length.”

We present measurements of microwave-induced Shapiro steps in a superconducting nanobridge weak link in the dissipative branch of a hysteretic current-voltage characteristic. We demonstrate that Shapiro steps can be used to infer a reduced critical current and an associated local temperature. Our observation of Shapiro steps in the dissipative branch shows that a finite Josephson coupling exists in the dissipative state. Although the nanobridge is heated, our model shows that the temperature remains below the critical temperature. This work provides evidence that Josephson behavior can still exist in thermally hysteretic weak-link devices and will allow extension of the temperature range over which nanobridge-based single-flux-quantum circuits, micron-sized superconducting quantum interference devices (i.e., nanoSQUIDs), and Josephson voltage standards can be used.

We present a piezoelectric transducer in thin-film lithium niobate that converts a 1.7-GHz microwave signal to a mechanical wave in a single mode of a 1-$\mu \mathrm{m}$-wide waveguide. We measure a $-12$-dB conversion efficiency that is limited by material loss. The design method we employ is widely applicable to transduction in wavelength-scale structures in emerging phononic circuits such as those needed for efficient piezo-optomechanical converters and spin-phonon transducers.

We present a method that combines continuous and pulsed microwave-radiation patterns to achieve robust interactions among hyperfine trapped ions placed in a magnetic field gradient. More specifically, our scheme displays continuous microwave drivings with modulated phases, phase flips, and $\pi$ pulses. This leads to high-fidelity entangling gates that are resilient against magnetic field fluctuations, changes in the microwave amplitudes, and crosstalk effects. Our protocol runs with arbitrary values of microwave power, which includes the technologically relevant case of low microwave intensities. We demonstrate the performance of our method with detailed numerical simulations that take into account the main sources of decoherence.

Monolithic optical cavities employing total internal reflections are appealing compact platforms for a wide range of applications from fundamental physics to industry applications. We characterize the transverse patterns of a millimeter-size monolithic nonplanar ring vortex laser pumped by a Gaussian beam. Vortex beams carrying orbital angular momentum (OAM) up to $30$ together with vortex crystals and petal patterns are observed. The rigid nature of the monolithic cavity brings along its low-noise lasing performance for these beams. A frequency noise floor as low as $0.1\mathrm{Hz}/{\mathrm{Hz}}^{1/2}$ corresponding to sub-Hz instantaneous linewidth is measured. Furthermore, we demonstrate low-noise dual-frequency lasing with phase noise as low as $-118\mathrm{dBc}/\mathrm{Hz}$ at 10-kHz offset for the beatnote at 8.2 GHz. We expect that such versatile and compact OAM laser sources can facilitate multiple applications including optical communications, optical tweezers, and laser metrology such as dual-frequency Doppler lidar.

A two-level quantum system can absorb or emit not more than one photon at a time. Using this fundamental property, we demonstrate how a superconducting quantum system strongly coupled to a transmission line can be used as a sensor of the photon flux. We propose four methods of sensing the photon flux and analyze them for the absolute calibration of power by measuring spectra of scattered radiation from the two-level system. This type of sensor can be tuned to operate in a wide frequency range, and does not disturb the propagating waves when not in use. Use of a two-level system as a power sensor enables a range of applications in quantum technologies, here in particular applied to calibrate the attenuation of transmission lines inside dilution refrigerators.

Single-shot helicity-independent all-optical switching of magnetization in ferrimagnets represents the fastest known approach for deterministic data recording. Recently, it was shown that 15-ps-long optical pulses could suffice in triggering the magnetic switching in certain $\mathrm{Gd}\text{−}\mathrm{Fe}\text{−}\mathrm{Co}$ alloys, generating enormous controversy about the underlying mechanism. Here, we demonstrate how the exact composition of the ferrimagnet affects the kinetics of the reversal process and facilitates the use of thermal pulses with a duration spanning all relevant timescales within the nonadiabatic limit. By modelling a generic ferrimagnet as two coupled macrospins, we show that the magnetization reversal can occur via distinctly different pathways, depending on the duration of the heater. We experimentally reveal that pulses with a duration below and above a critical pulse width respectively enable and disable the capability of all-optical magnetization switching in $\mathrm{Gd}\text{−}\mathrm{Fe}\text{−}\mathrm{Co}$ alloys, and that modest change of the alloy composition leads to drastic variation of the critical pulse width, by almost 2 orders of magnitude. Our interpretation and results resolve an urgent and outstanding technologically relevant controversy, and provide crucial but previously overlooked guidelines for how to engineer deterministic all-optical switching of magnetization in suitable ferrimagnets.

Any object with nonzero temperature emits thermal radiation described by the Stefan-Boltzmann law, and hence, thermal conduction is always accompanied by thermal radiation. Although a transformation theory has been proposed to control thermal conduction, a transformation theory has not yet been put forward to manipulate thermal radiation, let alone the thermal coupling between radiation and conduction (herein called multithermotics). To solve the problem, we establish the theory of transformation multithermotics to control radiation and conduction simultaneously. As model applications, we also design three devices with functions of cloaking, concentrating, and rotating. These devices are confirmed by transient finite-element simulations. As experimental suggestions, we further design transformation media with two homogeneous and isotropic materials to simplify fabrication. Our results provide general hints to multithermal management, and have potential applications in infrared camouflaging, solar-energy utilizing, etc.

We report a first-principles calculation for the wavelength dependence of a laser-excitation process on a silicon surface. Although a lower-frequency laser is reflected by a lower-density plasma, it can penetrate a thicker plasma sheet. Therefore, the depth of the laser processing depends on the width of the plasma at the surface and laser wavelength. The time-dependent density-functional theory and Maxwell’s equations are simultaneously employed to elucidate the effect of laser propagation on laser-matter interaction under ultrafast pulse lasers (FWHM: 12 fs). A longer-wavelength laser field facilitates deeper melting and ablation in silicon, despite a lower critical plasma density. Such a deeper excitation by a longer wavelength is because of the penetration of the laser field through the plasma on the surface. The plasma-formation depth is saturated at approximately half the wavelength in silicon.

Organic small molecule solar cells are used as a test bed to investigate the influence of film morphology on the density of charge-transfer (CT) states. CT states are considered as precursors for charge generation and their energy as the upper limit for the open-circuit voltage in organic donor-acceptor solar cells. In this study the influence of morphology for two perylene donors [crystalline diindenoperylene (DIP) versus amorphous tetraphenyldibenzoperiflanthene (DBP)] with almost identical ionization energy is investigated. As acceptor material, the fullerene ${\mathrm{C}}_{60}$ is used. By combining device measurements with optical and low-energy ultraviolet photoelectron spectroscopy, a comprehensive picture is obtained that describes how morphology and the connected density of states (DOS) affect device performance and the spectroscopic signature of CT states. Especially for the crystalline donor material DIP, strong exponential tail states reaching far into the gap are observed, which can be related to the presence of grain boundaries. A voltage-dependent filling of these states is identified as the origin of a blue shift of electroluminescence spectra with increasing applied voltage. Different approaches are compared to study the influence of static and dynamic disorder in the description of CT emission and absorption spectra of organic solar cells. Despite the fact that both donors yield almost identical CT energy (and, thus, the same open-circuit voltage) the Stokes shift between photocurrent and electroluminescence spectra and, concomitantly, the width of the CT DOS varies by more than a factor of 2. We discuss this observation in terms of the donor-acceptor reorganization energy as well as an additional line broadening by static disorder. Remarkably, the more crystalline donor DIP shows a significant deviation from a Marcus-type description, while this is not the case for the amorphous DBP. This highlights the importance of film morphology in organic solar cells.

Hot electron emission from waveguide-integrated graphene has been recently shown to occur at optical power densities multiple orders of magnitude lower than that of metal tips excited by sub-work-function photons. However, the experimentally observed electron emission currents are small, which limits the practical uses of such a mechanism. Here, we explore the performance limits of hot electron emission in graphene through experimentally calibrated simulations. Two regimes of nonequilibrium emission in graphene are identified, (i) single particle hot electron emission, where an electron is excited by a photon and emitted before losing significant energy through scattering; and (ii) ensemble hot electron emission, where the photon source causes nonequilibrium heating of the electron population beyond the electron lattice temperature. It is shown that, through appropriate selection of photon energy, optical power density, and applied electric field, hot electron emission can be used to create ultrahigh current electron emitters with ultrafast temporal responses in both the single particle and ensemble heating regimes. These results suggest that, through appropriate design, hot electron emitters may overcome the limitations of thermionic and field emitters.