Controlling and guiding optical spins and pseudospins over an interface is of great importance in the emerging fields of spintronics and valleytronics, holding the promise for new degrees of freedom in the quest for efficient information transport. Here we explore the general possibilities offered by metasurfaces to route optical momentum, spin and helicity through surface waves. We show how anisotropy and bianisotropy can be engineered to impart extreme directionality to local spin excitations. While most previous works have focused on out-of-plane spins, in-plane excitations have become particularly important for valleytronic applications relying on two-dimensional materials, and they offer specific challenges addressed here with suitably engineered nonlocality in metasurfaces.

The search for semiconducting materials with improved optical properties relies on the possibility to manipulate the semiconductors band structure by using quantum confinement, strain effects, and by the addition of diluted amounts of impurity elements such as $\mathrm{Bi}$. In this study, we explore the possibility to engineer the structural and physical properties of the $\mathrm{Ga}(\mathrm{As},\mathrm{Bi})$ alloy by employing different stress conditions in its epitaxial growth. Films with variable concentration of $\mathrm{Bi}$ are grown by molecular beam epitaxy on bare $\mathrm{Ga}\mathrm{As}(001)$ crystals and on partially relaxed $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ double buffer layers acting as stressors aiming to control the $\mathrm{Bi}$ incorporation into the alloy and improving the optical properties in terms of efficiency. A combination of several structural and electronic characterization techniques and dedicated density-functional-theory calculations allows us a systematic comparison between the samples grown under compressive and tensile strain. We demonstrate the possibility to grow $\mathrm{Ga}(\mathrm{As},\mathrm{Bi})$ under different strain conditions without affecting its crystal quality. The different strain conditions strongly impact the $\mathrm{Bi}$ incorporation in the $\mathrm{Ga}\mathrm{As}$ matrix and the luminescence properties of the sample. We find (i) a striking improvement of the photoluminescence with a strongly increased radiative efficiency when $\mathrm{Ga}(\mathrm{As},\mathrm{Bi})$ is grown under tensile strain and (ii) an interesting higher redshift with respect to $\mathrm{Ga}(\mathrm{As},\mathrm{Bi})$ grown compressively on $\mathrm{Ga}\mathrm{As}$. These two effects allow us to reach the important photoluminescence emission at 1.3 µm with a $\mathrm{Bi}$ concentration as low as 4.9% compared to 7.5% needed for samples grown directly on $\mathrm{Ga}\mathrm{As}$. This is a significant achievement for the application of the $\mathrm{Ga}(\mathrm{As},\mathrm{Bi})$ material in optoelectronic devices.

We introduce an optimal metagrating design for transverse-position metrology in presence of photon shot noise. The proposed working principle is closely related to the formation of a phase vortex in the diffraction orders in the parameter space. Using the topological robustness, we optimize the design and compress all the transverse-position information around a certain point into a small number of detected photons, saturating the shot-noise limit had all the photons used for probing the position been detected. The current scheme avoids the problem of detector saturation in the presence of high probe power while maintaining all the information detected, allowing one to make full use of the high power that is available. Besides, the direct link between the resonant property in the unit cell and the conditions to achieve the bound is given: one with the zeroth dipole resonance and the other one with the anapole condition of the first dipole. The connection between the metagrating design and the optimization using topological robustness along with the fundamental precision limit using classical light gives new insights in all of these fields.

The design of the scattering properties of electromagnetic structures is of fundamental interest in optical science and engineering. While there has been great practical success in applying local optimization methods to electromagnetic device design, it is unclear whether the performance of the resulting designs is close to that of the best possible design. This question remains open for absorptionless electromagnetic devices, since the absence of material loss makes it difficult to provide provable bounds on their scattering properties. We resolve this problem by providing nontrivial lower bounds on performance metrics that are convex functions of the scattered fields. Our bounding procedure relies on accounting for a constraint on the electric fields inside the device, which can be provably constructed for devices with small footprints or low dielectric contrast. We illustrate our bounding procedure by upper bounding the scattering cross sections of lossless dielectric and metallic particles.

Ferroicities in structural ferroic materials can effectively tune many different fascinating physical properties, leading to potential applications in the future. Usually, in order to break inversion symmetry in ferroelectrics, the material needs to be composed of multiple types of element. Here, we study a single element multiferroic material with time-reversal symmetry, monolayer $\mathrm{Bi}(110)$, which possesses multiple types of ferroic (ferroelectric and ferroelastic) order. By performing first-principles calculations combined with a group-theory analysis, we reveal that this multiferroic material exhibits various interesting properties, such as an out-of-plane persistent spin helix, valleytronics, and promising nonlinear optical properties, which depend on its orientation variant. We show further that under illumination with linearly polarized light, different orientation variants in the $\mathrm{Bi}(110)$ monolayer possess different free energies. Thus, one can use this noncontacting approach to control the thermodynamic stability of different types of order. The switching of the orientation variant is ultrafast and can occur in times on the order of picoseconds. During this process, no electron-hole pairs are generated. Therefore, one can eliminate waste heat during operation, ensuring good reversibility.

The random coupling model (RCM) has been successfully applied to predicting the statistics of currents and voltages at ports in complex electromagnetic (EM) enclosures operating in the short-wavelength limit. Recent studies have extended the RCM to systems of multimode aperture-coupled enclosures. However, as the size (as measured in wavelengths) of a coupling aperture grows, the coupling matrix used in the RCM increases as well, and the computation becomes more complex and time consuming. A simple power balance (PWB) model can provide fast predictions for the averaged power density of waves inside electrically large systems for a wide range of cavity and coupling scenarios. However, the important interference-induced fluctuations of the wave field retained in the RCM are absent in the PWB model. Here we aim to combine the best aspects of each model to create a hybrid treatment and study the EM fields in coupled enclosure systems. The proposed hybrid approach provides both mean and fluctuation information of the EM fields without the full computational complexity of the coupled-cavity RCM. We compare the hybrid model predictions with experiments on linear cascades of over-moded cavities. We find good agreement over a set of different loss parameters and for different coupling strengths between cavities. The range of validity and applicability of the hybrid method are tested and discussed.

We report the characterization of superlow-loss optics used in the second-generation gravitational-wave detectors currently in operation. The sapphire test-mass mirrors in the KAGRA detector are introduced as an example, but the techniques here are common to all detectors. In this work, we discuss mainly the surface topography obtained by interferometric techniques and the optical properties obtained with special setups.

Among some of the current uses of dc superconducting quantum interference devices (SQUIDs) are qubit readouts and sensors for probing the properties of quantum materials. We present a gradiometric niobium SQUID design with state-of-the-art sensitivity in the femtotesla range, which can be tuned to specific readout requirements. The sensor is a next-generation fractional SQUID with a tightly optimized input coil and a combination of various measures known for restraining parasitic resonances and other detrimental effects. Our design combines the practical usefulness of well-defined pickup loops for well-defined imaging kernel and tunable probing applications with a fractionalization approach to reduce undesired inductances. In addition, our modeling predicts small dimensions for these planar sensors. These features make them of relevance for material studies and for the detection of magnetic fields in small volumes, e.g., as part of a cryogenic scanning quantum imaging apparatus for efficient diagnostics and quantum device readouts. This manuscript will benefit scientists and engineers working on quantum-computing technologies by clarifying potential general misconceptions about dc SQUID optimization alongside the introduction of the flexible compact dc SQUID design.

We report on a giant enhancement in the thermal responsivity of doubly clamped $\mathrm{Ga}\mathrm{As}$ microelectromechanical-system- (MEMS) beam resonators by using the internal-mode-coupling effect. This is achieved by coupling the fundamental bending mode with the fundamental torsional mode of the MEMS-beam resonators through the cubic Duffing nonlinearity. In the mode-coupling regime, we find that when the input heat to the MEMS resonators is modulated at a particular frequency, the resonance-frequency shift caused by heating can be enhanced by almost 2 orders of magnitude. The observed effect is promising for realization of high-sensitivity thermal sensing by use of MEMS resonators, such as ultrasensitive terahertz detection at room temperature.

We propose a microscale dielectric laser-driven particle accelerator based on the inverse effect of Cherenkov radiation. It utilizes a parallel-polarized laser beam that is incident perpendicularly on a right-angle surface of a high-breakdown-threshold prism, at the hypotenuse surface of which the laser-induced waves are innately synchronized with incoming free electrons, providing them with a continuous acceleration force. Compared with radio-frequency accelerators, its acceleration gradient is remarkably higher and its size is orders of magnitude smaller. In contrast to previous dielectric laser accelerators based on the inverse Smith-Purcell effect, it is not susceptible to the spectral dispersion of the laser pulse, and it can provide a long duration of the synchronized accelerating field without using external pulse-front-tilting techniques. In addition, it effectively avoids electron deflection caused by transverse fields, greatly improving the acceleration capability. Thus, it affords a promising way of developing ultracompact and highly efficient accelerators on chip.

${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ ($\mathrm{YIG}$) thin films having a thickness of several nanometers and showing both strong perpendicular magnetic anisotropy (PMA) and low magnetic damping are reported. The films are deposited by magnetron sputtering at room temperature first and then annealed in ${\mathrm{O}}_2$ at high temperature. The substrates are ${\mathrm{Gd}}_3({\mathrm{Sc}}_2{\mathrm{Ga}}_3){\mathrm{O}}_{12}$, which share the same crystalline structure as $\mathrm{YIG}$, but have a lattice constant slightly larger than that of $\mathrm{YIG}$; the lattice mismatching gives rise to an out-of-plane compressive strain and PMA in the $\mathrm{YIG}$ films. The PMA is confirmed by vibrating sample magnetometer, magneto-optical Kerr effect, anomalous Hall effect, and angle-dependent ferromagnetic resonance (FMR) measurements. The damping of the films is analyzed through frequency-dependent FMR measurements. As an example, an 8-nm-thick $\mathrm{YIG}$ film shows an effective PMA field of about 2800 Oe, a nearly square hysteresis loop, and a damping constant of only $4.2\times {10}^{-4}$. As an illustration of possible applications of such films in spintronic devices, current-induced switching of the magnetization of the PMA $\mathrm{YIG}$ films is demonstrated by the use of $\mathrm{YIG}$/$\mathrm{Pt}$ bilayered Hall bar devices.

Magnetic and structural transitions are observed to coincide at around room temperature in transition-metal-based $(\mathrm{Mn}\mathrm{Ni}\mathrm{Si}{)}_{1\text{−}x}(\mathrm{Fe}\mathrm{Co}\mathrm{Ga}{)}_x$ (x = 0.15 and 0.16) alloys, which leads to a coupled first-order magnetostructural transition (MST) from paramagnetic hexagonal to ferromagnetic orthorhombic structure, and, as a result, a giant magnetocaloric effect is observed in these alloys. With subsequent doping for x = 0.17, the MST decouples into two separate transitions, structural and magnetic, although the transitions couple upon enhancing the applied magnetic field. The alloys with x = 0.15, 0.16, and 0.17 are found to exhibit isothermal magnetic entropy changes ($|\mathrm{\Delta }{S}_M|$) as large as about $25\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 323 K, about $31.1\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 281 K, and about $23.8\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 213 K, respectively, due to a field change of ΔH = 50 kOe. These low-cost materials may be considered as promising candidates for magnetic refrigeration around room temperature due to their giant magnetocaloric properties, with significantly large relative cooling power (RCP = 191.8, 209.6, and 139.2 J/kg, respectively, for x = 0.15, 0.16, and 0.17 due to ΔH = 50 kOe).

We report on the detection of diffraction gratings buried below a stack of tens of 18-nm-thick ${\mathrm{Si}\mathrm{O}}_2$ and ${\mathrm{Si}}_3{\mathrm{N}}_4$ layers and an optically opaque metal layer, using laser-induced, extremely high-frequency ultrasound. In our experiments, the shape and amplitude of a buried metal grating are encoded on the spatial phase of the reflected acoustic wave. This grating-shaped acoustic reflection of the buried grating is detected by the diffraction of a delayed probe pulse. A detailed understanding of our measurements is essential for nanometrology applications in the semiconductor manufacturing industry, such as wafer alignment. We show that the complex shape of the diffracted signal as a function of time can be reproduced using a comprehensive numerical model that includes the generation, propagation, and optical detection of the acoustic waves. This allows us to identify the salient features in our measurements such as the presence of acoustic-wave-induced gratings inverted with respect to buried grating, the interference between the optical fields diffracted off multiple grating-shaped acoustic waves, and multiple thin dielectric layers behaving as a single effective acoustic medium. Our results show that laser-induced ultrasound is a promising technique for subsurface nanometrology applications.

Efficient spin injection from epitaxial ferrimagnetic ${\mathrm{Ni}\mathrm{Fe}}_2{\mathrm{O}}_4$ thin films into a $\mathrm{Pd}$ layer is demonstrated via spin Seebeck effect measurements in the longitudinal geometry. The ${\mathrm{Ni}\mathrm{Fe}}_2{\mathrm{O}}_4$ films (60 nm to 1 $\mu$) are grown by pulsed-laser deposition on isostructural spinel ${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$, ${\mathrm{Mg}\mathrm{Ga}}_2{\mathrm{O}}_4$, and ${\mathrm{Co}\mathrm{Ga}}_2{\mathrm{O}}_4$ substrates with lattice mismatch varying between 3.2 and 0.2%. For the thinner films ($\le 330$ nm), an increase in the spin Seebeck voltage is observed with decreasing lattice mismatch, which correlates well with a decrease in the Gilbert damping parameter as determined from ferromagnetic resonance measurements. High-resolution transmission electron microscopy studies indicate substantial decrease of antiphase boundary and interface defects that cause strain relaxation, i.e., misfit dislocations, in the films with decreasing lattice mismatch. This highlights the importance of reducing structural defects in spinel ferrites for efficient spin injection. It is further shown that angle-dependent spin Seebeck effect measurements provide a qualitative method to probe for in-plane magnetic anisotropies present in the films.

Quantum sensing uses quantum systems as sensors to capture weak signals and is providing opportunities in science and technology. The biggest challenge in quantum sensing is decoherence due to the coupling between the sensor and the environment. The dissipation will destroy the quantum coherence and reduce the performance of the sensor. Here we show that quantum sensing can be realized under dissipation by engineering the steady state of the sensor. We demonstrate this protocol with a magnetometer based on ensemble nitrogen-vacancy centers in diamond, while neither high-quality initialization or readout of the sensor nor sophisticated dynamical decoupling sequences are required. Thus our method provides a concise and decoherence-resistant means of quantum sensing. The frequency resolution and precision of our magnetometer are far beyond the coherence time of the sensor. Furthermore, we show that the dissipation can be engineered to improve the performance of our sensor. By increasing the laser pumping, magnetic signals in a broad audio-frequency band from dc up to 140 kHz can be tackled by our method. Besides the potential applications in magnetic sensing and imaging on the microscopic scale, our results may provide opportunities for improvement of a variety of high-precision spectroscopies based on other quantum sensors.

Spin orbit torques are of great interest for switching the magnetization direction in nanostructures, moving skyrmions and exciting spin waves. The standard method of determining their efficiency is by spin torque ferromagnetic resonance (ST-FMR), a technique that involves analyzing the resonance linewidth or lineshape. On microstuctures these two analysis methods are quite consistent. Here we present ST-FMR results on permalloy (${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) nanowires—with widths varying from $150$ to 800 nm—that show that the standard model used to analyze the resonance linewidth and lineshape give different results; the efficiency appears greatly enhanced in nanowires when the lineshape method is used. A ST-FMR model that properly accounts for the sample shape is presented and shows much better consistency between the two methods. Micromagnetic simulations are used to verify the model. These results and the more accurate nanowire model presented are of importance for characterizing and optimizing charge-to-spin conversion efficiencies in nanostructures.

Absorption imaging is the most common probing technique in experiments with ultracold atoms. The standard procedure involves the division of two frames acquired at successive exposures, one with the atomic absorption signal and one without. A well-known problem is the presence of residual structured noise in the final image, due to small differences between the imaging light in the two exposures. Here, we solve this problem by performing absorption imaging with only a single exposure, where instead of a second exposure the reference frame is generated by an image-completion autoencoder neural network. The network is trained on images without an absorption signal, such that it can infer the noise overlaying the atomic signal based only on the information in the region encircling the signal. We demonstrate our approach on data captured with a quantum degenerate Fermi gas. The average residual noise in the resulting images is below that of the standard double-shot technique. Our method simplifies the experimental sequence, reduces the hardware requirements, and can improve the accuracy of extracted physical observables. The trained network and its generating scripts are available as an open-source repository (absDL.github.io).

Photon up-conversion processes are considered beneficial for energy-conversion devices. The recently proposed two-step photon up-conversion (TPU) solar-cell design employs an intermediate level with a high electron occupation probability. To understand the device physics, a quantitative evaluation of the different current-generation and loss mechanisms is required. In the present work, we use a TPU solar cell containing an $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ quantum-dot layer located 10 nm in front of an ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ heterointerface. We study the relation between the photocurrent (PC), radiative recombination, and nonradiative recombination as a function of the bias voltage. The radiative interband recombination is evaluated by integrating the photoluminescence (PL) over the range from 1000 to 1300 nm. The magnitudes of the PC and PL signals generated via interband excitation of the $\mathrm{Ga}\mathrm{As}$ layer depend on the bias voltage; a higher forward bias reduces the PC and increases the PL intensity. We verify that, under additional infrared light illumination at 1319 nm, which induces intraband transitions, the PC density increases while the PL intensity significantly decreases. This PC enhancement exhibits a maximum at −0.6 V, which reflects the optimum internal electric field strength for maximizing the TPU efficiency.