$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$-based solar cells exceed power conversion efficiencies of 23%. However, the fill factor of these solar cells, with best values around 80%, is relatively low ($\mathrm{Si}$ reaches 84.9%) mostly due to diode factors greater than 1. Recently, we proposed metastable defects, a general feature of the $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ alloy, to be the origin of the increased diode factor even in low injection. Here, we measure the diode factor of the bare absorber layers using excitation-dependent photoluminescence. The increased diode factor above 1 can be well described by the model of metastable defects, as well as a slight excitation dependence within the experimentally accessible range of excitation intensities. We discuss how the excitation dependence of the diode factor depends on the parameters of the metastable defects. Within the same model, we can additionally describe the experimental diode factors of n- and p-type epitaxial $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ films. We find that the diode factors measured optically by photoluminescence impose a lower limit for the diode factor measured electrically on a finished solar cell. Interestingly, the lowest diode factor (optical and electrical) and consequently highest fill factor of 81.0% is obtained by $\mathrm{Ag}$ alloying, i.e., an $(\mathrm{Ag},\mathrm{Cu})(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ absorber. This finding hints to a pathway to increase fill factors and thus efficiencies for $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$-based solar cells.

Optomechanical accelerometers promise quantum-limited readout, high detection bandwidth, self-calibration, and radiation-pressure stabilization. We present a simple, scalable platform that enables these benefits with nano-$g$ sensitivity at acoustic frequencies, based on a pair of vertically integrated ${\mathrm{Si}}_3{\mathrm{N}}_4$ membranes with different stiffnesses, forming an optical cavity. As a demonstration, we integrate an ultrahigh-$Q$ ($>{10}^7$), millimeter-scale ${\mathrm{Si}}_3{\mathrm{N}}_4$ trampoline membrane above an unpatterned membrane on the same $\mathrm{Si}$ chip, forming a finesse $\mathcal{F}\approx 2$ cavity. Using direct photodetection in transmission, we resolve the relative displacement of the membranes with a shot-noise-limited imprecision of $7\mathrm{fm}/\sqrt{\mathrm{Hz}}$, yielding a thermal-noise-limited acceleration sensitivity of $0.6\mu \mathrm{g}/\sqrt{\mathrm{Hz}}$ over a 1-kHz bandwidth centered on the fundamental trampoline resonance (40 kHz). To illustrate the advantage of radiation-pressure stabilization, we cold damp the trampoline to an effective temperature of 4 mK and leverage the reduced energy variance to resolve an applied stochastic acceleration of $50\mathrm{ng}/\sqrt{\mathrm{Hz}}$ in an integration time of minutes. In the future, we envision a small-scale array of these devices operating in a cryostat to search for fundamental weak forces such as dark matter.

While current-induced torque by orbital current has been experimentally found in various structures, evidence for its reciprocity has been missing so far. Here, we report experimental evidence of strong inverse orbital torque in $\mathrm{YIG}/\mathrm{Pt}/{\mathrm{Cu}\mathrm{O}}_x$ ($\mathrm{YIG}={\mathrm{Y}}_3{\mathrm{F}{\mathrm{e}}_5\mathrm{O}}_{12}$) mediated by spin-orbital mixed electronic states in $\mathrm{Pt}$. By injecting spin current from YIG to $\mathrm{Pt}$ by the spin pumping via ferromagnetic resonance and by the spin Seebeck effect, we find a pronounced inverse spin Hall effectlike signal. While a part of the signal is explained as being due to the inverse spin-orbital Hall effect in $\mathrm{Pt}$, we also find a substantial increase of the signal in $\mathrm{YIG}/\mathrm{Pt}/{\mathrm{Cu}\mathrm{O}}_x$ structures compared to the signal in $\mathrm{YIG}/\mathrm{Pt}$. We attribute this to the inverse orbital Rashba-Edelstein effect at the $\mathrm{Pt}/{\mathrm{Cu}\mathrm{O}}_x$ interface mediated by the spin-orbital coupled states in $\mathrm{Pt}$. Our work paves the way toward understanding of spin-orbital intertwined physics in nonequilibrium and provides a way for electrical detection of the orbital current in orbitronic device applications.

Nonlocal metasurfaces, by harnessing the nonlocality, have shown their unparalleled capacities for the efficiency of wave-front manipulation. Here, we demonstrate that nonlocality can be served as a powerful tool to achieve a dramatical enhancement in working bandwidth, showcased by an acoustic open metasurface encoding the exceptional property of allowing airflow while blocking sound transmission. Our design is a binary metaunit composed of two parts, a central orifice and a surrounding helical blade. We show that the transmission spectrum can be tailored by leveraging the hybrid nonlocal effect from the synergy of radiation coupling and direct coupling, the latter provided by a physical connection between the two parts. Verified by simulations and experiments, such a hybrid nonlocal open metasurface consistently blocks more than 90% of incident energy in the range from 600 to 1900 Hz across four adjacent transmission dips. Our finding may pave a way for noise blocking in a flowing-fluid-filled circumstance, more essentially, sets out an efficient approach for broadband wave manipulation in a hybrid nonlocal way.

The refractive index serves as an intrinsic contrast in light-scattering measurements for the estimation of the size, shape, and heterogeneities of microscopic particles. The refractive index of cells, intracellular organelles, and extracellular materials is an important input to the analysis of diffuse optical imaging of biological tissues. To improve the accuracy, the measurement is often performed at multiple discrete wavelengths or sometimes over a continuous wavelength range. Knowledge of the refractive index as a function of the wavelength is important, as the analysis of the acquired signal heavily relies on the relationship. Digital holographic tomography allows us to directly measure the three-dimensional (3D) refractive-index map of a heterogeneous microscopic specimen. Typically, a multitude of images are recorded for varying angles of illumination and a tomographic reconstruction algorithm is applied for the 3D reconstruction. Hyperspectral 3D refractive-index imaging has been demonstrated by combining the beam-rotation tomography with a wavelength-scanning light source; however, the data acquisition is slow due to the requirement of scanning both the orientation and the wavelength of the illumination. Recently, several strategies have been proposed to acquire the 3D tomogram in a single snapshot. Here, we combine snapshot holographic optical tomography with a wavelength-scanning laser to demonstrate hyperspectral 3D refractive-index imaging at high throughput. Using the developed system, we measure the refractive-index dispersion of polystyrene beads and single living HeLa cells.

Spin-charge interconversion (SCI) is a central phenomenon to the development of spintronic devices from materials with strong spin-orbit coupling (SOC). In the case of materials with high crystal symmetry, the only allowed SCI processes are those where the spin-current, charge-current, and spin-polarization directions are orthogonal to each other. Consequently, standard SCI experiments are designed to maximize the signals arising from the SCI processes with conventional mutually orthogonal geometry. However, in low-symmetry materials, certain nonorthogonal SCI processes are also allowed. Since the standard SCI experiment is limited to charge current flowing only in one direction in the SOC material, certain allowed SCI configurations remain unexplored. Here, we perform a thorough SCI study in a graphene-based lateral spin valve combined with low-symmetry ${\mathrm{Mo}\mathrm{Te}}_2$. Due to a very low contact resistance between the two materials, we can detect SCI signals using both a standard configuration, where the charge current is applied along ${\mathrm{Mo}\mathrm{Te}}_2$, and a recently introduced [three-dimensional- (3D) current] configuration, where the charge-current flow can be controlled in three directions within the heterostructure. As a result, we observe three different SCI components, one orthogonal and two nonorthogonal, adding valuable insight into the SCI processes in low-symmetry materials. The large SCI signals obtained at room temperature, along with the versatility of the 3D-current configuration, provide feasibility and flexibility to the design of the next generation of spin-based devices.

We explore the use of room-temperature ionic liquids (ILs) as Raman wavelength converters. ILs provide an engineerable framework to design suitable liquids for wavelength conversion over a broad spectral range, through careful selection of the molecular structures of the IL anions and cations so that specific characteristics can be obtained, such as a desirable Raman shift, low Brillouin scattering, and good optical transmission in the pump and Stokes wavelengths. Applying such criteria, we demonstrate that 1-ethyl-3-methylimidazolium dicyanamide (EMIM DCA) is an effective medium for conversion of 532-nm pulses from a Q-switched $\mathrm{Nd}:$YAG laser to 603 nm. This corresponds to an approximate 2200 ${\mathrm{cm}}^{-1}$ shift, which can be used to generate mid-infrared radiation through subsequent difference frequency generation for optical pumping of ${\mathrm{CO}}_2$ lasers. Threefold-higher Raman conversion efficiency is obtained in EMIM DCA compared with water under identical conditions in a proof-of-principle single-pass conversion setup, resulting in an efficient generation of multimillijoule, <6 ns duration, high-quality orange laser pulses in a wavelength region that is difficult to access at high energies. Consequently, we examine ILs representing two other classes of Raman-active functional groups and obtain conversion up to the fifth-order Stokes shift and first anti-Stokes shift. Through the tunable selection of their components and their useful dynamical properties, ILs provide a platform for efficient, simple, and alignment-tolerant high-energy Raman shifting with numerous industrial and technological applications.

Cloaking is typically reciprocal. Here, we introduce the concept of transmittable nonreciprocal cloaking, whereby the cloaking system operates as a standard omnidirectional cloak for external illumination but can transmit light from its center outward at will. We demonstrate a specific implementation of such cloaking that consists of a set of concentric bianisotropic metasurfaces, the innermost element of which is nonreciprocal and designed to simultaneously block inward waves and pass—either omnidirectionally or directionally—outward waves. Such cloaking represents a fundamental diversification of conventional cloaking and may find applications in areas such as stealth, blockage avoidance, illusion, and cooling.

We experimentally realize circularly polarized unidirectional cavity magnon polaritons in a torus-shaped microwave cavity loaded by a small magnetic sphere. At special positions, the clockwise and counterclockwise modes are circularly polarized, such that only one of them couples to the magnet, which breaks the mode degeneracy. We reveal the chiral nature of the spectral energy and angular-momentum flow by observing and modeling nonreciprocities of the microwave scattering matrix.

The fabrication of complex low-dimensional quantum devices requires the control of the heteroepitaxial growth at the subnanometer scale. This is particularly challenging when the total thickness of stacked layers of device-active material becomes extremely large and exceeds the multi-$\text{μ}\mathrm{m}$ limit, as in the case of quantum cascade structures. Here, we use the ultrahigh-vacuum chemical vapor deposition technique for the growth of multi-$\text{μ}\mathrm{m}$-thick stacks of high $\mathrm{Ge}$ content strain-balanced $\mathrm{Ge}$/$\text{Si-Ge}$ tunneling heterostructures on $\mathrm{Si}$ substrates, designed to serve as the active material in a THz quantum cascade laser. By combining thorough structural investigation with THz spectroscopy absorption experiments and numerical simulations we show that the optimized deposition process can produce state-of-the-art threading dislocation density, ultrasharp interfaces, control of dopant atom position at the nanoscale, and reproducibility within 1% of the layer thickness and composition within the whole multilayer. We show that by using ultrahigh-vacuum chemical vapor deposition one achieves simultaneously a control of the epitaxy down to the sub-nm scale typical of the molecular beam epitaxy, and the high growth rate and technological relevance of chemical vapor deposition. Thus, this technique is a key enabler for the deposition of integrated THz devices and other complex quantum structures based on the $\mathrm{Ge}$/$\text{Si-Ge}$ material system.

Natural structures conveying fluid flow exhibit an interplay between flow-mediated forces and long-term adaptation. This phenomenon is relevant in the cardiovascular system where the geometric remodeling of the heart chambers is the main mechanism underlying pathological progression leading to hearth failure. Cardiac adaptation is analyzed here in children with a single right ventricle (SRV) in their heart. In these patients, the left ventricle (LV) is not well developed and the healthy right ventricle (RV) is surgically reconnected, early after birth, to take the functional role of the systemic ventricle. Such a condition represents a special model to investigate cardiac adaptation and this study takes advantage of the availability of an uncommon dataset (64 normal RVs, 64 normal LVs, 64 SRVs with clinically normal function). The ventricular functional performance is analyzed in terms of fluid dynamics and tissue deformation with the objective of verifying to which degree the SRV configuration adapts from the original RV and progresses toward the function of a LV. Results show that the SRV immediately assumes a larger volume and a wider geometry due to the higher operating pressure. However, the fluid dynamics is weakly turbulent and produces a reduced propulsion. The surrounding tissue develops muscular thickening with multidirectional orientation of myofibers that mimic a LV. However, the reduced flow performance and a lower structural consistency makes the SRV at higher risk of progressive dysfunctional adaptations. This study demonstrates how the interplay between cardiac flow and tissue response represents the driving macroscopic factor underlying the development of heart failure. More in general, the combined evaluation of fluid dynamics and structural functional properties can be a requirement for the exploration of adaptation processes across the different time scales.

We study acoustic topological edge modes in a honeycomb phononic crystal composed of metallic nanopillars on a ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ substrate. Acoustic band calculations show that the topological surface acoustic wave (SAW) mode inhabits the edge of the honeycomb phononic crystal in spite of the hybridization with the internal acoustic modes of the substrate. Pulse-type microwave impedance microscopy realizes clear visualization of the gigahertz topological edge mode between two mutually inverted topological phononic crystals. A frequency-dependent image shows that the edge mode evolves as the bulk SAW modes are suppressed owing to the energy gap formation, consistent with the topological nature. The realization of a topological waveguide in a simple pillar structure on a substrate might pave a new path to the development of topological SAW devices for a wide range of usages such as quantum computing, sensing, and communication applications.

Thin films of disordered superconductors such as titanium nitride ($\mathrm{TiN}$) exhibit large kinetic inductance (KI), high critical temperature, and large quality factors at the single-photon level. KI nonlinearity can be exploited as an alternative to Josephson junctions to create novel nonlinear quantum devices with the potential to operate at higher frequencies and at elevated temperatures. We study a means of magnifying KI nonlinearity by confining the current density of resonant electromagnetic modes in nanowires with a small volume $V\simeq {10}^{-4}\mu {\mathrm{m}}^3$. Using this concept, we realize microwave-frequency Kerr cavities with a maximum Kerr shift per photon of $K/2\pi =123.5\pm 12$ kHz and report a nonlinearity-to-linewidth ratio $K/\gamma =21\mathrm{\%}$. With improved design, our devices are expected to approach the regime of strong quantum nonlinearity in the millimeter-wave spectrum.

As forward-looking electron devices increasingly adopt high-mobility low-band-gap materials, such as germanium ($\mathrm{Ge}$), questions remain regarding the feasibility of strain engineering in low-band-gap systems. Particularly, the $\mathrm{Ge}$ L-Γ valley separation (∼150 meV) can be overcome by introducing a high degree of tensile strain (ε ≥ 1.5%). It is therefore essential to understand the nature of highly strained $\mathrm{Ge}$ transport, wherein multivalley electron conduction becomes a possibility. Here, we report on the competitiveness between L- and Γ-valley transport in highly tensile-strained (ε ∼ 1.6%) $\mathrm{Ge}/{\mathrm{In}}_{0.24}{\mathrm{Ga}}_{0.76}\mathrm{As}$ heterostructures. Temperature-dependent magnetotransport analysis reveals two contributing carrier populations, identified as lower- and higher-mobility L- and Γ-valley electrons (in $\mathrm{Ge}$), using temperature-dependent Boltzmann transport modeling. Coupling this interpretation with electron-cyclotron-resonance studies, the effective mass (m*) of the higher-mobility Γ-valley electrons is probed, revealing m* = (0.049 ± 0.007)$m_e$. Moreover, a comparison of empirical and theoretical m* indicates that these electrons reside primarily in the first-two quantum sublevels of the $\mathrm{Ge}$ Γ valley. Consequently, our results provide an insight into the strain-dependent carrier dynamics of $\mathrm{Ge}$, offering alternative pathways toward efficacious strain engineering.

We study fluid-induced deformation and fracture of granular media and apply photoporomechanics to uncover the underpinning grain-scale mechanics. We fabricate spherical photoelastic particles of 2-mm diameter to form a monolayer granular pack in a circular Hele-Shaw cell that is initially filled with a viscous fluid. The key distinct feature of our system is that, with spherical particles, the granular pack has a connected pore space, thus allowing for pore-pressure diffusion and the study of effective stress in coupled poromechanical processes. We inject air into the fluid-filled photoelastic granular pack, varying the initial packing density and confining weight. With our recently developed experimental technique, photoporomechanics, we find two different modes of fluid invasion: fracturing in fluid-filled elastic media (with strong photoelastic response) and viscous fingering in frictional fluids (with weak or negligible photoelastic response). We directly visualize the evolving effective stress field and discover an effective stress shadow behind the propagating fracture tips, where the granular pack exhibits undrained behavior. We conceptualize the behavior of the system by means of a mechanistic model for a wedge of the granular pack bounded by two growing fractures. The model captures the pore-pressure build-up inside the stress shadow region and the grain compaction in the annular region outside. Our model reveals that a jamming transition determines the distinct rheological behavior of the wet granular pack, from a friction-dominated to an elasticity-dominated response.

We propose to incorporate fluids within multistable metamaterials to tailor a desired dynamic response, dictate the magnitude of rate-dependent dissipation, control the sequence and pattern of phase transition, and more. As a simple example we focus on straws, which are commonly used to transfer fluids and are constructed from a one-dimensional (1D) lattice of bistable elements. We first solve for the dynamics of a single element in the unstable spinodal state, reducing or increasing the pressure locally. Using a long-wave approximation, we then model viscous flow and solid phase change within a 1D lattice of bistable frusta. The model is compared with experiments, showing excellent agreement.

The prediction of measurement outcomes from an underlying structure often follows directly from fundamental physical principles. However, a fundamental challenge is posed when trying to solve the inverse problem of inferring the underlying source configuration based on measurement data. A key difficulty arises from the fact that such reconstructions often involve ill-posed transformations and that they are prone to numerical artifacts. Here, we develop a numerically efficient method to tackle this inverse problem for the reconstruction of magnetization maps from measured magnetic stray-field images. Our method is based on neural networks with physically inferred loss functions to efficiently eliminate common numerical artifacts. We report on a significant improvement in reconstruction over traditional methods and we show that our approach is robust to different magnetization directions, both in and out of plane, and to variations of the magnetic field measurement-axis orientation. While we showcase the performance of our method using magnetometry with nitrogen-vacancy center spins in diamond, our neural-network-based approach to solving inverse problems is agnostic to the measurement technique and thus is applicable beyond the specific use case demonstrated in this work.

Optically active spin defects hosted in hexagonal boron nitride ($h$-BN) are promising candidates for the development of a two-dimensional (2D) quantum sensing unit. Here, we demonstrate quantitative magnetic imaging with $h$-BN flakes doped with negatively charged boron-vacancy ($V_{\mathrm{B}}^{-}$) centers through neutron irradiation. As a proof-of-concept, we image the magnetic field produced by ${\mathrm{Cr}\mathrm{Te}}_2$, a van der Waals ferromagnet with a Curie temperature slightly above 300 K. Compared to other quantum sensors embedded in 3D materials, the advantages of the $h$-BN-based magnetic sensor described in this work are its ease of use, high flexibility, and, more importantly, its ability to be placed in close proximity to a target sample. Such a sensing unit will likely find numerous applications in 2D materials research by offering a simple way to probe the physics of van der Waals heterostructures.

The coherent one-way (COW) quantum key distribution (QKD) is a highly practical quantum communication protocol that is currently deployed in off-the-shelve products. However, despite its simplicity and widespread use, the security of COW QKD is still an open problem. This is largely due to its unique security feature based on the intersignal phase distribution, which makes it very difficult to analyze using standard security proof techniques. Here, to overcome this problem, we present a simple variant of COW QKD and prove its security in the infinite-key limit. The proposed modifications only involve an additional vacuum tail signal following every encoded signal and a balanced beam splitter for passive measurement basis choice. Remarkably, the resulting key rate of our protocol is comparable with both the existing upper bound on the COW-QKD key rate and the secure key rate of the coherent-state BB84 protocol. Our findings therefore suggest that the secured deployment of COW-QKD systems in high-loss optical networks is indeed feasible with minimal adaptations applied to its hardware and software.

Quantum dot arrays possess ground states governed by Coulomb energies, utilized prominently by singly occupied quantum dots, each implementing a spin qubit. For such quantum processors, the controlled transitions between ground states are of operational significance, as these allow the control of quantum information within the array such as qubit initialization and entangling gates. For few-dot arrays, ground states are traditionally mapped out by performing dense raster-scan measurements in control-voltage space. These become impractical for larger arrays due to the large number of measurements needed to sample the high-dimensional gate-voltage hypercube and the comparatively little information extracted. We develop a hardware-triggered detection method based on reflectometry, to acquire measurements directly corresponding to transitions between ground states. These measurements are distributed sparsely within the high-dimensional voltage space by executing line searches proposed by a learning algorithm. Our autonomous software-hardware algorithm accurately estimates the polytope of Coulomb blockade boundaries, experimentally demonstrated in a $2\times 2$ array of silicon quantum dots.