We study a rain-powered energy-harvesting device inspired by the natural impact of raindrops on leaves. In nature, a raindrop striking a leaf at high speed causes it to deform and vibrate. Inspired by this dynamic response, our device uses an elastic beam coupled to a piezoelectric material to convert mechanical vibrations from droplet impacts into electrical energy. We conduct experiments to analyze how beam length, droplet impact location, and residual droplet mass affect energy conversion. Our results, supported by a theoretical model, show strong agreement when the beam length exceeds 5 cm. Beyond this length, the energy conversion becomes independent of further increases, suggesting that 5 cm is optimal for maximizing output. To validate practical applicability, we also test the device under real rain conditions, demonstrating consistent performance. Understanding the interplay between raindrop dynamics and energy conversion can guide the design of efficient, scalable rain-powered energy-harvesting systems for environmental applications.

Efficient ultrasound transmission and focusing through acoustic barriers are crucial for advancing high-resolution imaging, targeted therapy, and acoustic manipulation. However, impedance mismatch and the geometric complexity of practical barriers, particularly stiff solids such as metal or bone, lead to strong reflections, wave-front distortion, and substantial energy loss. This work introduces an acoustic metamask that enhances transmission and enables flexible focusing through a stiff, curved barrier. The patterned surface structure of the metamask enables strong acoustic-solid coupling, where incident ultrasound excites resonant Lamb modes, and these guided modes subsequently reradiate the energy into the surrounding medium. This process mitigates impedance mismatch and enhances transmission, while the patterned structure simultaneously enables phase-modulated wave control for focusing. Full-wave simulations of curved brass shells reveal that the proposed metamask achieves a peak focal intensity over 2 orders of magnitude greater than that of a bare barrier, while maintaining 58.5% of the intensity observed in the barrier-free case and simultaneously producing a subwavelength focus. Our findings further demonstrate that the metamask design adapts to barriers with nonuniform thickness and complex geometries, enabling flexible multifocal control through stiff barriers. By utilizing the patterned structure to enhance acoustic-solid coupling, the design conceptually transforms the barrier into a functional acoustic device, facilitating efficient ultrasound transmission through acoustically opaque structures without requiring physical openings. The work establishes a design framework for wave manipulation through complex media, with broad potential implications for therapeutic ultrasound, nondestructive evaluation, and other advanced acoustic technologies.

Optically pumped atomic magnetometers have previously been used in arrays to reject interference from distant sources and enable the sensitive detection of local sources of radio-frequency (rf) signals, which is useful, for instance, in the detection of low-field NMR signals in an unshielded environment. Here, we demonstrate a complementary scheme in which four magnetometer measurements are used to locate a nearby rf source in three dimensions. The methodology relies on the measurement of an rf vector at two different positions and modeling the source as a magnetic dipole of known orientation. In contrast to coil detection, magnetometers have the advantage of measuring signals in a two-dimensional plane, and they do not inductively couple to their environment or each other, making them a strong candidate for localization of hidden rf sources. For this demonstration, we use only a single rf magnetometer to make four measurements of a synchronous and oriented dipole source, but it is expected that this could be replaced by four magnetometers working simultaneously. In addition, this work is greatly aided by the introduction of a fully integrated magnetometer, in which all optics, including lasers, are safely enclosed in a compact head with flexible wired connections. The portable and safe nature of the sensor make it quite valuable for work in the field.

Gadolinium ($\mathrm{Gd}$), a heavy rare-earth metal with a well-defined second-order magnetic transition near room temperature, serves as a benchmark for magnetocaloric materials. Extensive research on $\mathrm{Gd}$-based magnetocaloric materials has revealed an inherent inverse relationship between the breadth of the operating temperature range and the magnitude of the magnetic entropy change, thereby constraining further enhancement of the relative cooling power ($p_{\mathrm{RC}}$). Here, we report a pronounced enhancement of the $p_{\mathrm{RC}}$ of bulk polycrystalline $\mathrm{Gd}$ through high-pressure constrained deformation. Under applied stress, the easy magnetization axis [00l] of $\mathrm{Gd}$ gradually aligns toward the direction of the stress, promoting more efficient magnetic alignment. Simultaneously, grain refinement broadens the magnetic entropy change −Δ$S_M$(T) curve, extending the operational temperature window and boosting overall cooling capacity. At a magnetic field strength ${\text{μ}}_0$ΔH = 1.2 T, the value of $p_{\mathrm{RC}}$ increased from 90.42 to 121.05 J/kg, representing a 33.9% improvement. Our strategy provides an easy way to enhance the cooling ability compared with traditional methods such as alloying, coating, and element doping, and may provide fresh ideas for exploring advanced magnetocaloric materials.

Molecular spins offer a promising platform for quantum sensing, particularly in organic, supramolecular, and biological environments. Recognition of signals by these systems is of particular interest given their possible integration into more complex structures and their possible use as sensors in close proximity to analytes. In this work, we develop two quantum sensing protocols that enable discrimination between different time-dependent magnetic fields without requiring their periodicity to match the microwave manipulating sequence. These are based on the Hahn echo sequence and have been tested on VO(TPP) and ${\mathrm{VOPt}(\mathrm{SOCPh})}_4$ molecular spins embedded in a superconducting yttrium barium copper oxide (YBCO) microwave planar resonator. We report a magnetic field sensitivity up to $2.57\times {10}^{-7}$ T ${\mathrm{Hz}}^{-\frac{1}{2}}$ (with lower bounds approaching $2.87\times {10}^{-8}$ T ${\mathrm{Hz}}^{-\frac{1}{2}}$) for signals with a duration of a few microseconds. Under the given conditions, the minimum signal area that can be measured is in the ${10}^{-10}$ T s range, suggesting a potential trade-off between the minimum measurable field, and the required signal duration and the memory time.

Rashba-type spin-orbit coupling in two-dimensional electron gases (2DEGs) is of great interest for efficient charge-to-spin conversion. Ultrathin ${\mathrm{La}\mathrm{Ti}\mathrm{O}}_3$ films on ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ (LTO/STO) give rise to a 2DEG and have previously been shown to exhibit giant Rashba spin-orbit coupling, making this a promising platform for demonstrating efficient charge-to-spin conversion. For this reason, we fabricated $\mathrm{Co}\mathrm{Fe}\mathrm{B}/{\mathrm{La}\mathrm{Ti}\mathrm{O}}_3/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ heterostructures to understand how spin-charge interconversion occurs as a function of temperature in these systems. The spin-to-charge conversion in LTO/STO can be understood in terms of Rashba spin-orbit coupling and thermoelectric contributions. Ferromagnetic resonance shows spin-current absorption by the LTO/STO interface, which is reduced as the LTO layer thickness increases. Through second-harmonic Hall measurements, we observe a damping-like torque efficiency of approximately $0.30$ at 50 K, which increases sharply to approximately $8.6$ at 10 K. Our results characterize the factors contributing to spin-charge conversion in these heterostructures, which is important for the incorporation of 2DEGs with strong Rashba-type spin-orbit coupling into spintronic systems.

Quantum key distribution (QKD) enables information-theoretically secure communication against eavesdropping. However, phase instability remains a challenge across many QKD applications, particularly in schemes such as twin-field QKD and measurement-device-independent QKD. The most dominant source of phase fluctuation arises from the frequency offset between independent lasers. To overcome this limitation, we introduce a classical photodiode-based active compensation scheme for laser frequency mismatches and incorporate it into a mode-pairing QKD protocol. We have achieved an error rate approaching the theoretical limit and surpassing the linear key-rate bound over a fiber distance of 296.8 km. This approach provides a practical solution for frequency matching between independent lasers and can be extended to other fields requiring precise phase stabilization.

We propose a method to detect exoplanets based on their host star’s intensity centroid after it passes through a vortex filter. Based on our calculations with planets in face-on orbits, exoplanets with relative proximity to their host stars and with low mass ratios ($m_p/m_s$) can have discernible signals that can be amplified by the topological charge $\ell$ of the vortex filter. For exoplanets that have higher mass ratios and are far from their host stars, a clear signal can also be obtained but these are not affected by the value of $\ell$ and other planet detection methods may be more suitable. We present a simple tabletop optical experiment to support our calculations. Our proposed method adds to the arsenal of techniques for astrometric exoplanet detection.

We investigate the impact of missing input data on the construction of second-generation time-delay-interferometry (TDI) variables, which enable data analysis for the Laser Interferometer Space Antenna (LISA). TDI relies on the introduction of precise time delays into the raw interferometric data streams before they are combined to suppress otherwise dominant laser phase noise. We show that a single missing sample, corresponding to 0.25 s of data, will result in an effective data gap of approximately 90 s in the second-generation TDI output if further measures are not taken. This additional gap is largely independent of the initial gap duration, but it increases linearly with the order of the fractional-delay filter used for the computations. For a realistic gap scenario, incorporating both planned and unplanned data interruptions consistent with a target duty cycle of approximately $84\mathrm{\%}$, we find that frequent, short-duration gaps (e.g., a total of 1000 per year, each of which has a duration of approximately $100$ s) could result in an additional loss in the TDI variables of about one day per year, corresponding to a reduction of approximately $0.3\mathrm{\%}$ in duty cycle. This amounts to a loss of approximately one day of LISA data suitable for the global fit per year.

Mesa avalanche photodiodes (APDs) fabricated on $4H$ silicon carbide ($4H$-$\mathrm{Si}\mathrm{C}$) substrates have consistently produced spatially nonuniform optical responses, particularly at high gain. In this work, we propose that this nonuniformity can be explained by an interplay of several factors: the anisotropic band structure of the hexagonal material, the misalignment between the real-space and reciprocal-space axes induced by the miscut of $4H$-$\mathrm{Si}\mathrm{C}$ substrates, and the nonuniform electric fields of mesa APDs. Through full-band Monte Carlo (FBMC) simulations, field-dependent anisotropic impact ionization coefficients for a range of crystallographic directions between the principal axis ($⟨0001⟩$) and the basal plane ($⟨11\overline{2}0⟩$) are computed. In addition, the field-dependent carrier drift velocities are computed for the principal axis and the basal plane, primarily for model validation. Finally, the same FBMC model used to compute impact-ionization coefficients and drift velocities is used to perform a device simulation of a well-characterized APD for further validation of the model.

Sensors based on silicon micromechanical resonators play an important part in many aspects of daily life, such as motion sensors in mobile phones and navigation sensors in automobiles. Traditionally, a very small perturbation of the resonators induces a linear frequency shift with a scaling factor of about 0.5. Recently, an exceptional point that occurs in parity-time (PT)-symmetric systems featuring gain and loss has been utilized to amplify the response of the systems to perturbation. However, the frequency shift related to the perturbation displays a scaling factor considerably less than 1, which somewhat offsets the square-root singularity of the response in systems with linear gain, and the cube-root singularity of the response in systems with nonlinear gain. Meanwhile, asymmetric perturbations in systems with linear gain result in a spectrum broadening, while systems with nonlinear gain exhibit a reduced dynamic range of measurements. Here we theoretically and experimentally show that the phase difference between loss and gain resonators in PT-symmetric silicon micromechanical resonators with nonlinear gain follows a cube-root dependence on perturbation over a broad range of measurements, with a scaling factor substantially greater than 1. By using voltage-induced perturbations in PT-symmetric silicon micromechanical resonators with nonlinear gain, we demonstrate that the sensitivity, as characterized by the phase difference, is amplified by several orders of magnitude compared with that defined by the frequency shift. These findings set the stage for the development of highly sensitive silicon resonant sensors.

Second-harmonic injection (SHI) has emerged as a critical mechanism enabling networks of coupled oscillators to function as oscillator Ising machines (OIMs) capable of minimizing the Ising Hamiltonian. While SHI facilitates phase binarization, which is essential for mapping oscillator phases to spin states, we demonstrate that it can also induce spin freezing—where oscillator spins are unable to transition between spin states, even when such a transition would reduce the Ising energy. This freezing effect can impair the analog dynamics of the OIM, preventing it from reaching lower-energy spin configurations. Through theoretical analysis and numerical simulations, we show that the onset of spin freezing is highly sensitive to the initial phase configuration of the oscillators. Contrary to conventional practice, which favors random initialization, we find that initializing all oscillators at specific phase values ($\varphi =\pi$ or $\varphi =\pi /2$) delays the onset of spin freezing and consistently yields higher-quality solutions. These findings point to the need to carefully engineer SHI for optimal performance.

Spin polarimetry of low-energy electron beams is of considerable importance for a wide range of applications. However, an efficient method for two-dimensional, quantitative spin mapping is still lacking as state-of-art detectors rely on the sequential measurement of the spin polarization at individual points in energy and momentum space. In this work, we exploit the spin-dependent transmission of electrons through ultrathin magnetic layers embedded in a suspended matrix of a few graphene layers, fabricated in the form of micrometric magnetic freestanding membranes with an overall thickness below 10 nm, allowing significant transmission of low-energy electrons. We systematically investigate the role of deposition process, number of graphene layers, and magnetic materials with both in-plane and out-of-plane magnetization. We end up with optimized fabrication conditions for producing highly reliable elastic membranes with energy-dependent transmittivity suitable for spin filtering. We also present an analytical model that describes the detection of the spin polarization of an electron beam and outlines the experimental conditions under which such measurements can be performed using suspended magnetic membranes. This research paves the way for the development of spin filters that can be seamlessly integrated into existing detection systems, enabling spin-, angle-, and energy-resolved photoemission experiments as add-on functionality.

Quasi-phase-matching in ${\chi }^{(2)}$ media underpins many nonlinear conversion techniques, yet conventional sinc-squared gain models assume monochromatic pumps and wavelength-independent efficiency, limiting their accuracy for broadband sources. We introduce spectral-crystal efficiency mapping (SCEM), an analytical framework that combines input spectra with the nonlinear properties of a crystal to evaluate conversion potential. SCEM incorporates pump spectral profiles, higher-order interactions, and cascaded processes, providing an upper bound on efficiency that complements more computationally demanding numerical models. The resulting maps agree with experiment and simulation, while offering experimentalists an intuitive tool for assessing nonlinear gain media and guiding optimization of broadband frequency-conversion systems.

We study experimentally the nonlinear propagation of short pulses of forward volume spin waves in out-of-plane magnetized nanometer-thick yttrium iron garnet (YIG) films. We show that nonlinearity of the spin system can efficiently counteract dispersion broadening of the pulses, leading to the formation of envelope solitons. We demonstrate that in microscopic YIG systems, microwave powers of the order of 1 mW are sufficient to reach the soliton formation threshold. At powers slightly above this threshold, we achieve transmission of 3-ns spin-wave pulses over distances of up to 50 µm without increase in their temporal width. Our results demonstrate a promising way towards high-rate transmission of information in microscopic spin-wave circuits unaffected by detrimental dispersion effects.

Photon acceleration (PA) driven by ultrarelativistic electron beams offers a promising approach to generating high-power, high-frequency coherent radiation sources. While current methods typically rely on external optical laser pulses injected into beam-driven plasma wakefields, they face significant challenges in synchronization and alignment between electron accelerators and laser systems. We propose to utilize transition radiation (TR) generated by the drive electron bunch traversing the vacuum-gas interface as the seed photons for PA. Using a 1 GeV electron bunch, we demonstrate acceleration of TR from 4.4 $\text{μ}\mathrm{m}$ to 184 nm in 1.6 mm of two-stage uniform plasma, achieving more than a 20-fold frequency boost. Moreover, such techniques as the tailored density profile can further boost the wavelength to 137 nm. By increasing beam and plasma densities, this scheme can be scaled to produce coherent extreme ultraviolet radiation. This work addresses the synchronization and alignment issues occurring in previous approaches, providing a practical path toward beam-driven photon acceleration.

Piezoelectric micromachined ultrasonic transducers (pMUTs) hold great promise for applications ranging from medical imaging to range finding, yet their widespread adoption is hindered by low transmitting sensitivity. To address this limitation, we introduce a metasurface-integrated pMUT featuring a grooved hub-and-spoke structure, fabricated on an $\mathrm{Si}\mathrm{O}$${}_2$/$\mathrm{Pt}$/PZT/$\mathrm{Pt}$/$\mathrm{Si}\mathrm{O}$${}_2$/$\mathrm{Si}$ multilayer stack. In this design, the $\mathrm{Si}\mathrm{O}$${}_2$ vibration layer is strategically deposited above a patterned lead zirconate titanate (PZT) thin film with the precisely microfabricated geometry, a configuration that simultaneously enhances energy transduction and mitigates stress-induced performance deterioration. Experimental measurements validate the efficacy of the design, showing an increase in the electromechanical coupling coefficient from 0.68% to 2.95%. The device achieves a transmitting sensitivity of 0.71 Pa/V at 10 cm, representing a 2.36-fold improvement over conventional designs. Furthermore, the integrated stress-relieving grooves ensure exceptional wafer-scale uniformity, confining resonant frequency deviations to within 4% across an 8-in. wafer. These results, combined with a silicon-based CMOS-compatible fabrication process that emphasizes simplicity and cost-effectiveness, establish the proposed meta-pMUT as a highly viable and competitive technology for the commercialization of next-generation ultrasonic systems.