We investigated numerically the emission properties of an x-ray source based on direct laser acceleration of electrons interacting with an intense counterpropagating laser pulse. The source was realized by irradiating from both sides a high atomic number plasma-plume target. The resulting x-ray beam was analyzed through three-dimensional particle-in-cell simulations for its spectral content, source size, angular divergence, and temporal structure. For simulated experiments in which the total laser-pulse energy was 1.5 J, we obtained a peak brightness of approximately ${10}^{22}(\mathrm{s}{\mathrm{mrad}}^2{\mathrm{mm}}^{2}0.1\mathrm{\%}\mathrm{BW}{)}^{-1}$ of x-ray photons peaking at an energy of 25 keV. The results and their competitiveness in applications are discussed and compared with other laser-based x-ray generation methods.

Monoclinic $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$ films grown on $c$-plane sapphire have been shown to exhibit six $(\overline{2}01)$-oriented domains, which are equally spaced by rotation around the surface normal and equally sized by volume that render the film optical response effectively uniaxial. We derive and discuss an optical model suitable for ellipsometry data analysis of such films. We model mid- and far-infrared ellipsometry data from undoped and electrically insulating films with an effective uniaxial dielectric tensor based on projections of all phonon modes within the rotation domains parallel and perpendicular to the sample normal, i.e., to the reciprocal lattice vector ${\mathbf{g}}_{\overline{2}01}$. Two effective response functions are described by the model, and found sufficient to calculate ellipsometry data that best match measured ellipsometry data from a representative film. We propose to render either effective dielectric functions, or inverse effective dielectric functions, each separately for electric field directions parallel and perpendicular to ${\mathbf{g}}_{\overline{2}01}$, by sums of Lorentz oscillators, which permit determination of either sets of transverse optical phonon-mode parameters, or sets of longitudinal optical phonon-mode parameters, respectively. Transverse optical modes common to both dielectric functions can be traced back to single-crystal modes with $B_{\mathrm{u}}$ character, while modes with $A_{\mathrm{u}}$ character appear only within the dielectric function for polarization perpendicular to the sample surface. The thereby obtained parameter sets reveal all phonon modes anticipated from averaging over the sixfold-rotation domains of single crystal $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$, but with slightly shifted transverse optical, and completely different longitudinal optical phonon modes. Structural analysis of the film revealed virtually strain-free material. We suggest small crystal grains and high density of grain boundaries as a possible origin for the observed transverse optical phonon-frequency shifts with respect to bulk material. The differences in longitudinal optical modes here compared to the bulk are hypothesized to be caused by averaging of the electric-field-induced polarization over many long-range ordered rotation domains. Our model can be useful for future analysis of free charge-carrier properties using infrared ellipsometry on multiple domain films of monoclinic structure $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$.

The application of photoacoustic microscopy (PAM) in ocular imaging has been constrained by the stringent biosafety criterion and inhomogeneous tissue structure of eyes. In this study, we develop anti-interference PAM (AI PAM) to achieve high-quality images while upholding excellent biosafety standards. The system has two synchronized data acquisition channels. One channel captures photoacoustic signals, while the other channel concurrently collects electronic noise. Since the two-channel signals are highly correlated in their noise component, the noise component in the detected photoacoustic signals can be eliminated using adaptive noise cancellation. Furthermore, we develop an echo recovery mechanism based on the transfer function of the ultrasound reflection characteristics in the crystalline lens. Echo recovery not only enhances the signal-to-noise ratio (SNR) of the photoacoustic signals but also mitigates reflection artifacts arising from the eye tissue heterogeneity. The effectiveness of the proposed system and methodology are tested through phantom and in vivo mouse retinal imaging experiments. The results are compelling: AI PAM demonstrates an improvement of 14.1 dB in imaging SNR, or alternatively, it shows the potential to reduce the necessary laser energy by a factor of 4 without compromising imaging quality. AI PAM effectively reduces the risk of tissue damage due to high laser energy and extended exposure times, which may promote the application of PAM in clinics.

We present a method to stabilize the frequency of self-sustained vibrations in micromechanical and nanomechanical resonators. The method refers to a two-mode system with the vibrations at significantly different frequencies. The signal from one mode is used to control the other mode. In the experiment, self-sustained oscillations of micromechanical modes are excited by pumping at the blue-detuned sideband of the higher-frequency mode. Phase fluctuations of the two modes show near-perfect anticorrelation. They can be compensated in either of the modes by a stepwise change of the pump phase. The phase change of the controlled mode is proportional to the pump phase change, with the proportionality constant independent of the pump amplitude and frequency. This finding allows us to stabilize the phase of one mode against phase diffusion using the measured phase of the other mode. We demonstrate that phase fluctuations of either the high-frequency mode or the low-frequency mode can be significantly reduced. The results open new opportunities in generating stable vibrations in a broad frequency range via parametric down-conversion in nonlinear resonators.

Sorting elongated anisotropic particles, such as fibers, dispersed in suspensions poses significant challenges, as they present two characteristic dimensions: length and diameter. Fibers in suspensions usually align with the flow, leading to diameter-based filtration when passing through a sieve. Modifying the flow conditions by introducing more mixing, so that fibers are arbitrarily oriented, can lead to sorting by diameter and length simultaneously, resulting in a lower filtration quality. Here, we demonstrate that capillary filtration by dip coating can be utilized to selectively sort fibers by length or by diameter in a controlled manner. Using the withdrawal of a flat substrate from a fiber suspension, we demonstrate that fibers are primarily sorted by their diameters. When considering cylindrical substrates, fibers can be sorted by length under appropriate conditions due to the orientation adopted by the fibers during their entrainment. We report guidelines for designing this filtration process and obtaining good sorting efficiency.

We propose an interferometric device that generates spin-polarized electric currents based on an Aharonov-Bohm ring geometry. The device is driven by a temperature gradient and a magnetic flux without any voltage bias. Remarkably, no ferromagnetic contact is necessary once the two-path interference effects are present combined with a spin-orbit interaction. A large amplitude of spin current signal occurs where the charge current is small or even vanishing, thus easily creating pure spin currents by applying a magnetic field. Our proposal provides an ingredient of information processing devices in a generic caloritronic setup.

We present a semianalytical synthesis scheme for designing realistic printed-circuit-board- (PCB) based metagratings (MGs), capable of simultaneous beam steering and polarization conversion. These low-profile structures, comprised sparse periodic arrangements of subwavelength polarizable particles (meta-atoms), oriented by insightful analytical models leading to fabrication-ready designs, have gained growing interest in recent years, demonstrating exceptional diffraction engineering capabilities with very high efficiencies. Herein, to facilitate cross-polarization coupling, we introduce a tilted dogbone meta-atom, going beyond the popular vertical (loaded wire) or recently proposed horizontal (dipole) configurations used in common PCB-MG manifestations at microwave frequencies, strictly susceptible to either transverse-electric or transverse-magnetic fields, respectively. As shown, the anisotropy introduced by the in-plane rotation of the dogbones, integrated into the MG analytical model in the form of a rotated dipole line, serves as an additional degree of freedom that can be leveraged to nonlocally manipulate the polarization and trajectory of the incident fields. Verified experimentally, the resultant semianalytical synthesis framework enables valuable physical insights, opening the door to integration of efficient anisotropic MGs in applications requiring both polarization and wavefront control, such as satellite, radar, and advanced antenna systems.

Exploiting the strengths of different quantum hardware components may increase the capabilities of emerging quantum processors. Here we propose and analyze a quantum architecture that leverages the nonlocal connectivity of optics, along with the exquisite quantum control offered by superconducting microwave circuits, to produce entangled optical resource states. In contrast to previous proposals on optically distributing entanglement between superconducting microwave processors, we use squeezing between microwaves and optics to produce microwave-optical Bell pairs in a dual-rail encoding from a single microwave quantum processor. Moreover, the microwave quantum processor allows us to deterministically entangle microwave-optical Bell pairs into larger cluster states, from which entangled optical photons can be extracted through microwave measurements. Our scheme paves the way for small microwave quantum processors to create heralded entangled optical resource states for optical quantum computation, communication, and sensing using imperfect microwave-optics transducers. We expect that improved isolation of the superconducting processor from stray optical fields will allow the scheme to be demonstrated with currently available hardware.

Rydberg atomic sensors and receivers have enabled sensitive and traceable measurements of rf fields at a wide range of frequencies. Here, we demonstrate the detection of electric field amplitude in the extremely-high-frequency (EHF) band, at $131\mathrm{GHz}$. In our approach, we propagate the EHF field in a beam, with control over its direction and polarization at the detector using photonic wave plates. This way, we take advantage of the highest detection sensitivity, registered for collinear propagation and circular polarization. To exhibit the potential for applications in this kind of Rydberg-atom-based detection, we perform test measurements on the EHF field emitted from an on-chip radar, which is designed to be used in the automotive industry as a vital sign detector. Our work elucidates practical applications of Rydberg-atom media and photonic metamaterial elements.

Advanced simulations and calculations on quantum computers require high-fidelity implementations of quantum operations. The universal gateset approach builds complex unitaries from a small set of primitive gates, often resulting in a long gate sequence, which is typically a leading factor in the total accumulated error. Compiling a complex unitary for processors with higher-dimensional logical elements, such as qutrits, exacerbates the accumulated error per unitary, since an even longer gate sequence is required. Optimal control methods promise time- and resource-efficient compact gate sequences and, therefore, higher fidelity. These methods generate pulses that can directly implement any complex unitary on a quantum device. In this work, we demonstrate that any arbitrary qubit and qutrit gate can be realized with high fidelity, which can significantly reduce the length of a gate sequence. We generate and test pulses for a large set of randomly selected arbitrary unitaries on several quantum processing units (QPUs): the Lawrence Livermore National Laboratory Quantum Device and Integration Testbed’s (QuDIT’s) standard QPU and three of Rigetti’s QPUs: Ankaa-2, Ankaa-9Q-1, and Aspen-M-3. On the QuDIT platform’s standard QPU, the average fidelity of random qutrit gates is $97.9\pm 0.5\mathrm{\%}$ measured with conventional QPT and $98.8\pm 0.6\mathrm{\%}$ from QPT with gate folding. Rigetti’s Ankaa-2 achieves random qubit gates with an average fidelity of $98.4\pm 0.5\mathrm{\%}$ (conventional QPT) and $99.7\pm 0.1\mathrm{\%}$ (QPT with gate folding). On Ankaa-9Q-1 and Aspen-M-3, the average fidelities with conventional qubit QPT measurements were higher than 99% (see Appendix). We show that optimal control gates are robust to drift for at least 3 h and that the same calibration parameters can be used for all implemented gates. Our work promises that the calibration overheads for optimal control gates can be made small enough to enable efficient quantum circuits based on this technique.

Crystallization-induced stress within confined spaces of porous materials presents significant challenges across various disciplines, including conservation science, geomorphology, geotechnical engineering, and concrete durability. Despite its significance, understanding how localized, transient, and anisotropic stress is generated in pores due to salt crystallization, along with its mechanisms of damage, has remained elusive due to the lack of direct observation and quantitative measurement in porous media. This study introduces an innovative approach using a lab-on-a-chip (LOC) with a two-dimensional interconnected pore system, providing a customized porous domain that mimics real-world porous structures. It enables real-time spatially resolved observations of drying pattern evolution and sodium sulfate crystallization under repeated drying-wetting cycles. This LOC system allows us to explore the effect of efflorescence on drying and salt crystallization, quantitatively demonstrating that efflorescence influences the drying rate, particularly in hydrophilic porous media, which in turn affects salt growth kinetics. During drying at an elevated temperature (65 °C), thenardite (phase III) forms initially and then dissolves, and then thenardite (phase V) precipitates. During rewetting, the thenardite crystals formed during drying act as nucleation sites for accelerated mirabilite growth, leading to localized, transient, and anisotropic crystallization pressures up to 210 MPa. However, this high crystallization pressure alone does not guarantee macroscopic damage unless it is accompanied by substantial salt accumulation within pores, which is necessary for crack initiation and stress propagation. These findings underscore the significance of localized, transient, and anisotropic stress generation in porous media, providing critical insights into the long-term performance and deterioration mechanisms of porous materials in salt-laden environments. This work lays the groundwork for future research into the durability of porous structures and the development of targeted preservation strategies.

Long charge-carrier lifetimes are of great importance in designing highly efficient photocatalytic materials. In this work, we investigate the electronic structure and charge-carrier behavior of $\mathrm{Ga}\mathrm{Se}/\mathrm{WSSe}$ heterostructures using first-principles calculations. Different types of heterostructures with similar binding energies can be formed based on the polarization directions and stacking patterns. It is found that type-II band alignments, which are advantageous for water-splitting applications, can be found in some of the examined heterostructures. An intrinsic electric field exists at the interface of such double layers, promoting the separation of electron-hole pairs. The heterostructures show extremely high electron carrier mobilities and a large disparity of electron versus hole mobilities, with ${\mu }_n/{\mu }_p$ in the range of 39–52, inhibiting the recombination of electron-hole pairs and prolonging the carrier lifetimes. This unique behavior can be attributed to the interplay of intrinsic interface electric field, charge redistribution, effective carrier mass, and deformation potential. Our results reveal the potential of $\mathrm{Ga}\mathrm{Se}/\mathrm{WSSe}$ heterostructures as promising photocatalysts for overall water splitting with long carrier lifetimes.

Among all-inorganic metal-halide perovskites, mixed halides yield superior photovoltaic parameters. In this direction, we considered ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_3({\mathrm{Br}}_3)$ and observed that the ferroelectricity in the compounds appeared due to an overall distortion in the $[\mathrm{Pb}{X}_6{]}^{4-}$ octahedra and the consequent off-centered cesium ions. Apart from employing piezoresponse force microscopy to investigate the ferroelectric properties, we have shown that Kelvin probe force microscopy (KPFM), after applying suitable biases to pole the materials, can also be considered for probing the ferroelectricity of thin films. The band diagrams of the p-i-n heterojunctions were drawn by combining scanning-tunneling spectroscopy and KPFM results, and the devices were thereby cast and characterized. ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_2\mathrm{Br}$ and ${\mathrm{CsPbIBr}}_2$ have provided evidence of a piezoelectric modulus ($d_{33}$), and the devices based on the mixed-halide perovskites yielded better solar-cell parameters compared to those with the end members, namely, ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_3$ and ${\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_3$. The ferroelectricity in the compounds and the solar-cell parameters based on them were thereby found to be correlated.

The field of contrast-enhanced ultrasound (CEUS) combines nonlinearly oscillating microbubbles (MBs) with dedicated pulse sequences to reveal the vascular function of organs. Clinical ultrasound contrast agents consist of polydisperse MB suspensions with diameters ranging from 0.5 to 10 $\text{μ}\mathrm{m}$ and resonance frequencies ranging from 1 to 15 MHz. As a result, just a small fraction of MBs resonates at a given ultrasound frequency. MB suspensions with narrow size distributions can be tuned for a specific imaging frequency, boost CEUS sensitivity, and enable deeper vascular imaging. However, their enhanced nonlinear behavior makes imaging susceptible to nonlinear-wave-propagation artifacts. Here we numerically investigate the impact of the acoustic wavefront shape on the imaging of nonlinearly oscillating, monodisperse MBs. Specifically, our approach relies on an extension of the iterative nonlinear-contrast-source method that accounts for all nonlinear effects in CEUS. We demonstrate that supersonic X-shaped wavefronts referred to as “X waves” can be used to generate ultrasound images of monodisperse MBs without nonlinear-wave-propagation artifacts. In contrast, imaging based on focused, planar, and diverging wavefronts leads to significant nonlinear artifacts. Taken together, our results show that X waves can harness the full potential of monodisperse MBs by enabling their sensitive and specific detection in a tissue context.

Photonics is crucial for communication and computing, as it involves controlling how light scatters. Recent breakthroughs like bound states in the continuum (BICs) and parity-time-symmetric systems are linked to the scattering matrix, governed by passivity and causality. However, using complex-frequency excitations overcomes these limitations, allowing us to study systems with losses or gains and discover phenomena like virtual critical coupling and virtual gain. Applying complex-frequency-excitation concepts to more complicated systems is challenging. In this work, we extend the lumped-element model from circuit theory to analyze light-scattering anomalies in the complex-frequency domain. This approach helps us design and understand effects such as virtual perfect absorption, BICs, real and virtual critical coupling, exceptional points, and anisotropic transmission resonances. These insights broaden our knowledge of electromagnetic wave phenomena and open possibilities in photonics, offering innovative ways to create and optimize optical devices and systems for various applications.

Parity-time (PT)-symmetric systems featuring balanced gain and loss exhibit broken phases in which they have complex conjugate eigenvalues, yielding one eigenstate with exponential growth and the other with exponential decay. By biasing PT-symmetric silicon micromechanical resonators in the broken phase, we theoretically and experimentally demonstrate the nonreciprocal transmission of a pair of electrically coupled silicon micromechanical resonators. In our scheme, one resonator has a loss and the other has a nonlinear gain achieved by a positive feedback circuit loop. The coupling strength and gain can be tuned electrically. We observe 7.5 dB of nonreciprocal transmission with an isolation over 18 dB. Our results pave the way for isolators and circulators via silicon processing technology.