The operation of random lasers (RLs) is possible due to multiple light scattering, which provides optical feedback inside the gain medium. No conventional optical cavity is required for the operation of RLs and, therefore, RLs provide the best evidence to defy the conventional view that disorder is an unwanted phenomenon when present in physical systems. Mirrorless lasers demonstrate that the degree of disorder, associated with the spatial distribution of scattering particles, is the main factor responsible for generating a light source with low spatial coherence; this is reason why RLs are so attractive for modern imaging systems. Here, we demonstrate how to optimize the performance of typical diffusive RLs by exploiting a different degree of randomness in the system, which is loaded onto an induced disorder in the transverse intensity and wave-vector distributions of the light pattern that pumps the random-lasing medium. Quantitative analyses show that the RL’s intensity emission and threshold fluence can be optimized by more than 20 times, when excited by an optical field presenting a random spatial-intensity distribution, with high speckle contrast, becoming an efficient methodology to induce high photon degeneracy in RLs required for high sensitivity applications.

Next-generation millimeter-wave $(>30\mathrm{GHz})$ telecommunications electronics must be compact, energy efficient, and have good thermal management. Tunable materials may play a role in meeting these requirements for millimeter-wave front-end devices, but there are few models or even measurements of tunable dielectrics at these frequencies. Here, we report on the adaptation and development of high-frequency dielectric spectroscopy techniques for composition-spread thin films from 100 MHz to 110 GHz. Our comprehensive technique sequentially probes the composition, frequency, and electric field dependence of the complex permittivity in a combinatorial thin film library, which provides a platform to rapidly explore functional materials for emerging telecommunications electronics. This is achieved by modifying existing on-wafer transmission line permittivity measurement techniques to obtain a compact set of test devices that can be patterned to extract the complex permittivity in multiple regions of a thin film. We demonstrate this technique by applying it to composition-spread ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Ti}\mathrm{O}}_3$ thin films spanning compositions from $x=0$ to $x=1.$ The systematic approach to materials growth inherent in combinatorial synthesis allows for a comprehensive picture of the ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Ti}\mathrm{O}}_3$ system. Our continuous, quantitative measurements provide an encompassing view of the composition- and voltage-dependent trends in the room temperature dielectric properties at millimeter-wave frequencies—from strong, few-picosecond relaxations to no relaxation, and from large relative tunability ($n_r>50\mathrm{\%}$ at $75\mathrm{k}{\mathrm{V}}_{}\mathrm{c}{\mathrm{m}}^{-1}$) to zero tunability. Our work underscores both the utility of our technique, and the need to discover lower-loss, highly tunable electronic materials for next-generation telecommunications.

In the present paper, we propose and experimentally verify a concept of a magnonic reservoir computer. The system utilizes the nonlinear behavior of propagating magnetostatic surface spin waves in a yttrium-iron garnet thin film and the time delay inherent in the active ring configuration to process time-dependent data streams. Higher reservoir dimensionality is obtained through the time-multiplexing method, whereby inputs to the system are multiplied by a mask to drive complex dynamics in the ring and the output is sampled in time to create a series of “virtual” neurons for each sample. The input mask is implemented as a train of microwave pulses injected to the system. To demonstrate the efficacy of the concept, the reservoir computer is evaluated on the short-term memory and parity-check benchmark tasks, and the physical system parameters are tuned to optimize performance. By incorporating a reference line to mix the input signal directly onto the ring-resonator output, both the amplitude and phase nonlinearity of the spin waves can be exploited. The addition of a second spin-wave delay line increases the delay time of the ring and enhances the fading memory capacity of the system. Configuring the second delay line to transmit backward volume spin waves also partly compensates the dispersive pulse broadening that is present because of the large delay time.

A high-flux entangled-photon source is a key resource for quantum optical study and application. Here, it is realized in a lithium niobate on isolator (LNOI) chip, with 2.79 × ${10}^{11}$ Hz/mW photon-pair rate and 1.53 × ${10}^9$ Hz/(nm mW) spectral brightness. These data are boosted by over 2 orders of magnitude compared with existing technologies. A 160-nm-broad bandwidth is engineered for eight-channel multiplexed energy-time entanglement. Harnessed by high-extinction-frequency correlation and Franson interferences up to 99.17% visibility, such energy-time-entanglement multiplexing further enhances the high-flux data rate and warrants broad application in quantum-information processing on a chip.

Hyperdoping germanium with gold is a potential method to produce room-temperature short-wavelength-infrared radiation (SWIR; $1.4–3.0\mu \mathrm{m}$) photodetection. We investigate the charge carrier dynamics, light absorption, and structural properties of gold-hyperdoped germanium ($\mathrm{Ge}$:$\mathrm{Au}$) fabricated with varying ion implantation and nanosecond pulsed laser melting conditions. Time-resolved terahertz spectroscopy (TRTS) measurements show that $\mathrm{Ge}$:$\mathrm{Au}$ carrier lifetime is significantly higher than that in previously studied hyperdoped silicon systems. Furthermore, we find that lattice composition, sub-band-gap optical absorption, and carrier dynamics depend greatly on hyperdoping conditions. We use density functional theory (DFT) to model dopant distribution, electronic band structure, and optical absorption. These simulations help explain experimentally observed differences in optical and optoelectronic behavior across different samples. DFT modeling reveals that substitutional dopant incorporation has the lowest formation energy and leads to deep energy levels. In contrast, interstitial or dopant-vacancy complex incorporation yields shallower energy levels that do not contribute to sub-band-gap light absorption and have a small effect on charge carrier lifetimes. These results suggest that it is promising to tailor dopant incorporation sites of $\mathrm{Ge}$:$\mathrm{Au}$ for SWIR photodetection applications.

The acoustic analog of the quantum black hole for airborne sound in two dimensions was denominated as an omnidirectional acoustic absorber by Climente et al. [see Appl. Phys. Lett., 100, 144103 (2012)], who characterized its absorbing properties without providing any theoretical support. The viscothermal losses of the underlying structure, which consists of an absorbing core and a surrounding gradient-index (GRIN) lens both made of periodic distributions of cylindrical rods, are here comprehensively studied by using the boundary element method (BEM) in two dimensions. It is shown that the numerical simulations in two dimensions reproduce fairly well the increase in absorption of the core when the GRIN lens is added and reveal that the discrepancy between measured and calculated values of absorbance is an artifact of the experimental setup. The possibility of independent calculation of viscous and thermal losses contributions in the two-dimensional (2D) BEM algorithm is employed for the comparison with a homogenization theory in which the cluster of cylinders is represented by a single fluidlike viscous cylinder with effective parameters. We conclude that viscous losses represent about 90% of the total energy dissipated in the core. The homogenization approach results are only 2% below the results calculated with 2D BEM, indicating that the effective parameters obtained by the homogenization are very accurate.

Acoustic systems that are without limitations imposed by the Fermi level have been demonstrated as a significant platform for the exploration of fruitful topological phases. By surrounding the nontrivial domain with trivial “environment,” the domain-wall topological states have been theoretically and experimentally demonstrated. In this work, based on the topological crystalline insulator with a kagome lattice, we rigorously derive the corresponding Hamiltonian from the traditional acoustics perspective, and exactly reveal the correspondences of the hoppings and on-site terms within acoustic systems. Crucially, these results directly indicate that instead of applying the trivial domain, the soft-boundary condition precisely corresponds to the theoretical models, which always require generalized chiral symmetry. These results may provide a platform to construct desired acoustic topological devices hosting desired topological phenomena for versatile applications.

In ${\mathrm{Ta/(Co,Fe)B/HfO}}_2$ stacks, a gate voltage drives, in a nonvolatile way, the system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an optimum oxidation level resulting in perpendicular anisotropy (PMA) and further into an overoxidized state with IPA. The $\mathrm{IPA}\to \mathrm{PMA}$ regime is found to be significantly faster than the $\mathrm{PMA}\to \mathrm{IPA}$ regime, whereas only the latter shows full reversibility under the same gate voltages. The effective damping parameter also shows a marked dependence with gate voltage in the $\mathrm{IPA}\to \mathrm{PMA}$ regime, going from 0.029 to 0.012, and only a modest increase to 0.014 in the $\mathrm{PMA}\to \mathrm{IPA}$ regime. The existence of two magnetoionic regimes has been linked to a difference in the chemical environment of the anchoring points of oxygen species added to underoxidized or overoxidized layers. Our results show that multiple magnetoionic regimes can exist in a single device and that their characterization is of great importance for the design of high-performance spintronics devices.

We describe an experimental effort designing and deploying error-robust single-qubit operations using a cloud-based quantum computer and analog-layer programming access. We design numerically optimized pulses that implement target operations and exhibit robustness to various error processes including dephasing noise, instabilities in control amplitudes, and crosstalk. Pulse optimization is performed using a flexible optimization package incorporating a device model and physically relevant constraints (e.g., bandwidth limits on the transmission lines of the dilution refrigerator housing IBM Quantum hardware). We present techniques for conversion and calibration of physical Hamiltonian definitions to pulse waveforms programmed via Qiskit Pulse and compare performance against hardware default derivative removal by adiabatic gate (DRAG) pulses on a five-qubit device. Experimental measurements reveal default DRAG pulses exhibit coherent errors an order of magnitude larger than tabulated randomized-benchmarking measurements; solutions designed to be robust against these errors outperform hardware-default pulses for all qubits across multiple metrics. Experimental measurements demonstrate performance enhancements up to: about a $10$ times single-qubit gate coherent-error reduction; about a $5$ times average coherent-error reduction across a five-qubit system; about a $10$ times increase in calibration window to one week of valid pulse calibration; about a $12$ times reduction gate-error variability across qubits and over time; and up to about a $9$ times reduction in single-qubit gate error (including crosstalk) in the presence of fully parallelized operations. Randomized benchmarking reveals error rates for Clifford gates constructed from optimized pulses consistent with tabulated $T_1$ limits, and demonstrates a narrowing of the distribution of outcomes over randomizations associated with suppression of coherent errors.

We present a density-functional-theory-based study of a two-dimensional phase of chromium bismuthate ($\mathrm{Cr}\mathrm{Bi}$), a previously unknown material with exceptional magnetic and magneto-optical characteristics. Monolayer $\mathrm{Cr}\mathrm{Bi}$ is a ferromagnetic metal with strong spin-orbit coupling induced by the heavy bismuth atoms, resulting in a strongly anisotropic Ising-type magnetic ordering with the Curie temperature estimated to be higher than 300 K. The electronic structure of the system is topologically nontrivial, giving rise to a nonzero Berry curvature in the ground magnetic state, leading to the anomalous Hall effect with a conductivity plateau of about $1.5e^2/h$ at the Fermi level. Remarkably, the Hall conductivity and the magneto-optical constant are found to be strongly dependent on the direction of magnetization. Besides, monolayer $\mathrm{Cr}\mathrm{Bi}$ demonstrates the polar magneto-optical Kerr effect in the visible and near-ultraviolet spectral ranges with the maximum rotation angles of up to 10 mrad. Our findings suggest that monolayer $\mathrm{Cr}\mathrm{Bi}$ is a promising system for practical applications in magneto-optical and spintronic devices.

Topologically nontrivial nanowhirls, called magnetic skyrmions, are often considered attractive for spintronic applications like racetrack data storage devices. However, skyrmions do not move parallel to applied currents and typically do not coexist with other nano-objects, making the realization of such storage concepts difficult. Herein we consider materials with an anisotropic Dzyaloshinskii-Moriya interaction, like tetragonal Heusler materials, and show that four distinct types of topologically trivial bubbles can coexist. We show that each of them can be written by spin torques and that they can be distinguished using a single magnetic tunneling junction. Due to their trivial topology, the four types of bubbles move parallel to applied electrical currents and remain equidistant under motion. Still, the bubbles have a small size and high stability comparable to that of topologically nontrivial spin textures. This allows the construction of a quaternary-digit-based racetrack storage device that overcomes the two mentioned drawbacks of skyrmion-based racetracks.

Magnetic fields are recently being applied to metal additive manufacturing to control the molten pool dynamics and dendritic morphology. To study the molten pool dynamics under an external magnetic field, a thermoelectric magnetohydrodynamic (TEMHD) model is developed by incorporating the electrodynamic model with the Seebeck effect into the multiphysics thermal-fluid flow model. The Seebeck effect in the molten pool is analyzed with the simulation of stationary laser melting on a bare plate. Furthermore, different external magnetic fields are assigned in the TEMHD simulations with the stationary laser melting and the results show that the Lorentz force can damp the keyhole fluctuations. Finally, the laser scanning simulations on a bare plate under external magnetic fields indicate that the Lorentz force can smooth the fluid flow fields in the molten pool and increase the ratio of equiaxed grains in the solidification front.

Hybrid circuit QED involves the study of coherent quantum physics in solid-state systems via their interactions with superconducting microwave circuits. Here we present a crucial step in the implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bilayer, and implement voltage tunability of the quantum circuit’s frequency using a local electrostatic gate. We demonstrate a strong dispersive coupling to a coplanar waveguide resonator by investigating the gate-tunable resonator frequency. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of $10$–$200\mathrm{ns}$.

We report on experimental detection of 10 nm $\mathrm{Si}$ nanoparticles on $\mathrm{Si}$ surface using spatial modulation microscopy. We show that a simple optical layout with a galvo-mirror allows reliable detection of $9\times 10$ nm $\mathrm{Si}$ nanocylinders with signal-to-noise ratio 4.6 when the wavelength is tuned to the spectral range of direct-band optical transitions in silicon. We also show the possibility to use the spatial modulation microscopy principles for nonabsorbing particles. We demonstrate successful detection of $\mathrm{Si}$ nanocylinders down to $20\times 40\mathrm{nm}$ by scattered light ($\lambda =680$ nm) analysis. Our results pave the way to an inspection method for semiconductor metrology.

Highly conductive nanoscale electrical contacts suffer from strong current crowding at the contact edges, which can lead to nonuniform heat deposition; the formation of local hot spots, aggravation of electromigration; and, in the worst-case scenario, lead to thermal runaway and breakdown of the device. These effects severely affect the overall device properties, reliability, and lifetime. Devices based on thin-film junctions, nanotubes or nanowires, and two-dimensional (2D) materials are especially sensitive to current transport at electrical contacts, due to their reduced dimensions and increased geometrical confinement for current flow. Here, we demonstrate a method to mitigate current crowding, by engineering the interface layer properties and geometry. Based on a self-consistent transmission-line model, we show that the distribution of the contact current greatly depends on the properties of the interfacial layer between two contacting members. Current steering and redistribution can be realized by strategically designing the specific contact resistivity, ${\rho }_c$, along the contact length. For similar contact members, parabolically varying ${\rho }_c$ along the contact interface significantly reduces the edge-current crowding in ohmic contacts. Similarly, the nonuniform current distribution of 2D semiconductor-3D metal contacts can be decreased, and the current-transfer length can be increased by varying the Schottky barrier height along the interface. It is also found that introducing a nanometer- or subnanometer-scale thin insulating tunneling gap between contact members can greatly reduce current crowding, while maintaining a similar total contact resistance.

We develop an ultracompact high-resolution imaging system for cold atoms. Its only in-vacuum element is a multimode optical fiber with a diameter of 230 $\mu \mathrm{m}$, which simultaneously collects light and guides it out of the vacuum chamber. External adaptive optics allow us to image cold $\mathrm{Rb}$ atoms with approximately 1-$\mu \mathrm{m}$ resolution over a $100\times 100\mu {\mathrm{m}}^2$ field of view. These optics can be easily rearranged to switch between fast absorption imaging and high-sensitivity fluorescence imaging. This system is particularly suited for hybrid quantum-engineering platforms where cold atoms are combined with optical cavities, superconducting circuits, or optomechanical devices restricting optical access.

Recently, there has been increasing activity to build dedicated Ising machines to accelerate the solution of combinatorial optimization problems by expressing these problems as a ground-state search of the Ising model. A common theme of such Ising machines is to tailor the physics of the underlying hardware to the mathematics of the Ising model to improve some aspect of performance that is measured in speed to solution, energy consumption per solution, or area footprint of the adopted hardware. One such approach to build an Ising spin, or a binary stochastic neuron (BSN), is a compact mixed-signal unit based on a low-barrier nanomagnet-based design that uses a single magnetic tunnel junction (MTJ) and three transistors (3T-1MTJ design) where the MTJ functions as a stochastic resistor (1SR). Such a compact unit can drastically reduce the area footprint of BSNs while promising massive scalability by leveraging the existing magnetic RAM technology that has integrated 1T-1MTJ cells in approximate Gbit densities. The 3T-1SR design however can be realized using different materials or devices that provide naturally fluctuating resistances. Extending previous work, we evaluate hardware BSNs from this general perspective by classifying necessary and sufficient conditions to design a fast and energy-efficient BSN that can be used in scaled Ising machine implementations. We connect our device analysis to systems-level metrics by emphasizing hardware-independent figures of merit such as flips per second and dissipated energy per random bit that can be used to classify any Ising machine.

This study presents a method of producing thick (i.e., in the $\mu \mathrm{m}$ range) polycrystalline $\mathrm{Gd}$ free-standing flexible films. Preparation is carried out by sputtering on silicon conventional substrates using tantalum as a buffer and capping layer. Magnetic and magnetocaloric properties show good agreement with data from high-purity bulk $\mathrm{Gd}$, and are not altered by substrate removal. Moreover, the free-standing film is flexible and all the relevant magnetic properties (i.e., Curie temperature $T_c$, saturation magnetization $M_s$, and isothermal entropy change $\mathrm{\Delta }S$) are preserved under bending (up to a $\epsilon =\pm 0.78\mathrm{\%}$ strain over the two film sides). The technological opportunities heralded by availability of magnetocaloric flexible self-sustaining films are discussed in the conclusions with particular focus on energy-conversion applications (i.e., cooling and thermal energy harvesting). More precisely, the output-power upper bound of an thermal-energy harvester deploying a $\mathrm{Gd}$ flexible film with the reported properties is worked-out using a thermal switch model presented elsewhere. The calculations show a potential output able to supply the new generation of IoT wireless devices as well as small medical implants. The result moot $\mathrm{Gd}$ free-standing flexible films as a benchmark for a new generation of small high-throughput magnetocaloric energy-conversion devices.

We discuss the generation and measurement of a Bessel vortex beam carrying multiple orbital-angular-momentum (OAM) modes in the radio-frequency domain. Considering that a high-order Bessel beam inherently has high-order OAM modes, in this paper, we propose to superpose these high-order Bessel vortex beams together, which can combine the advantages of the orthogonality of the vortex beam’s multiple OAM modes and the Bessel beam’s nondiffraction property. A theoretical formula and the basic design method are presented. A reflective metasurface is designed as the beam launcher to generate a deflected Bessel vortex beam that carries two OAM modes of $\ell =1$ and $\ell =2$ simultaneously. The corresponding reflective metasurface is designed, fabricated, and measured. The full-wave electromagnetic simulations and experimental measurements both verify the effectiveness of the design. The proposed method could further promote the application of a Bessel vortex beam in related near-field scenarios such as near-field wireless communication and radar detection and imaging.