Vortex-field acoustic levitation is an application of acoustic vortices that allows the manipulation of a wide range of materials in various media. However, to date, its levitation capability in air is limited to relatively low-density objects. Here, by optimizing an array-tube setup, we levitate iridium ($22.56\mathrm{g}/{\mathrm{cm}}^3$) in a first-order acoustic vortex in air. This technique makes it possible to observe the orbital angular momentum transfer carried by acoustic vortices to freely levitated high-density objects, of which the highest rotation speed reaches up to 6500 revolutions per second and increases with a decrease in the object size and an increase in the source amplitude. It is revealed that the thermal-viscous boundary layer dissipation is the dominant effect and leads directly to the rapid rotation of high-density objects. This work can expand the application scope of acoustic vortices, especially in the fields of containerless processing of various materials and rapid rotation of objects, and help to understand the mechanisms of orbital angular momentum transfer to matter.

In contrast to software simulations of neural networks, hardware implementations have limited or no tunability. While such networks promise great improvements in terms of speed and energy efficiency, their performance is limited by the difficulty of applying efficient training. We propose and realize experimentally an optical system where highly efficient backpropagation training can be applied through an array of highly nonlinear, nontunable nodes. The system includes exciton-polariton nodes realizing nonlinear activation functions. We demonstrate high classification accuracy in the MNIST handwritten digit benchmark in a single-hidden-layer system.

Nonreciprocal transmission of structured light has possible applications in high-capacity optical communication and high-dimensional signal processing. Here, we demonstrate the magnet-free and cavity-free optical nonreciprocity of an orbital-angular-momentum (OAM) beam using optical-pumping technology with the assistance of the Doppler effect in hot atoms. By using optical pumping to induce a direction-dependent population transfer of the ground-state atoms, we realize optical nonreciprocity for the signal laser that is carrying OAM. Interestingly, an isolation ratio close to 30 dB and maximum transmission over 0.86 can be achieved simultaneously without the use of an optical cavity. Due to the unique amplitude and helical phase profile of the structured light, our proposal may open an avenue for exploring the singularity characteristics of optical nonreciprocity using structured light.

Broadband precision spectroscopy is an invaluable tool for high-fidelity species diagnostics as well as multispecies sensing in complex real-world environments. However, concurrent demands for broad spectral coverage and high spectral resolution present great challenges to both optical measurement systems and data acquisition and processing. We report methodological and experimental demonstration of a midinfrared direct optical frequency comb (OFC) spectrometer, using Fourier-transform spectroscopy (FTS) with a compressed sensing (CS) algorithm. Efficient data sampling and accurate spectral recovery are demonstrated with comb-teeth-resolving resolution of $0.0083{\mathrm{cm}}^{-1}$ (250 MHz) over a spectral range of $300{\mathrm{cm}}^{-1}$. The $v_3$ band of ${\mathrm{CH}}_4$ is measured, and validated against conventional FTS results and theoretical calculations. Studies on the effect of compression ratio show that a compressed interferogram using as few as 2% of the regular sample points can effectively recover the high-resolution broadband spectrum with an accuracy of 1.9%. The developed diagnostic system bears promising potential for accurate, real-time, multispecies sensing that is pressingly needed for studying the dynamics of complex multicomponent systems.

A coherent-synchrotron-emission regime of forward high-order harmonics generation (HHG) is proposed for the emission of an isolated unipolar half-cycle attosecond pulse by a three-color laser pulse impinging on ultrathin foil. A theoretical model is proposed for the electron nanobunching mechanism and the forward radiation, which is consistent with the numerical-simulation results. As the forward HHG does not need to penetrate from the front to the rear side of the target, the spectrum of the forward HHG has no low-frequency cutoff. The robustness of this regime is verified with different laser and foil plasma conditions as well as the two-dimensional effects. The robustness is also checked with similarity theory, which confirms that isolated attosecond pulses can be efficiently generated when the plasma density and the laser amplitude change simultaneously such that their ratio remains unchanged.

The ion microwave frequency standard is a candidate for the next generation of microwave frequency standard with the potential for very wide applications. The Dick effect and second-order Doppler frequency shift (SODFS) limit the performance of ion microwave frequency standards. The introduction of sympathetic cooling technology can suppress the Dick effect and SODFS and improve the stability and accuracy of the frequency standard. However, the sympathetically cooled ion microwave frequency standard has seldom been studied before. This paper reports a high-performance sympathetically cooled ion microwave frequency standard in a Paul trap. Using laser-cooled ${}^{40}{\mathrm{Ca}}^{+}$ as coolant ions, ${}^{113}{\mathrm{Cd}}^{+}$ ion crystal is cooled to below 100 mK and has a coherence lifetime of over 40 s. The short-term frequency stability reached $3.48\times {10}^{-13}/{\tau }^{1/2}$, which is comparable to that of the mercury ion frequency standard. Its uncertainty is $1.5\times {10}^{-14}$, which is better than that of directly laser-cooled cadmium ion frequency standard.

Deep learning is found to be a powerful data-driven force to transform the way we discover, design, and utilize photonics and metasurfaces. More recently, there has been growing interest in deep-learning-enabled on-demand structural design, as it can ease the limitations of low efficiency, time-consuming, and experience navigation in conventional design. However, training data is a valuable source, especially for high-dimensional scatterers. It is extremely challenging and costly to keep the pace of data collection with the increasing degrees of freedom. Here, we propose a transfer-learning-assisted inverse-metasurface-design method to relieve the data dilemma. A flexible transferrable neural network composed of an encoder-decoder network and a physical assistance network is constructed, the latter of which is attached to solve the nonuniqueness problem. Starting from the 5 × 5 metasurface, we successfully migrate the inverse design to a 20 × 20 metasurface, with a Pearson correlation coefficient that reaches 97%. Compared with direct learning, the data requirement is reduced by over 30%. In the experiment, we validate the concept via wave-front customization. Our work constitutes a green and efficient inverse-design paradigm for fast far-field customization and provides a key advance for the next generation of large-scale intelligent metadevices.

Gradient-index (GRIN) lenses have paved the way for electromagnetic wave tailoring due to their spatially varying permittivities, which enable continuous manipulation of the wave as it passes through the lens volume. However, traditional GRIN design methodologies, such as transformation optics or geometrical optics, often generate lens profiles that are limited to single-band performance. Here, we propose a multiobjective optimization strategy customized for designing dual-band GRIN lenses. Utilizing this powerful inverse-design tool, in conjunction with an advanced manufacturing technique, a GRIN lens with compact size and nonintuitive permittivity distribution is designed, fabricated, and characterized. The lens is capable of realizing two unique gain enhancement objectives at the L and C bands simultaneously, where different feeding sources are employed at the two bands.

A microscopic understanding of the mixed-functionalized MXenes ($M_2{X-T}_x$; $T_x=\text{O, OH,}$ and $\mathrm{F}$) are extremely important to the design of efficient ${\mathrm{CO}}_2$ catalytic activity. Here, we report a first-principles study of the ${\mathrm{CO}}_2$ activation on pure $\mathrm{O}$, $\mathrm{OH}$, and $\mathrm{F}$ and mixed-functionalized ${\mathrm{Sc}}_2\mathrm{C}$ MXene surfaces. We find that ${\mathrm{CO}}_2$ adsorption energy can be tuned by changing the coverage of $\mathrm{O}$, $\mathrm{OH}$, and $\mathrm{F}$ functional groups on the surface. Fully terminated $\mathrm{O}$ and $\mathrm{F}$ ${\mathrm{Sc}}_2\mathrm{C}$ forms weak interactions with ${\mathrm{CO}}_2$ molecules (binding energy $-0.136$ and $-0.168\mathrm{eV}$) whereas mixed-functionalized ${\mathrm{Sc}}_2\mathrm{C}$ surface exhibits higher binding energy($-0.364\mathrm{eV}$). In the mixed-functionalized ${\mathrm{Sc}}_2\mathrm{C}$ case, only $\mathrm{O}$ sites allow ${\mathrm{CO}}_2$ reduction ($\mathrm{F}$ and $\mathrm{OH}$ are inactive) and finally converts into methane (${\mathrm{CH}}_4$). Ab-initio-based Bader charge analysis and projected density of state calculations reveal strong bonding between the C atom of ${\mathrm{CO}}_2$ and $\mathrm{O}$ functional group. The Gibbs free-energy calculation confirms the conversion of $\mathrm{HCO}$ into ${\mathrm{H}}_2\mathrm{CO}$ to be a rate-limiting step with the limiting potential 1.387 eV. In the mixed-functionalized surface, as we increase the number of $\mathrm{OH}$ groups in the vicinity of $\mathrm{O}$ sites, the binding energy increases (transiting from a physisorption to a chemisorption regime). However, increasing the amount of O coverage turns out to be detrimental to the catalytic activity. Our study highlights the role of different functional groups in achieving efficient ${\mathrm{CO}}_2$ catalytic activity on ${\mathrm{Sc}}_2\mathrm{C}$ MXene, which can further help us to design experiments accordingly.

Multiport interferometers based on integrated beamsplitter meshes are widely used in photonic technologies. While the rectangular mesh is favored for its compactness and uniformity, its geometry resists conventional self-configuration approaches, which are essential to programming large meshes in the presence of fabrication error. Here, we present a configuration algorithm, related to the $2\times 2$ block decomposition of a unitary matrix, that overcomes this limitation. Our proposed algorithm is robust to errors, requires no prior knowledge of the process variations, and relies only on external sources and detectors. We show that self-configuration using this technique reduces the effect of fabrication errors by the same quadratic factor observed in triangular meshes. This relaxes a significant limit to the size of multiport interferometers, removing a major roadblock to the scaling of optical quantum and machine-learning hardware.

Realistic multiport interferometers (beamsplitter meshes) are sensitive to component imperfections, and this sensitivity increases with size. Self-configuration techniques can be employed to correct these imperfections, but not all techniques are equal. This paper highlights the importance of algorithmic stability in self-configuration. Naïve approaches based on sequentially setting matrix elements are unstable and perform poorly for large meshes, while techniques based on power ratios perform well in all cases, even in the presence of large errors. Based on this insight, we propose a self-configuration scheme for triangular meshes that requires only external detectors and works without prior knowledge of the component imperfections. This scheme extends to the rectangular mesh by adding a single array of detectors along the diagonal.

We investigate a W-Ta alloying route to reduce the auto-oscillation threshold current densities and the power consumption of nanoconstriction based spin Hall nano-oscillators. Using spin-torque ferromagnetic resonance measurements on microbars of ${\mathrm{W}}_{100-x}{\mathrm{Ta}}_x$(5 nm)/$\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$($t$)/$\mathrm{Mg}\mathrm{O}$ stacks with $t=1.4$, 1.8, and 2.0 nm, we measure a substantial improvement in both the spin-orbit torque efficiency and the spin Hall conductivity. We demonstrate a 34% reduction in auto-oscillation threshold current density, which translates into a 64% reduction in power consumption as compared with pure W-based spin Hall nano-oscillators. Our work demonstrates the promising aspects of W-Ta alloying for the energy-efficient operation of emerging spintronic devices.

Magnetic force microscopy (MFM) is a characterization method used to obtain images of the stray field emanating from the surface of magnetic samples with high spatial resolution. The MFM signal arises from a convolution of the magnetic moment distribution of the MFM tip with the stray field of the sample. Therefore, deconvolution of the stray field of a magnetic sample requires a well-defined micromagnetic structure of a tip. While the magnetic moment distribution of the tip remains challenging to assess, it is convenient to calibrate the tip-equivalent magnetic charge in a plane parallel to the sample surface running through the tip apex using a suitable tip-calibration method. Once the tip-equivalent magnetic charge is calibrated, MFM data can be deconvolved to obtain the stray field of the sample or the equivalent magnetic surface charge, for example, at the sample surface. Here we use low-noise MFM data obtained with high-quality-factor cantilevers and a modified L-curve method for the optimization of a Tikhonov deconvolution procedure used for the calibration of the MFM tip, and—once the tip is calibrated—for obtaining the stray field or magnetic surface charge of the sample.

We investigate the ferromagnetic resonance (FMR) response of microfabricated microwave resonators loaded with small ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ alloy rectangles. A major increase in the FMR signal-to-noise ratio is achieved by employing the microwave-resonator structure. A FMR peak shift similar to that of ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ continuous films is measured in the presence of hydrogen gas in the sample environment. We show that the very high sensitivity of the FMR signal of the ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ alloy rectangle to hydrogen exposure can be used to measure relatively small hydrogen-concentration steps near 100% ${\mathrm{H}}_2$. Additionally, we also demonstrate that this structure can measure hydrogen over a concentration range from 3% to 100% ${\mathrm{H}}_2$ in ${\mathrm{N}}_2$. In time-dependent FMR measurements, we discover a temperature dependence of the FMR signal, which we relate to intrinsic temperature-dependent changes in saturation magnetization and the magnetic anisotropy of the $\mathrm{Co}$-$\mathrm{Pd}$ alloy.

Quantum computing promises to provide exponential speed-ups to certain classes of problems. In many such algorithms, a classical vector $\mathbf{b}$ is encoded in the amplitudes of a quantum state $|b⟩$. However, efficiently preparing $|b⟩$ is known to be a difficult problem because an arbitrary state of $Q$ qubits generally requires approximately $2^Q$ entangling gates, which results in significant decoherence on today’s noisy intermediate-scale quantum (NISQ) computers. We present a deterministic (nonvariational) algorithm that allows us to flexibly reduce the quantum resources required for state preparation in an entanglement-efficient manner. Although this comes at the expense of reduced theoretical fidelity, actual fidelities on current NISQ computers might actually be higher due to reduced decoherence. We show this to be true for various cases of interest, such as the normal and log-normal distributions. For low-entanglement states, our algorithm can prepare states with more than an order of magnitude fewer entangling gates as compared with isometric decomposition.

Nonmagnetic nitride-based van der Waals magnetic heterostructures, introducing magnetism by constructing heterostructures with a two-dimensional room-temperature intrinsic ferromagnet and manipulating both the charge and the spin degrees of freedom, are important materials for developing high-performance low-dimensional spintronic devices. To obtain the unique physical properties of novel devices, a fundamental physical understanding of this material system is highly desired. The electronic and magnetic properties of the $\mathrm{Al}\mathrm{N}/{\mathrm{VSe}}_2$ van der Waals heterostructure are studied systematically by combining first-principles calculations and Schrödinger-Poisson simulations. The $\mathrm{Al}\mathrm{N}/{\mathrm{VSe}}_2$ van der Waals heterostructure presents superior physical properties such as a large conduction band offset, ferromagnetic ground state, magnetic anisotropy, and high Curie temperature. Meanwhile, the band alignment and Curie temperature of the $\mathrm{Al}\mathrm{N}/{\mathrm{VSe}}_2$ van der Waals heterostructure can be modulated by biaxial strain or an electric field, while the easy magnetization axis remains the in-plane direction. Varying the sheet carrier density changes the energy levels, shifts the average charge position, and tilts the conduction band profile. Our method can also be applied to study the interface properties of other van der Waals heterostructure systems.

Terahertz scanning tunneling microscopy (THz-STM) has enabled spatiotemporal imaging with femtosecond temporal resolution and angstrom spatial resolution. In THz-STMs, the junction bias is modulated by coupling THz pulses, which results in transient voltage bias and extremely high transient tunneling currents. In order to have efficient imaging of the sample surface, it is important to understand the nonlinear tunneling current response and its parametric dependence. In this work, we theoretically investigate the basic scaling of rectified electrons in a THz-STM junction. We use a self-consistent quantum model that includes both space-charge potential and exchange-correlation potential, which were ignored in previous studies. Since THz-STMs are operated at high transient voltage in field emission regime, the effects of exchange-correlation potential become crucial. We validate our calculation with recently reported experimental data and investigate the rectification property of the tip-sample junction for different parameters. We find that the time-dependent tunneling current and the electron transport can be manipulated by varying the dc bias voltage (polarity, amplitude), incident THz field (polarity, shape, peak amplitude), work functions of STM tip and sample—especially their difference $\mathrm{\Delta }W$, and the tip-sample separation. Our study provides an important framework that can be used in the future to characterize, control, and improve THz-induced currents and probing techniques at the nanometer scale over subpicosecond time periods.