Large-scale deployable solar panels are crucial for certain engineering applications. However, a complex network of actuators and power supplies are usually required to achieve deployment, and can be prone to failure. The single-degree-of-freedom design proposed here embeds shape-memory polymers within an elastic origami substrate, to achieve self-deployment through temperature change. The unexpected bifurcation during folding is studied by examining strain energy as a function of dihedral angle. By optimizing the geometry, tenfold self-deployment is achieved in under one minute. The results could benefit space exploration, as well as solar power generation in inaccessible areas.

The major challenge in improving deterministic single-ion sources is to control the position and momentum of each ion. Based on the extra information given by the electron created in a photoionization process, the trajectory of the correlated ion can be controlled, using a real-time feedback system. This versatile single-ion feedback control can be applied to different kinds of ion sources. This approach improves the spatial and temporal manipulation of charged particles (ions and electrons), and thus boosts applications in quantum technology and materials science, especially deterministic implantation.

Quantum processors based on spin qubits in silicon offer high-fidelity quantum control, with single- and two-qubit operation approaching the fault-tolerance threshold. Challenges in fabricating and controlling large quantum dot arrays in silicon have limited previous studies to only two qubits. Here the authors build on a scalable device design to fabricate and control all four spin qubits in a quadruple-quantum-dot device. This achievement paves the way to multiqubit quantum information processing in silicon.

Laser-driven particle accelerators are a promising avenue for creating tabletop accelerators and light sources, but their functionality is currently limited by a lack of controlled power delivery over a long acceleration length. This work demonstrates that reconfigurable photonic integrated circuits may be used as a control and delivery mechanism for such accelerators. Through the use of integrated Mach-Zehnder interferometers, the authors present a protocol for automatic, dynamic optimization of power delivery, and show that such an approach may enable significant performance enhancements, compared to conventional linear accelerators.

Reservoir computing is an increasingly popular approach to hardware implementations of neural networks. It does not require fine tuning of system parameters, and holds promise for high processing rates in photonic systems. The authors demonstrate how this concept can be applied in systems described by the complex Ginzburg-Landau equation, one of the fundamental models of wave phenomena. In particular, it is predicted that lattices of semiconductor microcavities could be used for information processing at data rates on the order of 1 Tbit/s, two orders of magnitude higher than the record to date in optical systems.

Though they are used widely across optics and photonics, nanofiber waveguides have a critical drawback: their inability to maintain the polarization state of guided light. This study reports a simple, very reliable method for complete polarization control via free-space compensation that is applicable to any nondichroic optical element, including adiabatically tapered single-mode nanofibers. Polarization control is realized by exploiting near-field directional coupling between two crossed nanofiber waveguides. These findings will have an impact on the vast range of systems based on optical nanofibers and evanescently coupled elements.

Quantum-dot-based nonlinearities are an important building block for logic operations in quantum information processing. The authors build on theoretical proposals by using the nonlinear effects of a spin-photon interaction and measuring the resultant phase shifts of scattered light pulses induced by a semiconductor quantum dot. Phase rotations of almost 80° are achieved at the single-photon level, and phase switching is demonstrated. These findings highlight the importance of semiconductor quantum dots as a nonlinear medium for developing quantum information processing and quantum photonic integrated circuits.

Bianisotropic gratings offer superior control of airborne sound via asymmetric wave transmission and reflection at an arbitrarily designed angle, leading to better technology in acoustic lensing, noise control, and high-intensity focused ultrasound therapies. Traditional bianisotropic materials depend on the resonance of deeply subwavelength particles, with intrinsic loss that limits grating efficiency. This study designs bianisotropic elements using a finite-element method to maximize scattering efficiency, and spatial Fourier analysis for verification. The result is a systematic process for designing gratings with 100% efficiency for an arbitrary scattering angle.

Magnetic nanoparticles have great potential for remote actuation at small scales, in a variety of applications. The magnetic properties of nanoparticles can be difficult to control and measure, though, due to their heterogeneity and interaction. This study develops a form of magnetometry, based on magnetic bubble expansion in sensor films with tunable properties, that provides hysteresis loops of $single$ magnetic nanoparticles with high throughput. The technique allows precise statistical analysis of nanoparticles for quality control, and direct characterization of the magnetic transition from single particles to agglomerates for cancer hyperthermia.

Spin-orbit torques (SOTs) show promise in efficiently driving magnetic memory, logic, and oscillators. However, low-resistivity spin Hall metals (including Pt, which has the highest intrinsic spin Hall conductivity yet known) are usually inefficient in generating SOTs. The authors demonstrate that enhancing interfacial scattering by inserting ultrathin layers within a spin Hall metal with intrinsic or side-jump mechanisms can significantly enhance the spin Hall ratio and dampinglike SOT efficiency. Inserting submonolayer Hf into Pt doubles the intrinsic spin Hall effect, and yields a very low critical switching current of 73 $\mu$A in memory devices.

Perpendicular magnetization switching induced by spin-orbit torque (SOT) continues to attract great attention as a promising writing method for ultrafast, high-density, energy-efficient spintronic devices. Unfortunately, this otherwise very attractive switching scheme often comes with an unfavorable external magnetic field. Here the authors achieve polarity-controlled, $field-free$ SOT switching via oriented columnar microstructures in sputtered films. It is demonstrated that Cr—a $3d$, not $4d$, metal—can induce strong perpendicular magnetic anisotropy and generate large spin current to deliver SOT. These findings point to highly efficient, nonvolatile SOT spintronic devices.

Quantum processors based on spin qubits in silicon offer high-fidelity quantum control, with single- and two-qubit operation approaching the fault-tolerance threshold. Challenges in fabricating and controlling large quantum dot arrays in silicon have limited previous studies to only two qubits. Here the authors build on a scalable device design to fabricate and control all four spin qubits in a quadruple-quantum-dot device. This achievement paves the way to multiqubit quantum information processing in silicon.

Time-resolved magneto-optical (TRMO) imaging with ultrashort laser pulses now allows direct observation of the excitation, propagation, and relaxation dynamics of magnetization, with the details of excitation still being studied. This article shows how to image the in-plane component of magnetization, modulated by propagating spin waves, via the birefringence that arises in a transverse magnetic field. The authors use this method to resolve the contribution from magnetoelastic coupling in an out-of-plane-magnetized film. This technique extends the scope of TRMO imaging for investigating laser-induced magnetization dynamics, which will enable work in spintronics.

As an emerging two-dimensional (2D) semiconductor, monolayer antimonene—think graphene, but made of Sb instead of C, and not so flat—is of great potential for (opto)electronic devices. High-quality electrode contact is critical to developing these devices. In this article, calculations of electronic structure and quantum transport are used to study the interfacial properties of antimonene-metal contacts. This comprehensive investigation provides a theoretical basis for selecting favorable electrodes in 2D antimonene devices. The optimal $n$-type electrode is 2D Hf${}_2$N(OH)${}_2$, while the optimal $p$-type electrode is graphene-Pt, both of which provide Ohmic contact.

Plasmonic transport is considered as a replacement for electronic transport in low-dimensional systems like field-effect transistors, as plasmons are faster than electrons. Plasmons in graphene are promising for use at THz frequencies, but this research is held back by the primitive state of circuitry for plasmonic devices. This study employs an asymmetric periodic grating to solve the problem of effective excitation of propagating plasmons in graphene. The important role of “dark” (weakly radiative) plasmon modes in efficient excitation is revealed. The approach outlined here could help to fill the need for THz plasmon sources, for tomorrow’s devices.

Monitoring the coherent evolution of a quantum system due to controlled interaction with its environment allows the probing of those surroundings. Such a quantum sensor can be improved by prolonging its coherence time, or using better state-readout techniques. This study shows how the choice of spin-readout technique impacts the performance of a single N-$V$ center in diamond. In particular, a technique based on spin-to-charge conversion significantly improves both readout noise per shot and sensitivity in ac magnetometry. The authors also identify applications where single-shot spin-readout noise, not sensitivity, is the limiting factor, such as some types of biomagnetometry.

Super-resolution imaging through contact microspherical lenses is often linked to the ability of dielectric microspheres to form photonic nanojets, and to the reciprocity of focusing and imaging. By rigorously solving Maxwell’s equations, the authors show that this common understanding of the origin of super-resolution is not valid. Furthermore, they apply the concept of the point-spread function in combination with magnification of the virtual image to provide a basis for quantifying the resolution in wide-field microspherical nanoscopy. These results are expected to strongly influence near-field imaging beyond the diffraction limit.

Wave plates are important optical components for controlling the polarization state of light, in scientific research as well as commercial applications. Traditional wave plates are not achromatic, and more elaborate designs typically rely on stacking of different wave plates and system optimization to extend the operating bandwidth. Here the authors demonstrate that broadband wave plates can be obtained by engineering the mode dispersion in a single layer of subwavelength gratings featuring high refractive-index contrast. This achievement will contribute to the design of compact, fully integrated polarization converters for practical applications.

The authors propose a method for determining the supply of nutrient molecules in the body, and establish a dual-modality imaging system to realize this method. In their approach, thermoacoustic imaging reveals the density of nutrient molecules (which are not visible to conventional techniques), while Doppler ultrasound imaging shows the velocity of blood flow. Combining these results, one can obtain the nutritional perfusion velocity, and thus the nutritional supply level of living tissue, which can be an indicator for tumor detection.

Multilayered metasurfaces represent a very promising technology for applications such as wireless communication and radar, but existing design methods fail to account for near-field coupling, which can lead to poor metasurface performance. The authors introduce an improved synthesis algorithm that quantifies and corrects near-field coupling, providing more physical insight and faster convergence than black-box optimization techniques. These findings enable more efficient synthesis and pave the way for innovative metasurface-based devices, especially for the millimeter-wave and terahertz regimes.

Mn-Al alloys are promising candidates to fill the performance gap between rare-earth permanent magnets and ferrites, but improving the intrinsic magnetic properties of Mn-Al alloys in the ordered $L{1}_0$ phase is not easy. Doping seems to be essential. This work investigates the influence of substitutional atoms on the alloy’s intrinsic magnetic properties. The occupation rules for substituents with different valence-electron structures are analyzed, and a strategy to boost the alloy’s magnetic properties is developed. This insight should help to promote the engineering of devices that do not rely on rare-earth-bearing magnets, which is of keen interest.

The reduction of magnetic damping is one of the biggest challenges in low-energy-consumption spintronics and magnonics, in the pursuit of $e.g.$ low switching current for spin-transfer-torque-based technology, and long-range spin-wave propagation. This experimental study highlights ultralow damping values in high-quality epitaxial Co${}_2$Mn$Z$ ($Z$ = Al, Si, Ga, Ge, Sn, Sb) Heusler half-metallic magnets. As predicted theoretically, these ultralow values are intrinsically coupled to the underlying electronic structure. The width of the spin gap, as well as the location of the Fermi energy within it, play key roles in the relaxation of precessing magnetization.

Enhancing the magneto-optical (MO) response of magnetic films and nanostructures is an important issue for a wide variety of applications, including biosensing. Past success via multilayer stacking has been focused primarily on symmetric or periodic geometries. Here, however, the authors find that a quasiperiodic structure exhibits a far larger MO response than a similar periodic structure, over a broad spectral range, even though both samples have otherwise very similar magnetic and optical properties. Model calculations based on local optical properties fail to describe this behavior; more fundamental, nonlocal effects must be the origin of the massively enhanced MO signal.

Topological quantum computing has traveled a long road, from theoretical proposals to recently becoming an experimental reality. However, most of the proposed state-readout techniques for topological qubits require state detection at time scales faster than the internal dynamics of the system. The authors develop fast nanowire-based charge sensors in a hybrid Majorana-compatible system that works in magnetic fields up to 1 T. They achieve a signal-to-noise ratio better than 3 for an integration time of 1 $\mu$s. Employing the same reflectometry method also speeds up gate-space mapping by a factor of 40, compared to low-frequency techniques.

While a wave’s spectrum generally can be manipulated by frequency-mixing methods or time-varying perturbations, as done in nonlinear optics, the low efficiency of acoustic nonlinearities makes this quite challenging for sound. Controlling the spectrum of sound is very desirable, though, especially for acoustic communication and voice encryption, where information is usually processed in the frequency domain. Thus the authors create synthetic gauge fields to generate Bloch oscillations of frequency in an acoustic waveguide, in which a time-varying bulk modulus can yield a frequency lattice. This allows $e.g.$ spectrum self-imaging, unidirectional transduction, and bandwidth engineering.

The titular technique, KTV, is an important development in the field of laser diagnostics for supersonic and hypersonic flows, as it gives access to unexplored regimes. KTV is not plagued by the fundamental limitations of traditional tracer-particle techniques. The authors investigate the boundary-layer profiles that form over a sharp, hollow cylinder in supersonic flows of air and N${}_2$ via a single-laser scheme. With the use of high-repetition-rate lasers, this simple, cost-effective evaluation tool for large facilities will allow for time-resolved measurements of turbulent flows, for $e.g.$ the development of high-speed vehicles such as bullet trains.

Laser-driven particle accelerators are a promising avenue for creating tabletop accelerators and light sources, but their functionality is currently limited by a lack of controlled power delivery over a long acceleration length. This work demonstrates that reconfigurable photonic integrated circuits may be used as a control and delivery mechanism for such accelerators. Through the use of integrated Mach-Zehnder interferometers, the authors present a protocol for automatic, dynamic optimization of power delivery, and show that such an approach may enable significant performance enhancements, compared to conventional linear accelerators.

Two-dimensional magnetic materials are being proposed as building blocks for future spin valves. The authors consider an insulating spacer of blue phosphorus, sandwiched between two hexagonal CrN monolayers, and calculate the lateral spin-dependent transport in the heterostructure using density functional theory plus Boltzmann transport theory. Spin transport in CrN/P/CrN is strongly dependent on doping, as well as the mutual orientation of magnetization in the individual CrN layers, with a resulting magnetoresistance of up to 12%.

The next disruptive step in nanoscale imaging will be the development of a label-free, far-field technique that beats the diffraction limit of resolution. To this end, the authors present an innovative far-field super-resolution metamaterial lens, composed of a planar array of discrete subwavelength resonant antennas. These antennas have individual, tailored scattering characteristics, for continuous amplitude and phase modulation. This system will enable label-free, super-resolved nonalgorithmic microscopies at harmless levels of intensity, without impregnating the imaging objects with fluorescent materials.

Planar quantum wires are important for interconnects in integrated optoelectronic circuits. The authors present high-quality quantum wires fabricated by molecular beam epitaxy on structured GaAs(001) surfaces. Unlike those defined by etching or electrostatic gating, these growth-defined quantum wires do not have free surfaces, and can be embedded within epitaxial structures. Their quality is confirmed by a systematic study of their structural and optical properties, and they are seen to efficiently transport electrons and holes in a moving surface acoustic field. These findings prove that such growth-defined quantum wires are promising as efficient charge and spin interconnects.

Understanding the physics of superconductor-ferromagnet transistors and pseudospin-valve devices adds remarkable capabilities to the practical design of large superconducting circuits, in which energy dissipation is minimized. To maintain the quality and speed of large-circuit simulations, the phenomenological models of individual devices must be simplified as much as possible. The authors show how to combine nonlinear devices in a working superconducting circuit, and how two types of nonlinear elements, described by different physics, interact with each other and behave in large, complex electronic circuits.

Voltage-torque (VT) switching of magnetization is an attractive method for low-power writing in magnetoresistive random-access memory (MRAM). While most studies of VT-MRAM have been performed under high-resistance conditions to eliminate any spin-transfer torque (STT), for practical applications it is necessary to investigate the impact of STT on switching characteristics, such as the write-error rate (WER). The authors theoretically analyze the impact of STT on the WER, and identify the minimum current density below which the impact of STT on the WER is negligible. Their results are important for developing VT-MRAMs with fast reading speed and low power consumption.

Techniques for scanning probe microscopy based on electrostatic forces have revealed the fates of charges in a broad array of semiconductor electronic and photovoltaic devices. Recent experiments, however, have called into question the equations widely used to describe these experiments. To remedy this situation, the authors conduct a rigorous analysis of the coupled electromechanical motion of cantilever position, cantilever charge, and sample charge during an electrostatic force microscope experiment. Their results should inspire researchers to rethink such experiments, and reconsider what the resulting data are telling them about charge motion in their materials.

Spin-orbit torque (SOT) has been demonstrated as a promising means to manipulate the magnetization of a ferromagnet in a device, but so far has been observed mainly in systems with broken bulk or structure-inversion symmetry. This study reports the observation of SOT in single Fe${}_{0.8}$Mn${}_{0.2}$ layers. Due to the scattering asymmetry for spin-up and spin-down electrons, the torques from the top and bottom surfaces simply add up instead of canceling each other out, which effectively removes the requirement for spatial-inversion asymmetry in generating SOT. This means SOT can be created in a ferromagnet without the need for an additional nonmagnetic layer.

Putting the proper spin on it: Using sound waves to manipulate particles is an ongoing theme of research, particularly in the life sciences and microfluidics. This work reveals the physical phenomena and mechanisms of the reversals of acoustic radiation forces in both axial and transverse components. The physical mechanism behind the spinning motion of a particle of general shape in an acoustic vortex is extended beyond the Rayleigh regime, allowing the desired control of spinning particles. Both theoretical and efficient numerical demonstrations yield the reversal parameter conditions over a broad range of frequencies.

Technologies based on laser-cooled atoms are leading candidates for the realization of mobile quantum devices for networking, timekeeping, and vacuum measurement. Moving laser-cooled atoms beyond the laboratory and into deployable devices requires a drastically smaller apparatus. The authors describe a compact laser cooling and trapping system for lithium vapor that is just the size of a coffee cup, and uses a nanofabricated diffraction grating to minimize external optics. This design can be adapted to trap other elements or even molecules, allowing the development of a host of field-usable quantum devices.

For various applications based on nitrogen-vacancy centers in diamond, high-contrast optical detection of the spin state is one of the most important techniques, and is limited by the probability of nonradiative intersystem crossing. Utilizing a time gate for fluorescence detection, this work shows that the spin-state signal’s contrast is improved by partially detecting the fluorescence photons. Subsequently, information hidden in the high-level background can be revealed. This technique can help to improve the performance of quantum sensing and imaging with nitrogen-vacancy centers, especially in noisy environments.

Understanding the mechanisms of phonon reflection and transmission at interfaces is fundamental to improving the sensitivity of cryogenic particle detectors, but remains an elusive goal. Even the preferred reflection mode (specular or diffuse) is unknown, and measuring transmission coefficients between materials is technically difficult. This study uses aluminum kinetic-inductance detectors (KIDs) deposited on silicon substrates to measure the athermal phonon flux, plus a Monte Carlo phonon simulation to obtain the Si-Al and Si-Teflon transmission coefficients, confirming the specular reflection mode. This could be a big step toward enabling direct detection of dark matter, for example.

The transport and device physics of the LaAlO${}_3$/SrTiO${}_3$ interface have mainly been investigated in terms of the system’s exotic two-dimensional electron gas. Here researchers observe something rather different: current flow, with strongly nonlinear $I-V$ characteristics, through narrow regions of nominally $insulating$ SrTiO${}_3$. Using a side gate, one can even create a transistor with surprisingly high subthreshold slope at low temperatures. While on one hand these results indicate limitations for standard microelectronic integration in LaAlO${}_3$/SrTiO${}_3$, on the other hand they may present a path toward another class of all-oxide nanoelectronics.

Dispersive readout is a promising technique for enabling scalable measurements of gate-defined semiconductor qubits, but the repeated observation of anomalous signals when using this technique has not yet been explained. The authors study these anomalies and propose that they are caused by charge pockets that appear as gates are depleted, in close proximity to the intentionally formed quantum dots. Understanding these signals gives us a path toward eliminating them in the next generation of devices, and may help in identifying sources of charge noise, the origin of which is not fully understood for these systems.

Rare-earth-doped microlasers featuring whispering-gallery modes are interesting for the integration of optical sensors or photonic functionalities, but their dynamical properties have not been investigated in detail. The authors measure relative-intensity noise and cross correlations involving the two counterpropagating modes in a glass-microsphere laser. It is shown that the laser’s operating regime strongly depends on the material constituting the microresonator. This result should facilitate all-optical microwave generation, or the miniaturization of laser gyroscopes, for example.

Reservoir computing is an increasingly popular approach to hardware implementations of neural networks. It does not require fine tuning of system parameters, and holds promise for high processing rates in photonic systems. The authors demonstrate how this concept can be applied in systems described by the complex Ginzburg-Landau equation, one of the fundamental models of wave phenomena. In particular, it is predicted that lattices of semiconductor microcavities could be used for information processing at data rates on the order of 1 Tbit/s, two orders of magnitude higher than the record to date in optical systems.

The performance of Nb superconducting radio-frequency (SRF) cavities in particle accelerators is often limited by breakdown events below the intrinsic limiting surface fields of Nb. Though excellent rf properties have been achieved, a detailed understanding of the causal links between surface treatment, defects, and ultimate performance is lacking. This study uses a magnetic writer probe from a conventional hard-disk drive as a near-field microwave microscope, to study the localized rf response of SRF-grade Nb samples. The probe reveals nonlinear response due to Josephson weak links at the Nb surface, possibly due to defects, which would evade detection using conventional techniques.

Borophene (a two-dimensional form of boron) monolayers have great potential for applications in nanoscale electronics and optoelectronics. From first principles, the authors explore the electronic transport and photoelectric properties of hydrogenated borophene, B${}_4$H${}_4$, which exhibits a perfect current-limiting effect with high and low levels, due to its strong electrical anisotropy along its zigzag and armchair directions. This material can also generate sizable photocurrent under illumination, with a strong photoelectronic response to blue (green) light in the zigzag (armchair) direction. Other device considerations are also discussed.

Metal/insulator/metal (MIM) tunnel diodes are promising for high-frequency rectifier systems, such as for energy harvesting in the infrared-to-terahertz range, where typical semiconductor devices cannot operate. For practical applications, though, better rectification is needed. The authors fabricate fully epitaxial Fe/ZnO/MgO/Fe tunnel junctions exhibiting 96% magnetoresistance at room temperature, as well as greatly enhanced rectification performance, due to magnon excitations at the Fe/barrier interfaces. This is an exciting result for energy harvesting at longer wavelengths.

Acoustic metasurfaces have drawn great interest for their flexibility in acoustic field manipulation, but are being held back in practice by their complex configurations and narrow frequency range. This study uses hornlike helices in the logical units of a compact coding metasurface that yields broadband acoustic bending and splitting. Such a simple, aperiodic design in compact coding metasurfaces could promote innovative techniques of acoustic wave control.

Early detection of thermoacoustic instabilities is of interest to both applied physicists and engineers, to avoid resonance leading to self-destruction of gas-based engines and turbines. This study shows how a combination of complex-network physics and machine learning can be used to detect a precursor of thermoacoustic instabilities, which can help to prevent the onset of a potentially destructive combustion-driven instability.

Acoustic waveguides are crucial elements for acoustic circuitry. The multimode nature of conventional acoustic waveguides is a limiting factor of their scalability, as the multimode propagation causes significant losses. This study shows how using a membrane-based acoustic waveguide makes it is possible to eliminate all but the out-of-plane modes, in a similar manner as microwave waveguides. Additionally, stressing this membrane also reduces propagation losses. These waveguides could become important in the development of an acoustic-circuit platform.

Acoustic techniques are used to overcome the problem of the longitudinal-transverse size mismatch of particle stream and x-ray beam in single-particle/single-molecule imaging with x-ray free-electron lasers (XFELs). This also enables synchronized injection of particles at kHz repetition rates. The acoustic manipulation is based on simple mechanical recoil, which could have advantages over light pressure, which relies on absorption. Data collection times could be reduced by a factor of 10${}^4$. This work not only provides efficient manipulation of streams of arbitrary gas-phase particles, but also opens wide avenues for acoustic-based particle optics.

In a mechanical network of solid-state spins, spin qubits in adjacent mechanical resonators are coupled via vibrations. This nearest-neighbor (NN) mechanical coupling, however, can also lead to the formation of spectrally dense mechanical modes, with crosstalk spoiling the required control of individual modes. With phononic band-gap engineering, a honeycomblike mechanical network is designed such that vibrations can be confined to any two adjacent resonators and the waveguide between them, enabling NN coupling without spectrally dense modes. This mechanical network can serve as an experimental platform for exploring topological quantum excitations and quantum computing.

Controlling dynamical mode coupling in magnetic nano-oscillators is essential to improving their microwave spectral properties for rf applications and neuromorphic computing. Progress here is held back by the lack of a suitable platform that provides controlled coupling. This study experimentally demonstrates control of dynamical mode coupling in spin Hall nano-oscillators with perpendicular magnetic anisotropy, by means of temperature, excitation current, and magnetic field. It is established that mode coupling in this system is dominated by thermal magnon-mediated scattering, suggesting fresh approaches to engineering device properties suitable for the desired applications.

Thin-film transistors (TFTs) are critical components in $e.g.$ flat-panel displays. In emerging semiconductors for TFTs, such as amorphous oxides and polymers, electrons are slower than in crystalline silicon (where they move in bands), but faster than in amorphous silicon (where they hop). Understanding electron and hole motion in these emerging semiconductors has been difficult so far. The author discuss a $transport$ $reduction$ $factor$ to rigorously enable use of the Boltzmann transport equation, to quantitatively understand electron motion in a range of thin-film semiconductors. This work fills an important gap in semiconductor physics that has lingered for decades.

Controlling the trajectory of general elastic waves with metamaterials is very difficult, because the classical elastic wave equations are not form-invariant. However, this study points out that the elastodynamic potential energy can nearly retain its form after conformal mapping, if the longitudinal wave velocity is much greater than the transverse. Based on this finding, an elastic-wave bender is designed and fabricated using two types of conventional rubber, and is verified as an efficient vibration isolator with a broad operating bandwidth. It even breaks the limit on damping ratios required by classical passive-vibration theory.

Though they are used widely across optics and photonics, nanofiber waveguides have a critical drawback: their inability to maintain the polarization state of guided light. This study reports a simple, very reliable method for complete polarization control via free-space compensation that is applicable to any nondichroic optical element, including adiabatically tapered single-mode nanofibers. Polarization control is realized by exploiting near-field directional coupling between two crossed nanofiber waveguides. These findings will have an impact on the vast range of systems based on optical nanofibers and evanescently coupled elements.

How might one design multioperator metasurface computers to realize multiple wave-based mathematical functions at the same time? The authors show in theory that using normal susceptibilities of a suitably engineered bianisotropic metasurface reveals much freedom to manipulate its nonlocal features, allowing a broader range of computing functionalities, particularly polarization- and angle-multiplexed optical signal processing. The proposed approach to optical signal processing overcomes substantial restrictions of previous designs, such as working with a single mathematical operation, slow responses, and most importantly supporting only the even symmetry operations for normal incidences.

Neural networks based on optoelectronics could be more than a million times as fast as electronic implementations, opening uncharted regimes of information processing. Despite advances in laser-based neurons and programmable Si photonic interconnects, a photonic neuron compatible with a photonic network is missing. These challenges could be addressed by photonic-modulator-based neurons that were integrable on the same platform as Si interconnects. The authors fabricate a Si photonic-modulator neuron and observe all essential networking properties of fan-in, cascadability, and high-gain nonlinearity, plus programmable, multi-input, time-resolved, and self-feedback processing behaviors.

Networks of tunable, integrated optical interferometers support quantum information processing and machine learning with much better energy efficiency than standard electronics. A network’s gridlike structure and imperfections localize optical signals propagating through the device, which ultimately slows training by gradient-based optimization. Here this problem is solved by proper initialization, combined with redundant and remotely interacting interferometers. The authors’ approach improves the convergence time of gradient-based optimization to random target operators by at least two orders of magnitude, at the scale of practical machine-learning applications (10${}^4$ to 10${}^6$ nodes).

The authors use density functional theory to study an ultrathick two-dimensional semiconductor, SnSb${}_2$Te${}_4$, which is predicted to have high mobility and a large optical absorption coefficient. Intriguingly, its electronic band gap exhibits an indirect-direct transition with increasing thickness. Importantly, compared to the monolayer, bilayer SnSb${}_2$Te${}_4$ possesses stronger light harvesting, higher mobility, and larger on-state current by about one order of magnitude, due to rehybridization of electronic states. These results suggest that this material is quite promising for high-performance infrared electronic and optoelectronic applications.

Let’s talk about your flaws… The authors present nondestructive examination of the crystallographic properties (including crystal size, facet shape, strain, and defects) of lone InP nanocrystals (NC) grown on Si nanostructures. This sort of three-dimensional structured imaging is of great significance in evaluating the quality of the active nanomaterials in fully processed nanoelectronic and nano-optoelectronic devices, even in an $operando$ manner.

The authors discuss a bipolar-unipolar transition in a spin diode, which would be interesting for logic or memory devices in spintronics. Generating the requisite on-off states for opposite spins could be difficult, though. This study proposes coupling the electronic valley degrees of freedom in monolayer silicene, germanene, or stanene to the spin degrees in a ferromagnet/antiferromagnet junction, such that the transition is valley-mediated but electrically switched. This approach would provide a feasible means of controlling spin current more comprehensively, and thus is expected to have an impact on engineering spin circuits for reprogrammable logic and nonvolatile memory.

A gravitational-wave detector in space can register signals in the low-frequency band below 1 Hz. The signal laser from a remote satellite has a typical power of only several picowatts, which hinders precise detection of the laser’s phase. With this in mind, the authors present an ultranarrow-band amplifier with high gain and low noise, which exploits four-wave mixing induced by radiation pressure. This mechanically mediated amplifier can work at the quantum noise limit, in principle. Moreover, this approach may allow one to generate quantum entanglement between the amplified and conjugate fields.

The major challenge in improving deterministic single-ion sources is to control the position and momentum of each ion. Based on the extra information given by the electron created in a photoionization process, the trajectory of the correlated ion can be controlled, using a real-time feedback system. This versatile single-ion feedback control can be applied to different kinds of ion sources. This approach improves the spatial and temporal manipulation of charged particles (ions and electrons), and thus boosts applications in quantum technology and materials science, especially deterministic implantation.

How ferroelectric (FE) and antiferroelectric (AFE) domains interact under an electric field is interesting, given the coexistence of FE and AFE phases in technologically important compositions. Work on AFE oxides has focused on the field-induced AFE-to-FE phase transition, though, as such a global event can be readily characterized with bulk measurements. This study employs $in$ $situ$ transmission electron microscopy to directly reveal the local action at an FE/AFE interface. The microscopic mechanism of the depolarization-field-assisted phase transition is observed and theoretically verified, which will help in engineering the phase interface to tailor the transition for applications.

Fluctuating thermal fields in antiferromagnets are seen as a potential means to realize a probabilistic distribution in an output signal that can also be tailored by electrical control. To this end, the dynamics of a 90-degree domain wall driven by spin-orbit torque are theoretically examined in an antiferromagnetic structure at finite temperatures. The calculations clearly illustrate that both the average displacement of the domain wall and its thermally induced dispersion can be electrically modulated, or “trained”, by tuning the driving spin-orbit torque. This unusual functionality could provide a key component in probabilistic computing and machine learning.

Space-time-modulated mechanical systems offer a different paradigm in the design of advanced nonreciprocal mechanical devices. Physically realizing materials with properties rapidly tuned in both space and time, however, presents several challenges. This study introduces an elastic metamaterial with coupling stiffness modulated in space and in time by programmably pumping ac currents into coils. It theoretically and experimentally demonstrates tunable, nonreciprocal propagation of flexural waves on a continuum. This device suggests interesting opportunities in structural dynamics, and in the design of advanced mechanical insulators, diodes, circulators, and topological insulators.

Superconducting coplanar waveguide resonators that can operate in strong magnetic fields are important for a variety of high-frequency superconducting devices. Magnetic fields degrade resonator performance by creating Abrikosov vortices that cause resistive losses and frequency fluctuations, or suppress the superconductivity entirely. To mitigate these effects, the authors investigate how device geometry and lithographically defined artificial defects can control vortex dynamics. These techniques allow the resonators to retain single-photon quality factors of about 10${}^5$ at $B_{\parallel }$ ≃ 6 T, for fast charge readout of a gate-defined double quantum dot at $B_{\parallel }$ = 1 T.

In laser-based welding and additive manufacturing, the interaction of the laser with the metal leads to the formation of a cavity known as a keyhole, which can fluctuate unstably during the process. This work significantly advances our understanding of laser-induced keyholes and their dynamics, by combining state-of-the-art dynamic x-ray radiography with multiphase, multiphysics modeling. Numerical simulations of keyhole morphologies are validated by experiment, then leveraged to predict transient nonuniform distributions of laser absorption, temperature, and flow velocity in the complex multiphase process.

Creating a multicolor laser on a single microchip has become a subject of great interest, with white-light lasers as the ultimate goal. This requires a high-quality-factor, multichannel cavity structure supporting the lasing of all elementary colors simultaneously, with strongly enhanced ultralow-threshold emission. The authors present a hybrid microcavity that eliminates background noise and integrates multiple capillaries on a slab, achieving multiwavelength lasing. This broadly tunable multichannel laser should find use in $e.g.$ optical interconnects and multiplexing, multiagent chemical and biological detection, solid-state lighting, solar cells, and superbright microdisplays.

In an industrial setting, only optical methods are fast enough to tease out the killer defects that may render a computer chip inoperable. Comprised of billions of periodic nanoelectronic devices, they yield optical responses like form birefringence, even for deep-ultraviolet (DUV) light. This study reveals how the form-dependent optical response changes as the wavelength approaches the periodicity, by realistically comparing five wavelengths numerically. Surprising results are obtained at 47 nm. Similarly optimizing sets of wavelengths, materials, and their optical constants may even foster improved optical materials in the far-ultraviolet regime.

$Plasma$ $photonic$ $crystals$ (PPCs) are promising for the manipulation of electromagnetic radiation from microwaves to terahertz waves. Applications are currently limited by PPC flexibility and controllability, since these structures are normally fixed once fabricated. This work shows how to make tunable PPCs via dielectric barrier discharge, by employing a lattice of water electrodes. A rich variety of plasma lattice structures are obtained, the symmetry, lattice constants, and dielectric constants of which can be dynamically controlled. Such PPCs may find broad application in, for example, precision radar rangefinding, signal processing, and wideband communication.

Going beyond two-state qubits, $qudits$ based on quantum states of high dimension constitute a rich resource in quantum information, and their exploitation will play a prominent role in next-generation technologies. Generation and manipulation of qudits have improved strongly over the last decades; their reliable transmission between remote locations remains the central challenge. The authors use an air-core fiber supporting orbital angular momentum (OAM) modes to faithfully transmit qudits. Four OAM quantum states and their superpositions are created, propagated over a 1.2-km fiber, and detected. Moreover, three quantum-key-distribution protocols are implemented.

Optical nondiffracting vector beams, with their invariant transverse profiles and longitudinal polarization states, have drawn interest in many areas, from optical tweezers to imaging and metrology, but bulky optical components still limit their utilization. The authors design ultrathin, ring-shaped plasmonic metasurfaces to produce nondiffracting Bessel, Mathieu, and Weber vector beams across a broad wavelength range. These metasurfaces present a compact, effective platform for producing complex optical beams, and thus for advancing numerous applications related to conversion of spin and orbital angular momentum, optical manipulation, and optical communication.

Is it hot in here? The recent redefinition of the kelvin unit of temperature, in terms of a fixed value of Boltzmann’s constant, prompts interest in primary-standard gas thermometers that could quantify possible differences from the International Temperature Scale (ITS-90). This study reports significant progress in the development of low-uncertainty Doppler-broadening thermometry. Successful operation of a comb-calibrated near-infrared absorption spectrometer is demonstrated by probing a line doublet of acetylene. This enables the authors to make thermodynamic temperature measurements with a statistical uncertainty of less than 10 parts per million.

Developing the next generation of free-electron lasers (FELs) depends on lowering the work function and increasing the quantum efficiency of the photocathode material. However, rules for designing appropriate materials are difficult to discern from observations of structure-composition relationships. This study uses high-quality electronic-structure calculations and a simple physical model to develop design rules for metallic alloys. Interestingly, Cu-Ba alloys terminating in a plane of Ba atoms bear a significant surface dipole, low work function, and high quantum efficiency. These alloys could yield better photocathodes than conventional Cu, and thus better FEL performance.

Optical nonlinearity due to intraband transitions (which is important for the application of epsilon-near-zero nonlinearity in metasurfaces and active plasmonics) requires a model to fully predict the spectrum, for design considerations. The extended Drude model, presented here, serves this purpose. It further elucidates the origin of the nonlinearity to be due to both band nonparabolicity and temperature-dependent mobility. The model matches experiment well for the representative compound indium tin oxide, and simplifies the description of spectrally resolved optical nonlinearities of transparent conductive oxides.

For applications in Si-based electronics, it would be interesting and significant to induce spontaneous polarization in some allotrope of pure Si. However, because of the symmetry protection in conventional Si forms, separating positive and negative charge states is difficult. This study shows that a desired Si structure, $T$ pentasilicene, can be realized by tilting the dimers in the two-dimensional pentasilicene sheet, which not only stabilizes the system but also leads to intrinsic ferroelectricity, with a high Curie temperature of 1190 K. This Si allotrope could have potential applications in nonvolatile random-access memory.

In assessing the reliability of metal-oxide-semiconductor devices, electron-hole recombination induced by a gate pulse, known as charge pumping (CP), is widely used to analyze dangling-bond defects at transistor interfaces. The authors perform CP under the magnetic-resonance mode at the Si(100)/SiO${}_2$ interface, and identify the bonding configuration of the defects responsible for the CP. In addition, they reveal the spin-dependent process in the CP sequence, from which they clarify the detailed mechanism of CP at this technologically important interface.

The authors provide a first-principles analysis of and methodology for easily estimating the surface piezoelectric response of materials that are not piezoelectric in the bulk. This study shows that the surface of sapphire ($\alpha$-Al${}_2$O${}_3$) is piezoelectric, in both Al-terminated and hydroxylated forms. If confirmed by experiment, this effect would be of high technological significance. For example, it would lead to a loss mechanism in the sapphire-based Josephson junctions used in a variety of quantum devices, and would allow for the fine tuning of $e.g.$ lasers and chemical catalysts. Furthermore, many technologically relevant materials are grown using sapphire as a substrate.

Electromagnetically induced transparency (EIT) and its acoustic analogue (AIT) are attractive for many applications in wave control. Autler-Townes splitting (ATS) is a similar phenomenon, but not the same, yet a quantitative distinction between ATS and AIT is unclear for acoustic systems. This study numerically investigates the interaction of Love shear waves with a pillared metasurface, revealing transmission zeros, Fabry-Perot resonances, cavity modes, and ATS and AIT resonances. ATS and AIT are distinguished via an analysis of the interaction between two pillars. These results should impact wave control, metamaterial design, biosensors, and other applications in acoustics.

Terahertz plasmonic devices based on two-dimensional systems often include alternating gated and ungated sections. Typically the frequency response of such a device is analyzed numerically, or with approximate analytical models. The authors show that several conventional analytical models provide only $qualitative$ agreement with numerical simulations, and proceed to offer one that instead provides $quantitative$ agreement. It is hoped and expected that this simple yet accurate method will facilitate the development of two-dimensional resonant plasmonic devices.

High-sensitivity magnetometry using ensembles of nitrogen-vacancy (N-$V$) centers in diamond has garnered broad interest lately. This technique typically requires a bias field to resolve magnetically sensitive features in the N-$V$ level structure—a requirement that has hindered the adoption of N-$V$ magnetometry in situations requiring zero ambient field. The authors overcome the need for a bias field by using circularly polarized microwaves to selectively address overlapping transitions in a ${}^{13}$C-depleted diamond. This approach offers a different avenue for applying N-$V$ magnetometry, from zero- and ultralow-field nuclear magnetic resonance (ZULF-NMR) to biomagnetic measurements.

Large-scale deployable solar panels are crucial for certain engineering applications. However, a complex network of actuators and power supplies are usually required to achieve deployment, and can be prone to failure. The single-degree-of-freedom design proposed here embeds shape-memory polymers within an elastic origami substrate, to achieve self-deployment through temperature change. The unexpected bifurcation during folding is studied by examining strain energy as a function of dihedral angle. By optimizing the geometry, tenfold self-deployment is achieved in under one minute. The results could benefit space exploration, as well as solar power generation in inaccessible areas.

The unusually high thermal conductivity recently found in semiconducting BAs makes it promising for thermal management in electronics, which becomes increasingly important as transistors shrink. To date, calculations of BAs have yielded inconsistent values for its thermal expansion coefficient $\alpha$, with actual measurements unavailable. Here the authors report measurements and $ab$ $initio$ calculations of $\alpha$ and the Grüneisen parameter $\gamma$ of BAs; experiment and theory agree only when long-range interatomic interaction is included. For heat management, BAs is a better match than $e.g.$ diamond or BN for common semiconductors.