Thermal runaway is a positive-feedback phenomenon observed in many contexts, including fluid boiling and self-heating in semiconductors. Leveraging the insulator-metal phase transition of VO${}_2$, the authors characterize the dynamics of thermal runaway when heat is dissipated from an emitter through far-field thermal radiation. By minimizing the emitter’s thermal mass, they show that rapid thermal switching is achievable. Further enhancements may be possible using near-field radiation. This description of the switching dynamics associated with radiative thermal runaway may enable the design of advanced infrared sensors, thermal diodes, and calorimeters.

Every computation technology dissipates heat while performing logic operations; the storage and conversion of such power for independent purposes is the main goal of energy harvesting. This study proposes a logic architecture, based on two innovative quantum systems (an ultraefficient thermoelectric element and a nanoscale heat valve), that reconverts the heat generated by any source into classic logic operations. This structure provides a functionally complete logic system, with operating frequencies up to GHz levels. Finally, this thermal logic scheme could be used to design hybrid thermal-electrical computing systems of higher total energy efficiency.

Highly doped multiple quantum wells (MQWs) are significant for infrared optoelectronics, plasmonics, and the physics of strong light-matter coupling. The authors reveal a multisubband plasmon (MSP) that arises from the couplings among several intersubband transitions in (Mg,Zn)O/ZnO MQWs, due to the outstandingly dense two-dimensional electron gas. Here the MSP energy is up to three times that of the constituent intersubband transitions, illustrating the potential of this system for optoelectronic applications in the infrared, which could be extended to the terahertz range by appropriate quantum-well design.

Could antineutrinos help to verify compliance with a ban on nuclear weapons testing, or provide useful information about illicit tests? Surprisingly, detectors proposed for basic physics may be capable of detecting antineutrinos from faraway nuclear tests. Seismic data is key to the analysis. This study explores how large, water-based antineutrino detectors could rapidly confirm the nuclear nature of an explosion, constrain estimates of the fission yield, and provide other forensic insight.

High-mobility transistors with two-dimensional channels support long-lived plasmons, and thus can act as resonant spectrometers or detectors of terahertz radiation. A major obstacle to developing such devices is the complexity of photoresponse modeling, which requires time-consuming electromagnetic simulations. This paper presents an exact solution for electric fields and responsivity in plasmonic transistors with realistic contact geometry. The solution reveals the previously underestimated role of near fields at the contacts, and provides clear recipes for maximizing photovoltage.

Thermal runaway is a positive-feedback phenomenon observed in many contexts, including fluid boiling and self-heating in semiconductors. Leveraging the insulator-metal phase transition of VO${}_2$, the authors characterize the dynamics of thermal runaway when heat is dissipated from an emitter through far-field thermal radiation. By minimizing the emitter’s thermal mass, they show that rapid thermal switching is achievable. Further enhancements may be possible using near-field radiation. This description of the switching dynamics associated with radiative thermal runaway may enable the design of advanced infrared sensors, thermal diodes, and calorimeters.

Low output power and limited spectral purity are two major challenges in developing spin-transfer nano-oscillators (STNOs) for applications in spintronics. The authors demonstrate that a combination of delayed magnetic field feedback and injection locking can be used to overcome both challenges, for significantly improved signal power and quality. Furthermore, with increasing feedback strength a phase is found in which oscillations are stabilized by the feedback itself, showing that a network of strongly coupled STNOs is feasible. By controlling the phase of oscillation, one can reduce the linewidth, and also can synchronize many oscillators.

Strong acousto-optical interaction can be a basis for efficient microwave-to-optical signal transduction (of interest in $e.g.$ quantum information processing), which is currently hindered by the weak intrinsic coupling strength. This study investigates the coupling between a surface acoustic wave (SAW) and the field inside an optical resonator, which leads to selective enhancement of the anti-Stokes scattering process. A well-known focusing technique is used to confine the SAW within a small region, for further improvement of the coupling strength. The polarization-dependent acousto-optical coupling identified in this study might also offer additional functionalities to a transducer.

Every computation technology dissipates heat while performing logic operations; the storage and conversion of such power for independent purposes is the main goal of energy harvesting. This study proposes a logic architecture, based on two innovative quantum systems (an ultraefficient thermoelectric element and a nanoscale heat valve), that reconverts the heat generated by any source into classic logic operations. This structure provides a functionally complete logic system, with operating frequencies up to GHz levels. Finally, this thermal logic scheme could be used to design hybrid thermal-electrical computing systems of higher total energy efficiency.

Dynamic switching of magnetization driven by subnanosecond voltage pulses would be a central technology for realizing ultralow-power magnetic random-access memory. For practical applications, however, there is a strong demand to reduce the write-error rate. This study shows that transitions of magnetization between different precession orbits, as induced by thermal fluctuations, play a dominant role in the probability of write error. The transitions can be avoided by choosing a proper width for the write voltage pulse. These findings will promote the development of voltage-driven spintronic devices.

Highly doped multiple quantum wells (MQWs) are significant for infrared optoelectronics, plasmonics, and the physics of strong light-matter coupling. The authors reveal a multisubband plasmon (MSP) that arises from the couplings among several intersubband transitions in (Mg,Zn)O/ZnO MQWs, due to the outstandingly dense two-dimensional electron gas. Here the MSP energy is up to three times that of the constituent intersubband transitions, illustrating the potential of this system for optoelectronic applications in the infrared, which could be extended to the terahertz range by appropriate quantum-well design.

Nonlinearity is a powerful tool for tuning the response of an elastic metamaterial and achieving wave-manipulation capabilities beyond those in the linear regime. This study explores nonlinear energy tunneling between wave modes in metamaterial waveguides. For large enough amplitudes of excitation, it is possible to generate conspicuous high-frequency wave packets dominated by axial cell-deformation mechanisms, even when the excitation is purely flexural. The most remarkable effect of this intermodal tunneling is the possibility to localize and trap the energy of long-wavelength excitations, for energy harvesting with subwavelength structures.

Fluctuations are an important spectroscopic tool for characterizing the single-electron sources relevant to metrology and quantum electron optics. The authors theoretically propose an experimental method to measure mesoscopic and nanoscale transport properties through macroscopic fluctuations. This proposal puts forward the solution to the open question of how to experimentally access energy-current fluctuations. The research team explains the physical mechanism behind the proposed setup, analyzes its feasibility, and provides guidelines for its optimization.

Understanding the microscopic origins of magneto-optoelectronic effects in organic semiconductors (as in the magnetoresistance of an OLED) is crucial to their technological application in magnetometers and other sensors. Here the magnetoresistance of a common conjugated polymer at low temperature and high magnetic field is studied, and contrasted with insight gained from magnetic resonance spectroscopy of charge-carrier pairs under identical conditions. These results imply that the high-field magnetoconductance is dominated by spin-orbit coupling, even though this compound contains only low-$Z$ elements.

Understanding the crystal structure, phase stability, and elasticity of magnetite (Fe${}_3$O${}_4$) at high pressure and temperature will explain the behavior of the Fe-O system deep within the Earth, and promote applications and engineering of Fe-O under extreme conditions. Using ultrasonic interferometry plus $in$ $situ$ x-ray techniques, the authors discover a temperature-driven anomaly in magnetite’s shear behavior and a pressure-induced softening of its shear modulus, due to local atomic distortions and charge ordering of Fe cations at the octahedral sites in the inverse-spinel structure. This work will interest researchers in physics, chemistry, materials science, and geoscience.

A great obstacle to technology using “forests” of highly ordered, vertically aligned carbon nanotubes (CNTs) is that collective properties of forests are inferior to those of individually grown freestanding CNTs. The pattern of changing growth rates seen in real-time measurements reveals that the carbon incorporation driving forest growth is governed by autocatalytic radical-chain polymerization of acetylene. Considering CNTs as a type of polymer renders the established methods of polymer science relevant for optimizing macroscopic platforms to retain the superior properties of individual CNTs in high-density energy storage, rapid heat dissipation, and advanced interconnects.

In spin-based hybrid quantum systems, the spatial modes of collective spin excitations can be used to encode a register of qubits. However, in superconducting circuits the wavelengths of microwave photons are much larger than the dimensions of the spin ensembles, which complicates direct coupling and limits information processing. This problem could be solved by exploiting the subwavelength nature of surface phonon-polaritons in a piezomagnetic superlattice, via coupling them to an ensemble of N-$V$ spins in diamond. Considering the excellent tenability and scalability of piezoactive superlattices, this approach opens routes to innovative hybrid quantum devices.

The authors propose a strategy to integrate $hierarchy$ into the design of elastic metamaterials and produce lightweight solutions for multiscale frequency attenuation, with applications ranging from nondestructive testing to seismic protection. This study shows the benefits of hierarchical structure in porous, viscoelastic single-phase materials, demonstrating efficient multifrequency attenuation—including low frequencies—while reducing structural weight. The results can be applied over a wide range of length scales, providing engineering solutions to vibration-damping problems.

Magnetization switching using spin-orbit torque (SOT) is an interesting candidate for future MRAM applications, and several studies have analyzed its performance versus standard technology based on spin-transfer torque (STT). However, progress in small devices is impeded mostly by challenges in making small interconnects and measuring tiny Hall voltages. This study uses conductive atomic force microscopy to measure tunnel magnetoresistance and demonstrate deterministic SOT switching in thermally stable 20-nm structures. Estimates based on measured switching currents suggest that SOT switching is more energy efficient than conventional STT technology.

Could antineutrinos help to verify compliance with a ban on nuclear weapons testing, or provide useful information about illicit tests? Surprisingly, detectors proposed for basic physics may be capable of detecting antineutrinos from faraway nuclear tests. Seismic data is key to the analysis. This study explores how large, water-based antineutrino detectors could rapidly confirm the nuclear nature of an explosion, constrain estimates of the fission yield, and provide other forensic insight.

The authors discuss the possible implementation of a single-photon detector based on a superconducting quantum interference device, or SQUID. The SQUID is initialized in an unstable state through the magnetic flux piercing it. A photon absorbed in a proximized normal-metal link induces a change in the critical current of the link, and a voltage pulse at the extremes of the device. Pulses of several tens of microvolts could result. Such a device allows the discrimination of a vast range of photon frequencies and is robust against thermal noise, making it a valuable candidate for detecting terahertz photons in particular.

Device-to-device variations in contact resistance currently hinder large-scale production of graphene-based electronics, making metal-graphene edge contacts an important research topic. The authors use density functional theory plus an advanced transport formalism to investigate the physical mechanisms behind the observed resistance variations. Contact quality depends intricately on the particular combination of metal and contaminant atoms present at the edge of the graphene flake. These results should help to refine fabrication processes for tomorrow’s electronic components.

Soft materials are $viscoelastic$ (exhibit time-dependent strain), and their interaction with the tip of an atomic force microscope (AFM) is capillary in nature. These facts are ignored in traditional nanomechanical analysis, where a tip indenting a purely $elastic$ solid is modeled. The authors present an alternative model that accounts for interfacial forces and the viscoelastic response of the moving interface. This rheological view of tip-surface force in dynamic AFM explains the relative magnitude, shape, and hysteresis of the measured amplitude dependence of both conservative and dissipative forces, in a significant refinement of a powerful measurement technique.

Metamaterials composed of carefully designed “meta-atoms” enable nearly arbitrary control of waves, but, particularly for electromagnetic waves at optical wavelengths, losses due to unwanted scattering and absorption within the meta-atoms can spoil performance. Compensation of losses in optical metamaterials and their devices is critical for broader impact in the field. This study describes a physical implementation of plasmon-injection loss compensation in the presence of noise, as an ideal means to overcome the limitations of conventional optical gain media. The most immediate implications of these results are for enhanced superresolution imaging and photolithography.

In the pursuit of practical quantum information processing, $quantum$ $annealing$ was devised as a quantum enhanced optimizer that aims to efficiently solve Ising problems. Various architectures using superconducting qubits have been proposed, but existing annealers are currently limited by qubit coherence time. The authors propose flexible, scalable hardware for implementing a quantum annealer, combining the long coherence times of three-dimensional circuit quantum electrodynamics with the recently proposed resonator-based Lechner-Hauke-Zoller scheme. This provides a promising physical platform to realize an annealer with improved coherence and noise resilience.

Modal diversity is an important advance in free-space optics, where propagation distances are limited by effects such as atmospheric turbulence. This experimental demonstration shows that orthogonal, coaxially propagating spatial modes of light from different bases are able to mitigate the effects of atmospheric turbulence. The different modes experience different perturbations, even when traveling along the same path, which removes the conventional need for physical separation of the transmitters, while also showing a significant improvement in bit-error rate. This approach could be applicable to a wide range of physical systems that are adversely affected by random aberrations.

Li${}_2$TiO${}_3$ is the most promising breeder material for a fusion reactor based on the lithium-deuterium nuclear reaction. Unfortunately, the kinetics of tritium release from this oxide is poorly understood, both experimentally and theoretically. This study uses density functional theory to analyze locally stable crystal sites and various diffusion paths for interstitial T${}^{+}$ in Li${}_2$TiO${}_3$. Strongly anisotropic diffusion of tritium along different crystalline directions is revealed. The results provide a first-principles theoretical guide for experiments, as well as key energetic parameters for further kinetic modeling and simulations.

Phosphorene is a promising channel material for next-generation electronics, due to its unique in-plane anisotropy, large density of states near the valence-band maximum, and high carrier mobility. Using $ab$ $initio$ quantum transport simulations, the authors investigate very small, double-gated MOSFETs based on monolayer phosphorene. It is predicted that these transistors can fulfill industrial requirements for ON current, delay time, and power dissipation until the gate length is scaled down to 2 nm.

In magnetic data storage, information can be corrupted by external magnetic fields or thermal fluctuations. A possible solution to both problems is reversibly altering the $magnitude$, rather than direction, of a bit’s magnetization. The authors propose such an alternative memory, based on chemical-order–disorder transformations in thin-film Fe-Al alloys where cycling (writing–erasing–writing) of disorder-induced ferromagnetism is found to occur under short-pulse heat treatments, e.g. laser irradiation, employed in information recording and storage technologies.

High-aperture-efficiency antennas are very desirable for applications such as satellite communications and automotive radars, as they can provide the narrowest beams at the smallest footprint. However, typical high-aperture-efficiency antennas are cumbersome to design, costly to fabricate, and do not exhibit a low profile. This paper proposes to engineer the resonances of simple 2D transmission-line grids, based on photonic crystal concepts, to achieve high-aperture-efficiency radiation. It was found that these grids can be realized with extremely low profiles, and they require the smallest possible real estate for a given beam size. This design can be a desirable compact antenna solution for the aforementioned applications of satellite communications and automotive radars, among a few others.

Phase estimation is a central goal of optical interferometry, with applications ranging from gravitational-wave detection to fiber-optic sensing. Recent progress has focused on the use of nonclassical light to improve performance, but that approach is often unreasonably complicated. The authors reexamine the simple, passive Mach-Zehnder interferometer by linearizing the phase-estimation problem, for straightforward computation of an efficient estimator, which is then used to compare several detection schemes for two- and four-output interferometers. Independent monitoring of all available output ports leads to the best overall sensitivity, and allows cancellation of amplitude noise.

Ce-doped ABO${}_3$ perovskites form an important class of scintillating materials, the performance of which intricately depends on a number of interrelated factors, such as host chemistry, synthesis conditions, Ce substitutional site, and charge state. Here, by employing state-of-the-art first-principles computations, the authors elucidate that, while Ce${}^{3+}$ or Ce${}^{4+}$ defects can be thermodynamically stable, depending on the choice of the substitutional site and synthesis conditions, only the Ce${}^{3+}$ dopant at the A site exhibits an electronic structure that can support scintillation. The present findings not only provide insights into past experimental observations for perovskites, but also are expected to be general and thus transferable to other chemistries.

Implementation of femtosecond fiber lasers has revolutionized the field of optical frequency combs. Even though Yb-fiber combs have a much more convenient wavelength range, the field is dominated by Er-fiber combs, due to the difficulty of satisfying multiple constraints at 1 μm. This study overcomes the difficulty by a design of the mode-locking dynamics that succeeds in generating energetic pulses 33 fs long, without using higher-order dispersion compensation, and while ensuring that the laser operates at a net cavity dispersion of zero, for low-noise supercontinuum generation.

We live in an age of designer materials, where device structure and material heterostructure become one. For example, inserting graphene into a van der Waals multilayer may open opportunities to design high-performance nanodevices. This theoretical study of the electronic structures and properties of MoS${}_2$/graphene/W$X_2$ ($X$ = S, Se) heterostructures reveal their high thermal stability, staggered band alignments, and intact Dirac cones. In particular, the transport current in systems with graphene is two orders of magnitude higher than in those without. These results point the way to designing high-performance optoelectronic devices based on two-dimensional semiconductors.

For practical use of spin-orbit torque (SOT) in magnetic memory applications, the writing current density must be reduced, not only for low power consumption and CMOS compatibility, but also for enhanced endurance and reliability. The authors study SOT-driven ferromagnetic resonance in ferromagnetic bilayers of cobalt and permalloy, sandwiched between Pt and MgO. Here the Pt/ferromagnet interface chiefly sets the spin Hall angle, but magnetic dissipation is also influenced by the ferromagnet/MgO interface. The configuration Pt/Co/Py/MgO seems to require less writing energy than Pt/Py/Co/MgO in SOT-driven magnetic switching.

A necessary, but not sufficient, condition for parity-time ($PT$) symmetry to hold in an optical system is that spatially separated gain and loss must be exactly balanced. This study introduces generalized $PT$-symmetric optical structures that can have asymmetric and unbalanced gain-loss profiles, yet offer similar scattering properties and phase transitions as traditional $PT$-symmetric ones. The concept of general $PT$ symmetry may help to reduce the threshold gain in recently discovered $PT$-enabled applications, such as the coherent-perfect-absorber laser and exceptional-point dynamics, and will facilitate optical and photonic devices by offering greater design freedom.

The Co-Fe-B/MgO system with perpendicular magnetic anisotropy finds extensive application in modern magnetic memories. Critical issues for the practical usage of magnetic tunnel junctions (MTJs) include the limited thermal stability of bit storage, and low switching efficiency. This study elucidates the deterministic influence of Co-Fe-B composition and the MgO interface on an MTJ’s fundamental magnetic properties, which affect both thermal stability and switching current. Furthermore, the authors observe an anomalous trend in saturation magnetization, which is attributed to a change in magnetic anisotropy.

Basic elements for two-dimensional photonics include the beam splitter, launcher, reflector, polarization rotator, and the photonic single-pole double-throw switch. This study uses one-dimensional grooves inscribed on a dielectric multilayer to manipulate the polarization state and propagation path of a Bloch surface wave. Most of the basic elements above can be realized with such grooves, which are accessible, controllable, and easily produced, so this approach is bound to facilitate integrated optics for $e.g.$ lab-on-a-chip applications, or optical computing.

Nondegenerate two-photon absorption is important for integrated, broadband nonlinear optics, and has been exploited for $e.g.$ sensitive midinfrared photodetectors of direct-band-gap semiconductors. However, little is known about nonlinear absorption in semiconductors with $indirect$ band gaps, such as silicon. This study uses the nonlinear interaction between short pulses to probe two-photon absorption and phase modulation in silicon-wire waveguides. While dispersion cannot be neglected, nonlinear phase modulation surprisingly may not change much over a very wide wavelength range. This could impact supercontinuum and frequency-comb generation, or all-optical signal manipulation.

Nonlinear thermal transport could provide an avenue to controlling heat flow, and a basis for innovative thermal devices. This study offers a theoretical prescription for realizing thermal rectification via heterojunctions of phase-change materials, a long-sought result in thermal physics. This general theory handles the complex heat flow arising at such heterojunctions, and indicates that a diode comprised of known phase-change materials should exhibit a thermal rectification ratio surpassing that of any individual material by a factor of 10.

Antiferromagnets attract keen interest for use in spin valves, magnetic tunnel junctions, and high-density data storage. There are difficulties, though: Bulk antiferromagnets need very large external fields to induce a spin-flop transition, and synthetic antiferromagnets made of multilayers of magnetic metals or alloys are tough to integrate with functional oxides. By constructing a superlattice of two oxides, this study realizes an all-perovskite synthetic antiferromagnet with high Curie temperature, interlayer exchange coupling, and layer-resolved magnetic switching at modest magnetic fields. These results could carry wide implications for next-generation magnetic devices.

Manipulating microparticles is important to various processes, from large-scale synthesis to small-scale applications in lab-on-a-chip systems for analysis and sensing. This work employs low-frequency capillary waves to continuously focus microparticles in a millmeter-scale channel. The technique is scalable, simple to set up and operate, and capable of collecting particles at flow rates of up to 4 liters/hour. This approach is a bridge between microscale collection methods, such as dielectrophoresis and inertial microfluidics, and macroscale methods, such as centrifugation and sedimentation.

High-mobility transistors with two-dimensional channels support long-lived plasmons, and thus can act as resonant spectrometers or detectors of terahertz radiation. A major obstacle to developing such devices is the complexity of photoresponse modeling, which requires time-consuming electromagnetic simulations. This paper presents an exact solution for electric fields and responsivity in plasmonic transistors with realistic contact geometry. The solution reveals the previously underestimated role of near fields at the contacts, and provides clear recipes for maximizing photovoltage.

Transient photovoltage measurements are widely used to characterize organic solar cells, but the validity of this method has been established only in a limited range of cases. The authors simulate these measurements over a broad range of conditions, and offer a generalized method for assessing validity. The approach that they propose significantly extends the range of valid measurement conditions, which ought to be quite useful in the drive toward clean, cheap power.

A Raman laser based on a high-$Q$ nanocavity in silicon has potential as a light source in photonic integrated circuits, but it is unclear how the significant enhancement of nonlinear optical effects by the cavity affects the lasing dynamics. Spectrally resolved time-domain measurements reveal that free carriers generated by two-photon absorption induce various dynamical effects during the initial lasing process, even at very low threshold power. Surprisingly, the Raman laser signal exhibits large oscillations at high excitation power. This insight will be useful for incorporating such Raman nanocavity lasers into device designs.

Artificial neural network (ANNs) are of great interest for applications such as recognition and scene detection. Traditional ANNs consist of software; this work proposes hardware for an ANN based on spin-torque nano-oscillators. By tuning two exchange interactions, $e.g.$ the Heisenberg and Dzyaloshinskii-Moriya interactions, the relative phase of the oscillators can be controlled in an external oscillating magnetic field. The suitability of these components for ANNs is simulated for a standard recognition task. This simple hardware could combine the oscillatory activity of biological neurons with the perceptron model, which is the workhorse of present-day ANNs.

Single crystals of the high-temperature superconductor Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$ are promising terahertz emitters, but for operating temperatures above 70 K they either show no emission or radiate well below 500 GHz, limiting their applicability. To improve their emission properties, especially above liquid-nitrogen temperature, a standalone stack of overdoped material with high critical temperature and critical current density is embedded in a sandwich structure, for efficient heat removal. Emission of 0.577-THz light is achieved at 80 K, and the experimental results are compared to simulations based on three-dimensional coupled sine-Gordon equations plus heat-diffusion equations.

The magnetic damping factor $\alpha$ (and particularly its upper limit) is the key parameter in the spin dynamics of a magnetic structure. The authors’ calculations predict the upper limit of $\alpha$ to be on the order of 1 in an ultrathin Co${}_2$MnSi-based spin valve, due to the huge angular dependence of spin-dependent scattering that is related to an interfacial resonant state. Giant $\alpha$ is related to the half-metallicity of this material, which is sensitive to point defects, interfacial disorder, spin disorder, and thermal lattice disorder. These results are quite interesting for the design of spintronic devices.

Multiferroics and two-dimensional materials with concurrent ferroic and topological order are important research topics, in part for promising applications in high-density nonvolatile memory. Here first-principles calculations show coexisting ferroelectric and ferromagnetic order in recently synthesized CH${}_2$OCH${}_3$-functionalized germanene, as well as coexisting ferroelectric and topological order in ligand-functionalized arsenene. A feasible approach to a ferroelectric switch is proposed. These findings shine a light on avenues to realizing exotic properties of matter, for innovative memory or logic devices.

Polar molecules (PMs) are ubiquitous and serve many important functions in living organisms, and abnormal PM metabolism is often associated with disease onset. The authors show that PMs other than water contribute substantially to microwave absorption by blood. This phenomenon is important for thermoacoustic functional imaging in a biomedical context, which has been held back by a lack of understanding of the relevant physics. This work establishes an $in$ $vivo$ noninvasive functional imaging system, for real-time monitoring of concentration changes of polar molecules and imaging of blood vessels in a large field of view.

Harvesting flexural energy has emerged as a viable means for driving low-power electronic devices. This study presents an optimized phononic gradient refractive index (GRIN) lens, which has important applications in focusing and harvesting elastic waves in fields such as structural health monitoring and wearable electronics. Also presented is a general framework for optimizing GRIN lenses, using multiple-scattering computations and ray-tracing analysis. Experimental results for an actual, optimized lens exhibit significant increase in focused power, with significant shortening of focal distance.

High-precision optical frequency transfer through free-space links will enable advances in fields ranging from coherent optical communication and satellite Doppler ranging, to tests of general relativity and fundamental physics. Previous work has focused on two-way time and frequency transfer techniques using optical frequency combs. Here the authors show that a continuous-wave coherent link can exceed the fractional frequency stability of frequency-comb methods by an order of magnitude. They also highlight and discuss the technical hurdles to implementing similar links over the distances required to reach low Earth orbit.

Despite the fact that metastable $\alpha$-Ga${}_2$O${}_3$ becomes increasingly interesting for applications in $e.g.$ high-power electronics, its basic material properties remain unclear. The authors employ plasma-assisted molecular-beam epitaxy to grow $r$-plane-oriented thin films of this birefringent semiconductor, as that crystal orientation allows the measurement of its anisotropic optical properties. The firm knowledge of these properties allows device design to proceed, including band-gap engineering by alloying with $\alpha$-In${}_2$O${}_3$ and $\alpha$-Al${}_2$O${}_3$.

Enhancing the electrocaloric effect (ECE), by which a solid’s temperature changes in an external electric field, is important for advancing solid-state cooling. However, little is known about the impact of $internal$ bias fields, which exist widely in ferroelectrics with defects or dopants, and composites, gradients, and thin films. Here researchers reveal that internal fields can tailor and enhance the ECE, and that even an inverse ECE exists. Through analytical modeling and $ab$ $initio$ molecular dynamics they find that, due to a shift in hysteresis, negative work loss appears in part of the caloric cycle. This insight will benefit a thermodynamic approach to device design.

Determining the level of uranium enrichment is often essential in nuclear nonproliferation and safeguarding. Conventional nondestructive techniques for determining enrichment depend on measuring the ratio of $\gamma$-rays emitted by uranium isotopes, but this requires prior knowledge—or assumptions—about sample and shielding. This study presents an alternative method that exploits the differences in the characteristic profile of emission over time for delayed neutrons from fission. The method uses a custom scintillation detector and requires no calibration standards. A fully engineered system based upon these principles could improve our capability for isotopic measurements.

They say no two are alike… Inversion-symmetric fluorescent color centers in diamonds, such as the germanium vacancy, are desirable for solid-state single-photon emitters in integrated quantum systems, but their complex mesoscopic environments make it challenging to obtain multiple $identical$ emitters. These experiments use the large strain gradient in a diamond microcantilever to align the spectra of two germanium vacancies within a 50-nm spot. This approach makes it possible to obtain identical single-photon emitters in diamond.

Understanding the structural origin of the magnetic proximity effect (MPE) at the interface of Pt and Y${}_3$Fe${}_5$O${}_{12}$ (YIG) is critical to developing YIG-based spintronic devices. This work reveals that the MPE mechanism at this interface is due to band hybridization and exchange coupling between tetrahedral Fe${}^{3+}$ ions and Pt atoms. Controlled substitution of nonmagnetic Al${}^{3+}$ for Fe${}^{3+}$ in YIG’s tetrahedral lattice is the key to these experiments. MPE depends linearly on the concentration of tetrahedral Fe${}^{3+}$ in YIG and plays the main part in the anomalous Hall effect at lower temperatures, while at higher temperatures the spin Hall effect is dominant.

A common task in quantum information science is to embed classical data into entangled multiqubit states. The author offers a practical embedding circuit for a chain geometry that is especially well suited for first-generation superconducting architectures. Measuring a variety of fidelity and information-theoretic quantities provides a detailed, quantitative assessment of actual performance using the IBM Quantum Experience ibmqx5 device. To check the expressiveness of this general-purpose circuit, the author also measures its ability to generate Haar random unitaries and quantum chaos, as reckoned by Porter-Thomas statistics and out-of-time-order correlation functions.

High-resolution imaging at long working distances is very important in optical microscopy, providing great freedom in sample and imaging conditions. However, this is challenging for a conventional refraction-based optical lens, due to optical aberrations. The authors propose a microscopic technique based on multiple scattering of light instead of refractive optics, where the complex amplitude of light is retrieved using a scattering layer. This approach will impact engineering solutions for high-resolution long-working-distance imaging in optical microscopy, and can be extended to holography for $e.g.$ particle tracking or cell pathology.

The room-temperature broadband photoresponse of silicon in the infrared region is of great interest for on-chip photonic platforms, but is fundamentally limited to the near infrared, due to the particular value of the band gap. The authors combine ion implantation with pulsed laser melting in a CMOS-compatible approach to introducing Te dopant into the Si crystal, at concentrations orders of magnitude above the solid solubility limit. This leads to the formation of an intermediate band in the upper half of silicon’s band gap, extending the photoresponse of Te-hyperdoped $p-n$ photodiodes to the midinfrared range.