Forget inventory control in big-box stores—what if we had RFID chips to monitor $individual$ $cells$ in a living body? A team of researchers has started us on this road, by creating radio-frequency devices just 22 µm across, which can be naturally taken up by live cells. Despite their size, these devices emit strong signals and can be sensed wirelessly by their corresponding transceivers. Their potential for continuous tracking of intracellular activities is quite significant for research and practice in biology and medicine.

The semiconductor Zn${}_3$N${}_2$, composed of inexpensive, abundant, nontoxic elements, shows a very high electron mobility and is quite appealing for many applications. However, real-world samples remain puzzling, with reported band gaps ranging from 0.85 to 3.2 eV. Using advanced first-principles calculations, the authors solve this puzzle: Gaps of up to 2 eV are mainly due to either hydrogen interstitials or oxygen substitution at nitrogen sites, while larger values seem to be due to flawed interpretation of experimental data. This insight is key to the development of devices based on this interesting, but tricky, compound.

Energy-efficient lighting is an important goal, but what is the ultimate limit that we can expect from our technology, and how close are we? Thermodynamic analyses of LED efficiency have been discussed in the reversible case. Incorporating the concept of passive optical extraction, the authors propose an $irreversible$ model for LED operation, for a more realistic view of theoretical efficiency. Even so, the maximum wall-plug efficiency is $unbounded$ as emission intensity diminishes, and in the range useful for indoor lighting, output can significantly exceed the electrical input power, as the LED cools the room while it shines.

Controlling magnetically polar domains with an electric field, rather than a magnetic field, is a promising route to the next generation of fast, energy-efficient devices for data storage. However, electrical switching of magnetic polarity has been directly observed in bulk single crystals of just a few multiferroics, at low temperatures. Through cutting-edge fabrication and synchrotron x-ray techniques, the authors demonstrate complete reversal of magnetic polarity by an applied electric field in a BiFeO${}_3$-based device, $at$ $room$ $temperature$. This is a key result in the quest for practical applications of this class of materials.

In electronics, circuit theory has been tremendously successful in translating physical equations into circuit elements in an organized manner, for further analysis and the proposing of creative designs for applications. Considering that spintronics is seen as a replacement for electronics, it might similarly benefit from such a framework. The author develops the spin-circuit representation of spin pumping and verifies that it reproduces literature results, and furthermore shows how complex multilayers can be analyzed. Circuits can be simply solved analytically by hand, or programmatically for complex devices.

Flexible electronics offer exciting prospects for devices that could be rolled up, folded, or worn. Conventional methods for printing electronics on plastic films, such as gravure or inkjet techniques, use special inks made of organic semiconductors in environmentally unfriendly solvents. The authors demonstrate that thin-film transistors can be fabricated by the principle of laser printing, based on toner rather than solvent, in a digital process akin to that of 3D printing. As a next step, the authors envision all-toner printing of not just the semiconductor, but also metal electrodes and insulator layers.

What makes some double-helix structures so rigid? Both inspiration and insight may be found in a rather mysterious wooden staircase in Santa Fe, New Mexico. The authors study the elastic behavior of an unsupported helical staircase using continuum elasticity theory, and identify conditions under which it becomes very rigid. The understanding obtained for the macroscale structure extends down to artificial micro- and nanostructures.

“Smart” gels that respond to external stimuli are interesting and useful in soft microfluidic valves, or for drug delivery, but measuring their mechanical properties can be tricky. For temperature-sensitive responsive gels, the authors exploit the thermomechanical connection to probe elastic properties via differential scanning calorimetry. This approach could be particularly valuable for studying systems with very small suspended particles (microgels), or oddly shaped ones, to which standard methods are not readily applicable.

Although quantum annealing has received considerable interest as a potential computing paradigm, it has not met the key criterion of improved scaling over classical methods. Qubit coherence is currently a major limitation. Starting from a detailed theoretical understanding of the system, the authors demonstrate tunably coupled flux qubits with a circuit geometry similar to those used in existing annealers, but with coherence times that are roughly two orders of magnitude longer. Their design concepts are not limited to superconducting qubits, and could be generalized to other types being considered for quantum annealing.

Transformation optics is a powerful tool for designing devices to manipulate light waves. Traditionally, device functions designed with this approach rely on a specific manipulation of wave $paths$ through a coordinate transformation. This study instead analyzes how the wave $vector$ changes under certain coordinate transformations, and therefore which transformation should be chosen to manipulate the wave vector as desired. The results are used to design flat lenses that can achieve perfect convergence of emergent rays.

Steering waves is slick, but what if one wants to change the waveguide to achieve a different response? The authors propose on-demand guiding of acoustic waves in a solid phononic crystal with holes, by filling a selected sequence of holes with a fluid. Waves are conducted by the coupling of adjacent resonating fluid cavities, while propagation in the solid is completely forbidden. Arbitrary phononic circuits could be reconfigured at will, simply by filling or emptying the chosen holes.

Germanium is eyed as a channel material in next-generation CMOS transistors. By developing high-quality contacts between Ge and a highly spin-polarized Heusler alloy in a spin valve, the authors demonstrate electrical spin injection, transport, manipulation, and detection up to 250 K, with pure spin-current transport observable at $roomtemperature$. The spin-relaxation mechanism in Ge near room temperature is found to be dominated by a phonon-induced intervalley spin-flip process.

Waste not, want not: In considerations of efficiency, the recovery of energy dissipated as heat addresses a large chunk of present-day production. Here we often think of thermoelectric modules, but the author points out that a different approach could be better. This comprehensive analysis of solid-state thermionic generators includes analytic calculations of their efficiency and power in the nonlinear regime, and provides guidelines for designing such devices. In contrast to earlier studies, it shows that these thermal-to-electrical energy converters could be more efficient than state-of-the-art thermoelectric modules.

Coatings much thinner than the wavelength of incident light not only suppress reflection from the substrate, but also can absorb almost all of the light, if the optical properties of film and substrate are perfectly matched. This research offers a general strategy to find all suitable film-substrate combinations, and demonstrates that low-loss, epsilon-near-zero substrates like (Al,Zn)O are a promising platform for wavelength-tunable ultrathin-film perfect absorbers, which could serve as alternatives to plasmonic metasurfaces in the near infrared, for example.

They say that life imitates art, but in physics, imitating real life is the art of computational work. For example, simulating a melt-quenched semiconductor to study its defects must yield realistic structures, if it is to have true predictive power. The authors develop a procedure to identify semiconductor defects in an unbiased way, using $ab$ $initio$ molecular dynamics to produce structures at chosen, fixed Fermi levels within the band gap, and to reveal the associated charge-trapping mechanisms. Especially where physical intuition has failed, this scheme could be invaluable in helping experimentalists recognize and neutralize performance-degrading defects.

Want a better motor in your electric car, or windmill generator? Know your magnets. In a permanent magnet, local strain near grain-boundary interfaces plays a crucial role in determining the effective coercivity. The authors use both first-principles and micromagnetic calculations to examine strain effects in Nd-Fe-B, and find that in-plane shrinkage reduces its magnetocrystalline anisotropy. These results should stimulate interest in terms of tailoring microstructures, understanding the vast differences between molecular dynamics simulations, and spurring high-resolution experiments on local strain.

When developing a metasurface, researchers commonly consider the subwavelength condition in real space, but this limits functionality. This study proposes four guidelines for designing an arbitrary metasurface in $momentum$ $space$. As illustrations, three lenses with exotic properties are detailed. This methodology offers a wide platform for engineering tailored multifunctional metasurfaces, thus broadening the application of artificial nanostructures in optical, acoustic, and mechanical metamaterials.

It would be terrific to use semiconductor quantum dots to produce entangled photons in a quantum communication network, but entanglement is complicated by the exciton spin splitting of typical dots. The authors present a growth strategy that improves the in-plane aspect ratio of $III-V$ quantum dots by 72%, greatly reducing the fine-structure splitting of exciton eigenstates that is the root of the problem. Their approach can be implemented with either molecular-beam or vapor-phase epitaxy, to yield dots that emit at telecommunication wavelengths and are easily incorporated into optical cavities.

Splicing optical fibers for good transmission is important, and can be tricky. This study presents a photonic device for launching light from a single-mode fiber to a hollow-core photonic-crystal fiber, based on inserting a glass-fiber “nanospike” into the hollow core. Optomechanics and the adiabatic evolution of the optical mode are exploited to achieve self-aligned, efficient light coupling in a compact, ready-to-use integrated device. This hardware would naturally see application in telecommunications, light-gas interaction and spectroscopy, and nonlinear and quantum optics.

The semiconductor Zn${}_3$N${}_2$, composed of inexpensive, abundant, nontoxic elements, shows a very high electron mobility and is quite appealing for many applications. However, real-world samples remain puzzling, with reported band gaps ranging from 0.85 to 3.2 eV. Using advanced first-principles calculations, the authors solve this puzzle: Gaps of up to 2 eV are mainly due to either hydrogen interstitials or oxygen substitution at nitrogen sites, while larger values seem to be due to flawed interpretation of experimental data. This insight is key to the development of devices based on this interesting, but tricky, compound.

The limit of precision in optical measurements is due to quantum fluctuations, which are governed by Poissonian statistics. With a clever setup, however, the authors achieve a precision in transmission measurements that beats this limit, without the selective analysis of recorded data (post-selection). This shows that the single-photon-detector approach, used by many groups worldwide, is viable for real-world optical quantum metrology, not just proof-of-principle tests. In turn, this demonstration unlocks an array of techniques for quantum-state engineering that crucially depend on single-photon detection.

Energy-efficient lighting is an important goal, but what is the ultimate limit that we can expect from our technology, and how close are we? Thermodynamic analyses of LED efficiency have been discussed in the reversible case. Incorporating the concept of passive optical extraction, the authors propose an $irreversible$ model for LED operation, for a more realistic view of theoretical efficiency. Even so, the maximum wall-plug efficiency is $unbounded$ as emission intensity diminishes, and in the range useful for indoor lighting, output can significantly exceed the electrical input power, as the LED cools the room while it shines.

Acoustic streaming (fluid flow due to acoustic absorption) is observed in many devices designed to manipulate cells and particles with ultrasound. The authors show that higher-order patterns exist in previously reported transducer-plane streaming, and investigate the conditions under which they occur. Interaction of both standing and traveling waves is required to generate the vorticity in acoustic intensity that underlies this phenomenon. This study illustrates how subtleties in the underlying physics can significantly affect the behavior of acoustofluidic devices typically used in bioengineering applications.

Controlling the optical activity of terahertz waves is crucial to unlocking applications in the region of the spectrum between infrared and microwave light. The authors show how a fundamental optical property, the Blaschke phase, can be manipulated to improve device performance. With a judicious choice of geometry for a nearly lossless chiral metamaterial, in which Blaschke-phase contributions dominate the optical response, one can obtain strong rotation of light’s circular polarization without sacrificing bandwidth.

In multiferroic heterostructures, regular ferroelectric-domain patterns can be imprinted in a continuous ferromagnetic film without any nanoscale patterning of its own. The authors demonstrate that confined and propagating spin waves are excited by a microwave-frequency magnetic field in such structures. The wavelength of the emitted spin waves is tunable via the rotation of an external bias field. Excitation and active tuning of short-wavelength spin waves is essential to miniaturizing magnonic and spintronic devices for signal processing and wireless communication, so the authors’ approach is most welcome.

Spintronics is not the only alternative to traditional electronics. The $fluxon$ (magnetic flux quantum) could be used as a vehicle for ultralow-energy data processing and storage, in fluxonic devices. First, though, we need to figure out how to create circuitry. The authors demonstrate a technique for fabricating submicrometer patterns over a wide area in a cuprate superconductor, where fluxons are carried by nanoscale magnetic vortices. Being able to create these “pinning centers” at will allows for trapping and manipulating vortices in a controlled fashion.

Spin pumping in ferromagnet/normal-metal bilayers is usually studied at resonance, but this study points out that spin current can be generated under $nonresonant$ conditions, due to the thermal effects of microwave heating as well as spin pumping. In turn, this suggests that thermal effects also should be addressed when quantifying the spin Hall angle at resonance. These findings can explain some unexpected observations in the spin-pumping literature, and are expected to facilitate spintronic devices that work in the absence of an external magnetic field.

Understanding the flow of suspensions is crucial to applications in geophysics, biology, and industry, for example. In such a system the fields describing velocity and concentration are coupled, so modeling and analysis requires simultaneous knowledge of both quantities. The authors use ultrafast ultrasound echography to quantify both fields in suspensions of particles 20–80 µm in size. Their technique is well suited for opaque flows, time scales down to 10 ms, and volume fractions up to 40%, and is easily adapted to existing apparatus in industrial and research settings.

Interlayer exchange coupling of two ferromagnetic layers through a nonmagnetic spacer is an integral part of many magnetic sensors and devices. The authors use broadband ferromagnetic resonance to investigate the oscillatory interlayer exchange coupling between two Co-Fe layers, as a function of Ru spacer thickness and of temperature. The results show that the temperature dependence of the coupling is governed by the thermal spin-wave model—important insight for engineering magnetic and spintronic components.

Nanoscale aluminum nitride waveguides transform near-infrared laser light into a broad rainbow of colors, from midinfrared to visible frequencies. The asymmetric crystal structure of AlN endows the material with both ${\chi }^{(2)}$ and ${\chi }^{(3)}$ nonlinearities and allows the same color of light to be generated via different nonlinear pathways, enabling stabilization of the frequency comb and a rich, complex output spectrum. This development is of significant practical importance, as basing such waveguides on a lithography-compatible material allows for customization and mass production, greatly reducing their cost and boosting access to them worldwide.

In silicon heterojunction solar cells, the differences in valence- and conduction-band energies of crystalline and hydrogenated amorphous silicon are key parameters that govern charge transport. This study calculates these band offsets with particular care, using models of the interfacial structure obtained from extensive first-principles molecular dynamics simulations. These results are important for accurate modeling and optimization of this class of energy-harvesting devices.

Optical clocks combined with frequency combs are the future of precision timekeeping, as well as a key enabling technology for applications in communication, navigation, relativistic geodesy, quantum physics, and fundamental measurements. Using a silicon nitride waveguide to spectrally broaden a telecom-frequency comb, the authors perform an optical-clock comparison. Additionally, they present measurements and analysis to support an “all-in-one” waveguide design for a next-generation clock network. Such devices will be very useful not just in metrology labs, but also in the field—including space missions.

Precise, absolute measurement of radio-frequency electric fields is an important task in metrology and testing applications. Starting from principles of quantum optics and atomic physics, the authors calibrate an rf electric field and measure its polarization with a robust, all-optical laser technique. Their method is about an order of magnitude more precise than traditional field-calibration tools, nicely filling a clear need in rf technology.

Fin-based acoustic structures that have been specifically designed for double-negative behavior ($i.e.$ both bulk modulus and mass density < 0), based on models with no losses, instead dissipate nearly all of the transmitted energy inside the metamaterial region. This comprehensive study demonstrates a fundamental limit, due to viscothermal losses, for some artificial structures with corrugations. Scaling up the structure does not solve the problem. On the plus side, this extraordinary property suggests these structures as an alternative to conventional absorbers, for low frequencies or narrowband excitation.

A thermophotovoltaic (TPV) device, composed of a photovoltaic cell that absorbs infrared photons from a hot emitter, converts heat directly to electricity. While the cell’s external luminescence is not affected by the radiation source in a solar cell, the authors explain how it $is$ impacted by the presence of the emitter in a TPV device subject to near-field radiation effects. Near-field enhancement of conversion performance is more significant in cells with higher nonradiative recombination rates, due to a larger amplification of the external luminescence efficiency. This study offers good insight for improving TPV waste-heat recovery systems.

Forget inventory control in big-box stores—what if we had RFID chips to monitor $individual$ $cells$ in a living body? A team of researchers has started us on this road, by creating radio-frequency devices just 22 µm across, which can be naturally taken up by live cells. Despite their size, these devices emit strong signals and can be sensed wirelessly by their corresponding transceivers. Their potential for continuous tracking of intracellular activities is quite significant for research and practice in biology and medicine.

(In,Ga)N nanostructures are keenly investigated for their potential as light-emitting diodes. The authors investigate the radiative and nonradiative recombination processes in both Ga-polar and N-polar structures, and observe that carrier recombination occurs between individually localized electrons and holes with varying spatial separation. This experimental side-by-side comparison of a state-of-the art planar structure and a nanowire, combined with reaction-diffusion simulations, enables the evaluation of polar, axial (In,Ga)N/GaN nanowires for practical use in solid-state lighting.

Controlling magnetically polar domains with an electric field, rather than a magnetic field, is a promising route to the next generation of fast, energy-efficient devices for data storage. However, electrical switching of magnetic polarity has been directly observed in bulk single crystals of just a few multiferroics, at low temperatures. Through cutting-edge fabrication and synchrotron x-ray techniques, the authors demonstrate complete reversal of magnetic polarity by an applied electric field in a BiFeO${}_3$-based device, $at$ $room$ $temperature$. This is a key result in the quest for practical applications of this class of materials.

Destinations are fine, but getting there is half the fun—and a map can be crucial. Case in point: Spin-torque oscillators (STOs) are the subject of much research to functionalize steady-state precession of magnetization. They exhibit rich dynamical behavior arising from interactions between multiple spin-wave modes. The authors map out the phase diagrams of a nanocontact STO and show how control parameters, such as external-field angle and temperature, drive the evolution of the system’s dynamics. This physical understanding provides guidance for controlling STO multimode dynamics in applications ranging from communication to computation.

Nobody likes micromanaging—never mind nanomanaging. Instead of complex processing, it is very handy to have nanostructures form on their own, according to physical law. This presumes that we know how to set them up to succeed, by engineering their circumstances. Systematic simulations elucidate the kinetics responsible for intriguing, symmetrical patterns of self-assembled islands. This insight enables processing strategies for complex nanopatterning by precise control of a $macroscopic$ force, with potential applications in optoelectronics and data storage.

Periodically patterned composite ferromagnetic nanostructures known as magnonic crystals (MCs) are promising for spintronic and other spin-based technologies, as they manipulate spin waves in analogy to photonic crystals’ manipulation of light. The authors create an array of asymmetric sawtooth waveguides that presents two different one-dimensional MCs in perpendicular directions, yet not a two-dimensional MC. Its magnonic band structure and gap are tuned by rotating the applied magnetic field, and its edge modes may also be controlled, to yield, say, a GHz-frequency nanoscale filter.

To bring magnetic refrigeration to market, superior magnetocaloric materials are still needed. In particular, it is important to tune alloy composition to promote abrupt, second-order magnetic phase transitions, for suitable cooling performance in a magnetic field that can vary as rapidly as 1000 T/s in a real device. Studying candidate quaternary alloys in very high pulsed magnetic fields, the authors find that magnetoelastic coupling determines the order of the transition, and that both candidates offer a large magnetocaloric response, which promises good performance under realistic conditions.

The spin-torque ferromagnetic resonance (ST-FMR) technique offers great potential for simplified measurement in rf spintronics. However, there has been little discussion of relevant heating effects due to dissipation from magnetization dynamics or anisotropic magnetoresistance. The authors show how to observe FMR heating through a sensitive mechanical thermometer. Monitoring magnetization dynamics independently via ST-FMR and this stress-induced mechanical resonance allows separation of any thermoelectric signal, and a refined outlook on thermal contributions to the spin Hall effect.

Quantum circuits based on superconducting nanowires have recently made an impact in the development of devices harnessing phase slip, superinductance, and Josephson effects. Exploiting this rich physics is difficult, though, due to fabrication-induced, parasitic two-level systems, which cause high levels of loss and decoherence. Using a focused beam of neon ions, the authors embed superconducting nanowires into coplanar waveguide resonators and realize much lower losses than in previous devices. This achievement suggests excellent prospects for this class of circuitry.