Guest Editors Morinobu Endo and David Tománek introduce a body of papers to memorialize a pillar of the community, in the context of her life’s work.

Wireless, implanted devices for biotelemetry, telemedicine, and neural interfacing are an emerging technology with powerful capabilities for medicine and clinical research, but are being held back by unreliable communication with external equipment. This study uses full-wave-problem formulations to study the mechanisms of electromagnetic propagation through tissue, and to derive optimal radiation conditions. Surprisingly, 80–99% of radiation efficiency is lost due to tissue-air impedance mismatch, not due to tissue absorption, as is commonly believed. Efficiency could be improved by an order of magnitude, compared to existing systems.

$Quantum$ $plasmonics$ is an emerging field with a wide range of applications in quantum information science. Despite significant progress so far, it is not known how decoherence affects quantum plasmonic systems. This experimental study shows that damping of either amplitude or phase can lead to decoherence in these systems, and provides important information for designing plasmonic waveguide systems for loss-tolerant and phase-sensitive applications, such as quantum sensing and imaging. The techniques developed here may be useful for studying decoherence in other plasmonic structures, too, including nanoantennas, unit cells in metamaterials, and nanotraps for cold atoms.

In studying the practical design of a quantum antenna with given spatial correlations, the authors show that the antenna’s initial quantum state is at least as important as the spatial current distributions. Applying their state-inference procedure to a simple antenna (a linear one-dimensional array of equidistant quantum dots, trapped atoms, or superconducting qubits), they synthesize the initial states to generate drastically different emitted fields, for co- and contradirectionally entangled photons, complete suppression in the far field, or a nearly homogeneous far-field distribution—pointing to a host of applications in quantum optics and photonics.

Ghost modalities are by no means “spooky actions”, but are metrology schemes exploiting intensity correlations of light, based on the fundamentals of quantum optics. The authors demonstrate ghost spectroscopy based on classical photon correlations in the wavelength domain of broad-band light emitted by an ultraminiaturized superluminescent semiconductor diode. Comprehensive proof of principle is provided by a real-world experiment on chloroform at 1214 nm. This demonstrated analogy between ghost imaging and ghost spectroscopy is expected to further fertilize the field, to promote a deeper understanding, and to lead even to other protocols in ghost metrology.

$Kirigami$, the extension of origami to include cutting as well as folding of a sheet, continues to gain attention as a design principle for functional materials. Using theoretical analysis, numerical simulations, and experiments, the authors apply kirigami patterns to enhance both the piezoelectric energy generation and stretchability of polyvinylidene difluoride (PVDF) films. Compared to everyday, cut-free PVDF films, kirigami films with patterned cuts can withstand much higher axial stretch, while maintaining the same level of voltage output. This approach enables, for example, tunable generators for integration into biomedical devices that are powered by the patient’s own movements.

“Bowtie” nanoantennas are not unusual in the field of plasmonics, yet they still can be used in unusual ways. Harnessing the near-field enhancement due to locally resonant surface plasmons, the authors demonstrate material ablation and concurrent deposition of high-quality film at the subwavelength scale, all driven by a bowtie-based oscillator, rather than a laser. This gentle ablation is mediated by hot-electron emission and electrostatic acceleration—a different mechanism than for thermal ablation or optical breakdown. This mechanism can be made to work for either metallic or dielectric materials, rendering this approach even more appealing for nanotechnology.

Unlike everyday applications on the ground, electronics in space are exposed to damaging radiation, and could misbehave unpredictably. This study employs atomistic molecular-dynamics simulations and many-body theory to analyze the effects of radiation-induced defects in semiconductors on the performance of satellite-borne optoelectronic devices. This technical approach, spanning initial material damage to final performance degradation, is important for space applications, as it allows the prediction of lifetime for space-based imaging technologies.

$Quantum$ $plasmonics$ is an emerging field with a wide range of applications in quantum information science. Despite significant progress so far, it is not known how decoherence affects quantum plasmonic systems. This experimental study shows that damping of either amplitude or phase can lead to decoherence in these systems, and provides important information for designing plasmonic waveguide systems for loss-tolerant and phase-sensitive applications, such as quantum sensing and imaging. The techniques developed here may be useful for studying decoherence in other plasmonic structures, too, including nanoantennas, unit cells in metamaterials, and nanotraps for cold atoms.

For intense femtosecond bursts of coherent light, one can look to electrons accelerated almost to the speed of light. In this context, the authors analyze the performance of electro-optical sampling crystals under the action of the near and far electromagnetic fields produced by relativistic electron bunches passing nearby. Different features are observed for a linac and a laser-plasma accelerator, due to the different beam characteristics in each case. This systematic study of the physical principles underlying the diagnostic technique is expected to interest researchers from accelerator physics and beam diagnostics to plasma physics and laser optics.

The current interest in materials supporting topological states calls for detailed electronic-structure characterization, via magnetometry in intense magnetic fields. A quartz tuning fork (QTF) could be an advantageous platform for this very challenging task, but the technique has not been widely studied. The authors demonstrate that a particular QTF configuration allows $torque$ $differential$ $magnetometry$, with good sensitivity. Observations of hysteresis in a ferromagnet and quantum oscillations in bismuth show that QTF-based magnetometry is promising for studying even small samples in high fields.

A lack of understanding of the quantum and even semiclassical interactions in two-dimensional electron gas (2DEG) systems continues to hold back many applications. This study of transport in high-electron-density semiconductor heterostructures reveals a complex interplay of mobility-limiting mechanisms. Scattering due to phonons is highly temperature-dependent, whereas charged impurities produce small-angle scattering that is important at low temperatures. Also observed is $weak$ $localization$, a counterintuitive effect of quantum interference. This insight will help in suppressing scattering and boosting the performance of GaN-based power electronics.

Efficient generation of harmonics in nonlinear optical resonators, particularly in whispering-gallery-mode (WGM) resonators, has led to successes such as parametric oscillators, narrow-band single-photon sources, and versatile frequency converters. A subtle problem, though, has strongly limited efficiency and hindered precise out-coupling of the generated light. Implementing their own prior theory and a polarization-selective birefringent coupling prism, the authors demonstrate an improvement of more than an order of magnitude in the amount of out-coupled light. The results are particularly interesting for applications in quantum optics relying on WGMs.

Manipulating the polarization state of an optical surface wave is interesting and important for next-generation information and biophotonics technologies. The authors theoretically show that the polarization of the two-dimensional Bloch surface wave (BSW) on a dielectric multilayer can be transformed between the transverse-electric (TE) and transverse-magnetic (TM) states, using the laterally continuous grooves inscribed on the multilayer. Due to this transformation, anomalous reflection of BSW beams can also be realized. These findings provide a different approach to tuning reflected beams, for opportunities in optical devices where metasurfaces are currently employed.

Molybdenum disulfide is important for a variety of future (opto)electronic devices, thanks to its atomically thin layered structure, good mobility, high current on/off ratio, and large optical absorption. However, stable and controllable realization of $p$-type MoS${}_2$, which is prerequisite for most applications, is a still major issue. Here mechanical strain is investigated to address the problem: Strain can enhance $p$ doping, and suppresses the formation of sulfur vacancies that could be potential hole compensators. This approach should be be of considerable interest to the large community studying transition-metal dichalcogenides to create tomorrow’s technology.

Modifying graphene is important for exploiting sulfides as excellent electrode materials in high-capacity Li-ion and Li-S batteries, but a physical, mechanistic picture is needed to interpret the enhancement of electrochemical properties induced by different dopants. The authors find Li storage with intercalation character in the Li${}_2$S—doped graphene system, and reveal two mechanisms to enhance interfacial lithium adsorption and diffusion, plus the synergistic effect of codoping. These calculations give a theoretical basis for materials engineering using doped graphene for advanced energy storage.

They say an $elephant$ never forgets…but your $dessert$? In this study, energetic electron beams are employed to induce the shape-memory effect in pure off-the-shelf gelatin, $i.e.$ unadulterated by undesirable additives. This is experimentally demonstrated by programming actuators to perform complex shape changes. Also presented is a semi-empirical force field that is capable of predicting the experimental observations in molecular-dynamics calculations. This combined experimental and theoretical framework opens the potential for large-scale use of this suddenly smart material, in biomedical applications and beyond.

Spintronics relies on generating, manipulating, and detecting spin current. In addition, converting waste heat into electricity is an essential topic for current and future technologies. To explore spin caloritronics, the authors propose a thermoelectric spin-current generator, consisting of a zigzag germanene nanoribbon with thermal leads partially exposed to noncollinear exchange fields. The spin polarization of the current in the leads can reach 100%, 0%, or even ∞ (pure spin current with no charge flow), but only under a temperature difference, and the giant magnetoresistance can reach extremely large values just by adjusting the relative directions of the exchange fields.

Controlling the state of polarization (SoP) of light could find use in many fields, such as materials processing, polarimetry, microscopy, and optical communication, but applications typically involve interaction with absorbing media, which limits a light beam’s range. Current methods for longitudinal control of SoP cannot overcome medium losses, and are either limited or not systematic in terms of SoP variations. The authors present a fully analytic, systematic methodology to engineer both the SoP and intensity of nondiffracting, attenuation-resistant beams in both lossless and absorbing media. This method is envisioned as an important advance for applications of structured light.

Following the lead of modern optics, $transformation$ $acoustics$ is a promising approach to manipulating sound waves, yet the complexity of material parameters for actual devices hinders progress. Simplified parameters are sorely needed, so that accessible materials can be used. This study uses Mie theory to show that concentrators with a simplified set of acoustic parameters from Fabry-Pérot resonances are perfectly inaudible, and that such a version could be realized using “holey” metamaterials. Going beyond traditional transformation acoustics, this theory is important for designing undetectable devices in three dimensions, for real-world acoustic cloaking.

In semiconductor-based quantum computing, gate operations beyond the two-qubit limit are important, but have remained extremely challenging. The authors realize gate-voltage control of interqubit couplings in a specially designed three-qubit quantum-dot device. They exercise coherent control of both amplitude and phase of a target qubit using the prepared states of two control qubits, and demonstrate the basic functionalities of the universal Toffoli gate. This initial effort to achieve controlled three-qubit operations provides useful insight for research on multiqubit systems in semiconductor devices.

For sensing temperature by measuring infrared radiation (in $e.g.$ night-vision goggles), most often bolometers are used, but there is another, possibly even better way. The authors show how to use nanoelectromechanical resonators as pixels for thermal sensing, including a complete electrical transduction chain, and the anomalous phase noise that ultimately limits the technique. A low-temperature process enables integrated-circuit fabrication, for dense and inexpensive imager production, and several suggestions for tenfold improvement of temperature sensitivity could allow these devices to surpass today’s detectors.

In electronics, cooling a device would seem to be as straightforward as putting a good thermal conductor on top. Graphene is one of the best thermal conductors and thus a natural choice, but realize that its thermal conductivity is a function of sample size $L$, and also strongly depends on temperature. Through extensive phonon-mode calculations, the authors show that graphene’s maximum thermal conductivity occurs around 100 K, and that $L$ of just 10 $\mu$m is sufficient for maximum effect. With these results, one can engineer a device with optimal $L$ for a required thermal conductivity at a given temperature.

Reverse-osmosis membranes made of composites of carbon nanotubes and aromatic polyamide have been exhaustively studied in recent years, due to their high salt rejection and permeability. However, there is still not complete agreement on the mechanism of diffusion of water through these interesting membranes. This study combines experiment and theory to propose a mechanism based on water hopping through the nanotubes dispersed within the membrane, and shows the great utility of multiscale simulations in this context. This understanding of water diffusion through nanocomposites is necessary for the optimization of next-generation desalination membranes.

Thermoelectric performance is expressed by the figure of merit $ZT$. However, as the electrical conductivity $\sigma$ in the numerator of the formula $ZT=S^2\sigma T/\kappa$ is related to the thermal conductivity $\kappa$ in the denominator, we still do not have a good guiding principle for obtaining a high $ZT$. The authors propose that $ZT$ is set by a universal function of a dimensionless parameter $\alpha$, which can be experimentally measured. The function reproduces most experimental results for most materials, whether they are one-, two-, or three-dimensional. From this we conclude that any semiconductor with $\alpha >4.5$ will show $ZT>2$, which could dramatically improve technology.

Given the importance of heat management in batteries and electronic circuitry, enhancing the efficiency of thermoelectric cooling materials could have a great technological impact. This research presents an approach for nanostructuring bismuth antimonide alloys, and yields an enhanced Seebeck coefficient for Bi${}_{1-x}$Sb${}_x$ nanocomposites. Thermopower clearly increases compared to that of bulk samples, even though the electron concentration is higher than in the bulk, which suggests that, consistent with theory, the increase results from size quantization. This result provides a promising strategy for improving the properties of this appealing thermoelectric alloy.

In studying the practical design of a quantum antenna with given spatial correlations, the authors show that the antenna’s initial quantum state is at least as important as the spatial current distributions. Applying their state-inference procedure to a simple antenna (a linear one-dimensional array of equidistant quantum dots, trapped atoms, or superconducting qubits), they synthesize the initial states to generate drastically different emitted fields, for co- and contradirectionally entangled photons, complete suppression in the far field, or a nearly homogeneous far-field distribution—pointing to a host of applications in quantum optics and photonics.

The massless surface states of topological insulators (TIs) are of great interest for tomorrow’s electronics. However, excessive defect doping of bulk bands significantly hinders the studies that would enable applications of TIs, in particular as efficient channel materials for high-frequency transistors. The authors take an important step toward resolving this problem, by realizing a Bi${}_2$Se${}_3$-based capacitive device operating at radio frequencies. Judicious choice of growth technique, substrate, and gate dielectric allow the depletion of bulk carriers and quantitative measurement of the rf quantum capacitance of the topological surface states.

Reducing the reliance on rare-earth elements for permanent magnets is seen as a key technological issue. Enhancing ferromagnetism in a material by adding a magnetic element seems intuitive, but this work reports a rare case where magnetism is enhanced by doping with a $nonmagnetic$ component. Starting from the Pauli paramagnet CeCo${}_3$, when 45% of Ce atoms are replaced by Mg, the compound becomes a rare-earth-lean ferromagnet that is promising for applications like motors for electric cars. This discovery illustrates that the Stoner mechanism should not be overlooked as a means to induce ferromagnetism by substituting nonmagnetic elements.

Epigenetic modifications (chemical changes to the four canonical DNA bases) are responsible for important processes like cell differentiation, yet epigenetic abnormalities are associated with diseases like cancer. Thus the epigenome is a better indicator of cell health than is the genome itself. Familiar sequencing techniques, however, are insensitive to base modifications. The authors show that the most common epigenetic modifications can be detected via their tunneling-current signatures, right alongside the usual DNA bases, in a single run through a nanopore. Using current-current correlations, the modified bases can be identified with any preselected accuracy.

Charge-carrier transport in polycrystalline thin films is affected by inhomogeneity, including high- and low-conductivity regions. This effect appears considerably in the transfer characteristics of organic thin-film transistors (TFTs) with lateral conduction paths. Using a time-resolved microscopic imaging technique, the authors reveal the temporal variation of charge and field distributions in the channel layer of an organic TFT after a gate bias is applied. Their comprehensive discussion of the temporal response of TFTs and the transient features of charge accumulation around grain boundaries is expected to stimulate research on organic electronics.

Carbon nanomaterials are now in the spotlight because of their prospective applications in photonics and optoelectronics. Although graphene metasurfaces and individual carbon nanotubes (CNTs) have been shown to be active plasmonic materials, studies of the plasmonic properties of CNT thin films are lacking. The authors demonstrate that a free-standing metasurface made of a film of single-walled CNTs supports propagation of surface plasmons in the technologically interesting midinfrared range. This result should be particularly useful for developing CNT-based photodetectors and photonic devices.

The simultaneous displacement of fluids through a porous medium and deformation of that host medium are crucial in applications as varied as hydraulic fracturing, methane venting from organic-rich sediments, volcanic eruptions, and desiccation cracking of soil. The authors present quantitative three-dimensional imaging of a deforming porous pack under immiscible fluid-fluid displacement. The data are modeled as the onset and evolution of cavity formation by overcoming the frictional resistance in the granular pack. These findings connect pore-scale observations to macroscopic behavior, and elucidate the physics at play in important natural processes and engineering applications.

$Magnonics$—the generation, manipulation, and detection of magnons (quantized spin waves)—is important for next-generation data storage and processing, where low energy consumption is a key concern. Conventional magnonics, though, uses magnetostatic spin waves that are difficult to propagate in complex nanoscale geometries. This study uses topologically protected edge spin waves to neatly address the issue. The suggested paradigm for designing robust, reconfigurable, on-chip magnonic devices like diodes, beam splitters, and interferometers could have quite an impact on engineering solutions in information technology.

The development of chip-scale frequency combs would enable precision optical-frequency measurements outside the laboratory, in real-world operating environments. Microresonators supporting soliton pulses provide an important step towards this goal, but reliable soliton stabilization and supercontinuum generation is challenging. The authors demonstrate streamlined soliton generation in a 15-GHz silica resonator, and chip-scale supercontinuum generation at 15-GHz pulse rates in a silicon nitride waveguide. Self-referencing this frequency-comb system with $f-2f$ interferometry has allowed them to measure the relative drift between two optical references, an important advance in metrology.

In the variant of atomic force microscopy (AFM) known as photoinduced force microscopy (PiFM), amplitude-modulation techniques have been used to detect the photoinduced force. In such approaches the signal is also affected by other forces, which damp the motion of the probe’s cantilever. This study proposes a heterodyne (sideband-detection) $frequency$-modulation technique that enables one to obtain the photoinduced force without artifacts from the photothermal force and the changes in the resonance frequency of the cantilever, and thus to realize true optical imaging at the nanoscale.

The surface tension of a magnetic domain wall (DW) is an important, fundamental parameter that affects its static and dynamic processes. However, direct observation and accurate quantification of this property have been missing. This series of experiments demonstrates the governing effect of DW surface tension on the stabilization of magnetic-domain bubbles and on DW depinning. Two methods to quantify DW surface energy are developed. These results can facilitate the design of magnetic devices for effective manipulation of DWs, as well as quantification of intrinsic magnetic parameters, such as the strength of the Dzyaloshinskii-Moriya interaction.

Wireless, implanted devices for biotelemetry, telemedicine, and neural interfacing are an emerging technology with powerful capabilities for medicine and clinical research, but are being held back by unreliable communication with external equipment. This study uses full-wave-problem formulations to study the mechanisms of electromagnetic propagation through tissue, and to derive optimal radiation conditions. Surprisingly, 80–99% of radiation efficiency is lost due to tissue-air impedance mismatch, not due to tissue absorption, as is commonly believed. Efficiency could be improved by an order of magnitude, compared to existing systems.

Reverse osmosis (RO) is the most energy-efficient means of harnessing the ocean as a source of water for drinking, agriculture, or industry. The basic principles have been long understood, and practiced for 50 years, but perfecting this technology takes on new importance in the face of a burgeoning world population. The authors present a comprehensive transport theory for species transiting the pores of a RO membrane, and their calculations closely match experimental data. Of particular interest is understanding and improving the rejection of boron-bearing ions, which are a health concern at typical input concentrations.

$Thermophoresis$ refers to the movement of particles under the influence of a temperature gradient, and could be exploited as a separation technique. In this study, pattern formation and separation of microparticles by on-chip thermophoresis in a microfluidic device is demonstrated. Particles of equal size yet different bulk characteristics and surface modifications show opposite responses. This clearly shows the potential of thermophoretic manipulation in microfluidic devices by adding controllable parameters, laying the foundation for innovative separation and filtration techniques that can be easily integrated into existing device designs.

Suppressing hydrodynamic instability in coflow two-phase systems is interesting for the continuous fabrication of microfibers, interfacial chemical reactions, and on-chip transport. Though there are plenty of strategies to trigger the convective hydrodynamic instability, ones to suppress it are rare. This study uses an expansion-contraction functional unit to passively reduce the convective noise on the jet interface and extend the convectively unstable jet, so that it does not break up into droplets. By implementing the functional unit repeatedly, a two-phase jet can be extended to a surprising length.

In diverse applications employing very thin, single-crystalline semiconductor sheets, including flexible electronics and photonics, energy-storage devices, and solar cells, the impacts of surfaces and interfaces on charge transport need to be understood. The authors study the electrical conductance of very thin Si(001) sheets using a special, sensitive back-gating method, to quantify the influence of H termination on conductance for sheets of different thickness, and confirm that surface states play a dominant role in these conductance properties. Their approach is extensible to other important systems, $e.g.$ $III-V$ semiconductors and transition-metal dichalcogenides.

Metasurfaces offer a convenient means to control the polarization state of light. Converting linear polarization (LP) and maintaining circular polarization (CP) in reflection mode have been widely realized, but these two types of polarization control have not been achieved using a single metasurface. This article reports an anisotropic metasurface that allows both multiband LP conversion and CP-maintaining reflection, due to the anisotropy of the metamaterial’s unit cell. This system facilitates ultrawide-band polarization converters for $e.g.$ antennas, wireless communication, and radar technology.

The performance of organic solar cells is limited by trap states, which increase energetic disorder, lead to trap-assisted recombination of charge carriers, and lower carrier mobility. To increase solar-cell efficiency, it is important to understand the origin and energetic distribution of those states. Using impedance spectroscopy, the authors find that the intermixing of C${}_{60}$ and the donor molecule DCV5T-Me in bulk heterojunction solar cells introduces notably deep electron-trap states in the C${}_{60}$ phase, decreasing power-conversion efficiency. As C${}_{60}$ is a widely used acceptor material, these findings are of general interest for fullerene-based solar cells.