Point defects can seriously affect the performance of scintillator materials. Therefore, the fabrication of defect-free scintillators is an exciting goal for materials scientists. By applying an advanced optical ceramic preparation process, $({\mathrm{Lu}}_{1-x}{\mathrm{Y}}_x{)}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}:\mathrm{Pr}$ ($\mathrm{LuYAG}:\mathrm{Pr}$) ceramics free of defect-induced host luminescence are produced. Moreover, a detailed low-temperature thermoluminescence (TSL) study reveals that all TSL peak intensities and trap depths decrease in the Y admixture. The whole phenomenology indicates that a joint effect of “band-gap engineering” and “defect engineering” works in $\mathrm{LuYAG}:\mathrm{Pr}$ ceramics, as confirmed by the comparison between TSL data, radioluminescence temperature-dependence measurements, and first-principles calculations. As a consequence of material optimization, the light yield of $\mathrm{LuYAG}:\mathrm{Pr}$ ceramics with 25% Y is found to be $24400\mathrm{ph}/\mathrm{MeV}$ with a short $1.0\text{−}\mu \mathrm{s}$ shaping time, which is 20% higher than the value obtained for Y-free $\mathrm{LuAG}:\mathrm{Pr}$ ceramics. We close with an explanation of the role of Y that also suggests future research directions concerning other oxide scintillators.

Metamaterials are engineered materials which exhibit fascinating properties unreachable by traditional materials. Here, we report on the design, fabrication, and experimental characterization of a three-dimensional single-port labyrinthine acoustic metamaterial. By using curled perforations with one end closed and with appropriate loss inside, the metamaterial can perfectly absorb airborne sounds in a low-frequency band. Both the position and the relative width of the band can be tuned flexibly. A trade-off is uncovered between the relative absorption bandwidth and thickness of the metamaterial. When the relative absorption bandwidth is as high as 51%, the requirement of deep-subwavelength thickness ($0.07\lambda$) can still be satisfied. We emphasize that the perfect absorption with large tunability in relative bandwidth (from 9% to $>180\%$) was not attainable previously and may find applications ranging from noise reduction to sound imaging.

Incorporation of gain media in plasmonic nanostructures can give the possibility to compensate for high Ohmic losses in the metal and design truly nanoscale optical components for diverse applications ranging from biosensing to on-chip data communication. However, the process of stimulated emission in the gain medium is inevitably accompanied by spontaneous emission. This spontaneous emission greatly impacts the performance characteristics of deep-subwavelength active plasmonic devices and casts doubt on their practical use. Here we develop a theoretical framework to evaluate the influence of spontaneous emission, which can be applied to waveguide structures of any shape and level of mode confinement. In contrast to the previously developed theories, we take into account that the spectrum of spontaneous emission can be very broad and nonuniform, which is typical for deep-subwavelength structures, where a high optical gain (approximately $1000{\mathrm{cm}}^{-1}$) in the active medium is required to compensate for strong absorption in the metal. We also present a detailed study of the spontaneous emission noise in metal-semiconductor active plasmonic nanowaveguides and demonstrate that by using both optical and electrical filtering techniques, it is possible to decrease the noise to a level sufficient for practical applications at telecom and midinfrared wavelengths.

The coalescence in dense arrays of spontaneously formed GaN nanowires proceeds by bundling: adjacent nanowires bend and merge at their top, thus reducing their surface energy at the expense of the elastic energy of bending. We give a theoretical description of the energetics of this bundling process. The bending energy is shown to be substantially reduced by the creation of dislocations at the coalescence joints. A comparison of experimental and calculated x-ray–diffraction profiles from ensembles of bundled nanowires demonstrates that a large part of the bending energy is indeed relaxed by plastic deformation. As a consequence, spontaneously formed GaN nanowire ensembles contain boundary dislocations with a density on the order of $10^{10}{\mathrm{cm}}^{-2}$.

The tetragonal ${\mathrm{Fe}}_4\mathrm{N}/{\mathrm{BiFeO}}_3(001)$ heterostructures aimed at simultaneously gaining the perpendicular magnetic anisotropy (PMA) and high spin polarization have been investigated by the first-principles method. It is found that ${\mathrm{Fe}}_4\mathrm{N}$ with ${\mathrm{Fe}}_A{\mathrm{Fe}}_B$ termination is better for achieving interfacial and inner PMA simultaneously than ${({\mathrm{Fe}}_B)}_2\mathrm{N}$ termination. When the positions of interfacial ${\mathrm{Fe}}_A$ and ${\mathrm{Fe}}_B$ relative to Fe in ${\mathrm{BiFeO}}_3$ are changed, the PMA in ${\mathrm{Fe}}_4\mathrm{N}$ transforms into the in-plane magnetic anisotropy. Especially, PMA in ${\mathrm{Fe}}_4\mathrm{N}$ near the heterointerfaces depends on the direction of ferroelectric polarization in ${\mathrm{BiFeO}}_3$. Finally, the interfacial and inner PMA of ${\mathrm{Fe}}_4\mathrm{N}$ along with high spin polarization appear in the stable ${\mathrm{Fe}}_A{\mathrm{Fe}}_B/\mathrm{Fe}{\text{−}\mathrm{O}}_2$ case owing to the $3d\text{−}3d$ orbital hybridization. These results provide the opportunities for developing multifunctional spintronic devices.

A significant deviation from the normal mode expansion is observed in the optomechanically measured thermal noise of a graphene membrane suspended on a silicon nitride nanoresonator. This deviation is due to the heterogeneous character of mechanical dissipation over the spatial extent of coupled eigenmodes, which is tuned through an avoided anticrossing. We demonstrate that the fluctuation-dissipation theorem permits a proper evaluation of the thermal noise of the coupled nanomechanical system. Since a good spatial homogeneity is delicate to ensure at the nanoscale, this approach is fundamental to correctly describing the thermal noise of nanomechanical systems which ultimately impact their sensing capacity.

Using capacitance-frequency ($C\text{−}f$) analysis to characterize the density-of-states (DOS) distribution of defects has been well established for inorganic thin-film photovoltaic devices. While $C\text{−}f$ analysis has also been applied to bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices, we show that the low carrier mobility in the BHJ material can severely alter the $C\text{−}f$ behaviors and lead to misinterpretations. Because of the complicated nature of disorders in organic materials, artifacts from an erroneous $C\text{−}f$ analysis are difficult to identify. Here we compare drift-diffusion simulations with experiments to reveal situations when the validity of $C\text{−}f$ analysis for defect characterization breaks down. When a flat-band region is present in the low-mobility active layer, the capacitive response cannot follow the electrical modulation and behaves as if the active layer is a dielectric at frequencies higher than the characteristic frequency determined by carrier mobility and thickness. The transition produces a fictitious shallow defect when defect analysis is applied. Even in fully depleted devices, the defect distributions derived from $C\text{−}f$ analysis can appear at spuriously deeper energies if the mobility is too low. Through simulations, we determine the ranges of mobility and thickness for which the $C\text{−}f$ analysis can effectively yield credible defect DOS information. Insight from this study also sheds light on transport limitation when using capacitance spectroscopy for defect characterization in general.

The interplay of strong infrared photon-phonon coupling with electromagnetic confinement in nanoscale devices is demonstrated to have a large impact on ultrafast photon-assisted tunneling in metal-oxide-semiconductor (MOS) structures. Infrared active optical phonon modes in polar oxides lead to strong dispersion and enhanced electric fields at material interfaces. We find that the infrared dispersion of ${\mathrm{SiO}}_2$ near a longitudinal optical phonon mode can effectively impedance match a photonic surface mode into a nanoscale tunnel gap that results in large transverse-field confinement. An integrated 2D nanoantenna structure on a distributed large-area MOS tunnel-diode rectifier is designed and built to resonantly excite infrared surface modes and is shown to efficiently channel infrared radiation into nanometer-scale gaps in these MOS devices. This enhanced-gap transverse-electric field is converted to a rectified tunneling displacement current resulting in a dc photocurrent. We examine the angular and polarization-dependent spectral photocurrent response of these 2D nanoantenna-coupled tunnel diodes in the photon-enhanced tunneling spectral region. Our 2D nanoantenna-coupled infrared tunnel-diode rectifier promises to impact large-area thermal energy harvesting and infrared direct detectors.

CdO-based transparent-conducting oxide thin films have great potential applications in optoelectronic devices due their high mobility, low resistivity, and high transparency over a wide spectral range. In this paper, we report the results of a comprehensive study of optical properties of CdO thin films doped with different donors (In, Ga, V, Ti) with a carrier concentration in the range of ${10}^{20}$ to $>{10}^{21}/{\mathrm{cm}}^3$. Variable angle spectroscopic ellipsometry (SE) studies reveal that the complex dielectric function of CdO thin films drastically depends on the carrier concentration. Specifically, with increasing carrier concentration, (1) the net effect of Burstein-Moss shift and band-gap renormalization gives rise to an increase in the optical band gap from 2.6 to 3.2 eV; (2) the free-carrier absorption coefficient at a wavelength of 1200 nm increases from ${10}^2$ to $1\times {10}^4{\mathrm{cm}}^{-1}$; (3) the refractive index decreases from 2.4 to 2.05 at 600 nm; (4) the high-frequency dielectric constant reduces from 5.5 to 4.8. The SE results are analyzed with results from Hall measurements to obtain information on the electron effective mass and optical mobility of CdO thin films. The significantly higher effective mass of V- and Ti-doped CdO thin film is attributed to the modification of the conduction band due to an anticrossing interaction between the localized $d$ levels of V and Ti atoms and the CdO conduction-band extended states. The effective mass of In- and Ga-doped CdO increases with the electron concentration, consistent with the prediction from the nonparabolic conduction-band model. We also find that the optical mobility ${\mu }_{\text{opt}}$ is close to the Hall mobility ${\mu }_{\text{Hall}}$ when the ${\mu }_{\text{Hall}}<60{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$, while ${\mu }_{\text{opt}}<{\mu }_{\text{Hall}}$ for materials with higher ${\mu }_{\text{Hall}}$.

Significant successes have recently been reported in the study of the generation of a spectrally pure state in group-velocity-matched (GVM) nonlinear crystals. However, the GVM condition can be realized only in limited kinds of crystals and at limited wavelengths. Here, we investigate pure-state generation in the isomorphs of the PPKTP crystal: i.e., periodically poled RTP, KTA, RTA, and CTA crystals. By numerical simulation, we find that these crystals from the KTP family can generate pure photons with high spectral purity (over 0.8), wide tunability (more than 400 nm), and reasonable nonlinearity at a variety of wavelengths (from 1300 to 2100 nm). It is also discovered that the PPCTA crystal may achieve a purity of 0.97 at 1506 nm. This study may provide more and better choices for quantum-state engineering at telecom wavelengths.

Substituting noble metals for high-index dielectrics has recently been proposed as an alternative strategy in nanophotonics to design broadband optical resonators and circumvent the Ohmic losses of plasmonic materials. In this paper, we demonstrate that subwavelength silicon nanoantennas can manipulate the photon emission dynamics of fluorescent molecules. In practice, we show that dielectric nanoantennas can both increase and decrease the local density of optical states at room temperature, a process that is inaccessible with noble metals at the nanoscale. Using scanning probe microscopy, we analyze quantitatively, in three dimensions, the near-field interaction between a 100-nm fluorescent nanosphere and silicon nanoantennas with diameters ranging between 170 and 250 nm. Associated with numerical simulations, these measurements indicate increased or decreased total spontaneous decay rates by up to 15% and a gain in the collection efficiency of emitted photons by up to 85%. Our study demonstrates the potential of silicon-based nanoantennas for the low-loss manipulation of solid-state emitters at the nanoscale and at room temperature.

Identifying limiting factors is crucial for a better understanding of the dynamics of the resistive switching phenomenon in transition-metal oxides. This improved understanding is important for the design of fast-switching, energy-efficient, and long-term stable redox-based resistive random-access memory devices. Therefore, this work presents a detailed study of the set kinetics of valence change resistive switches on a time scale from 10 ns to ${10}^4\mathrm{s}$, taking $\mathrm{Pt}/{\mathrm{SrTiO}}_3/\mathrm{TiN}$ nanocrossbars as a model material. The analysis of the transient currents reveals that the switching process can be subdivided into a linear-degradation process that is followed by a thermal runaway. The comparison with a dynamical electrothermal model of the memory cell allows the deduction of the physical origin of the degradation. The origin is an electric-field-induced increase of the oxygen-vacancy concentration near the Schottky barrier of the $\mathrm{Pt}/{\mathrm{SrTiO}}_3$ interface that is accompanied by a steadily rising local temperature due to Joule heating. The positive feedback of the temperature increase on the oxygen-vacancy mobility, and thereby on the conductivity of the filament, leads to a self-acceleration of the set process.

An array of four ${}^{87}\mathrm{Rb}$ vector magnetometers is used to detect nuclear quadrupole resonance signals in an unshielded environment at 1 MHz. With a baseline of 25 cm, the length of the array, radio-frequency interference mitigation is also demonstrated; a radio-station signal is suppressed by a factor of 20 without degradation to the signal of interest. With these compact sensors, in which the probe beam passes through twice, the fundamental limit to detection sensitivity is found to be photon-shot noise. More passes of the probe beam overcome this limitation. With a sensor of similar effective volume, $0.25{\mathrm{cm}}^3$, but $25\times$ more passes, the sensitivity is improved by an order of magnitude to $1.7\pm 0.2\mathrm{fT}/\sqrt{\mathrm{Hz}}$.

Diamond nanoparticles with negatively charged nitrogen-vacancy (NV) centers are highly efficient nonblinking emitters that exhibit spin-dependent intensity. An attractive application of these emitters is background-free fluorescence microscopy exploiting the fluorescence quenching induced either by resonant microwaves (RMWs) or by an applied static magnetic field (SMF). Here, we compare RMW- and SMF-induced contrast measurements over a wide range of optical excitation rates for fluorescent nanodiamonds (FNDs) and for NV centers shallowly buried under the (100)-oriented surface of a diamond single crystal (SC). Contrast levels are found to be systematically lower in the FNDs than in the SC. At low excitation rates, the RMW contrast initially rises to a maximum (up to 7% in FNDs and 13% in the SC) but then decreases steadily at higher intensities. Conversely, the SMF contrast increases from approximately 12% at low excitation rates to high values of 20% and 38% for the FNDs and SC, respectively. These observations are well described in a rate-equations model for the charged NV defect using parameters in good agreement with the literature. The SMF approach yields higher induced contrast in image collection under commonly applied optical excitation. Unlike the RMW method, there is no thermal load exerted on the aqueous media in biological samples in the SMF approach. We demonstrate imaging by SMF-induced contrast in neuronal cultures incorporating FNDs (i) in a setup for patch-clamp experiments in parallel with differential-interference-contrast microscopy, (ii) after a commonly used staining procedure as an illustration of the high selectivity against background fluorescence, and (iii) in a confocal fluorescence microscope in combination with bright-field microscopy.

Dielectric elastomers (DEs) are a category of soft materials that are capable of large deformation, however, the actuation performance is affected by snap-through instability. In this article, a dynamics model is developed to investigate the dynamic snap-through instability of DEs by applying the triangle and sinusoidal voltages. The DE materials with different limiting stretches are considered. When the DE is under a triangle voltage or a sinusoidal voltage, the snap-through behavior may emerge during the response in time domain, and the DEs with a large value of limiting stretch are susceptible to dynamic snap-through instability. By tuning the mechanical tensile force, the occurrence of dynamic snap-through instability can be controlled. With the increase of tensile force, the dynamic snap-through behavior initially does not emerge during vibration, then accompanies the vibration, and eventually disappears. Phase paths and Poincaré maps are utilized to explore the dynamic stability evolution of the DE systems.

Quantum confinement and coherence effects are considered the cause of many specific features for systems which are generally low dimensional, strongly disordered, and/or situated in the vicinity of the metal-insulator transition. Here, we report on the observation of anomalous resistance peak and specific heat peaks superimposed at the superconducting transition of heavily boron-doped polycrystalline bulk diamond, which is a three-dimensional system situated deep on the metallic side of the boron-doping-driven metal-insulator transition in diamond. The anomalous resistance peak and specific heat peaks are interpreted as a result of confinement and coherence effects in the presence of intrinsic and extrinsic granularity. Our data, obtained for superconducting diamond, provide a reference for understanding the superconductivity in other granular disordered systems. Furthermore, our study brings attention to the significant influence of granular disorder on the physical properties of boron-doped diamond, which is considered a promising candidate for electronics applications.

Swelling is a volumetric-growth process in which a porous material expands by spontaneous imbibition of additional pore fluid. Swelling is distinct from other growth processes in that it is inherently poromechanical: local expansion of the pore structure requires that additional fluid be drawn from elsewhere in the material, or into the material from across the boundaries. Here, we study the swelling and subsequent drying of a sphere of hydrogel. We develop a dynamic model based on large-deformation poromechanics and the theory of ideal elastomeric gels, and we compare the predictions of this model with a series of experiments performed with polyacrylamide spheres. We use the model and the experiments to study the complex internal dynamics of swelling and drying, and to highlight the fundamentally transient nature of these strikingly different processes. Although we assume spherical symmetry, the model also provides insight into the transient patterns that form and then vanish during swelling as well as the risk of fracture during drying.

The rapid progress seen in the development of quantum-coherent devices for information processing has motivated serious consideration of quantum computer architecture and organization. One topic which remains open for investigation and optimization relates to the design of the classical-quantum interface, where control operations on individual qubits are applied according to higher-level algorithms; accommodating competing demands on performance and scalability remains a major outstanding challenge. In this work, we present a resource-efficient, scalable framework for the implementation of embedded physical layer classical controllers for quantum-information systems. Design drivers and key functionalities are introduced, leading to the selection of Walsh functions as an effective functional basis for both programing and controller hardware implementation. This approach leverages the simplicity of real-time Walsh-function generation in classical digital hardware, and the fact that a wide variety of physical layer controls, such as dynamic error suppression, are known to fall within the Walsh family. We experimentally implement a real-time field-programmable-gate-array-based Walsh controller producing Walsh timing signals and Walsh-synthesized analog waveforms appropriate for critical tasks in error-resistant quantum control and noise characterization. These demonstrations represent the first step towards a unified framework for the realization of physical layer controls compatible with large-scale quantum-information processing.

We demonstrate the invisibility of a homogeneous ferrite rod and photonic crystals composed of such rods when an appropriate external static magnetic field (ESMF) is applied. As a naturally occurring effect, no external tool, including metamaterial and plasmonic covers or active sources, is used to hide the rod. In the invisible state, the response of the rod to the impinging wave becomes unique, with a $1/4$-cycle response delay of the induced magnetic dipole which lowers its scattering efficiency to the maximum possible extent. The visible and invisible states can be manipulated by an ESMF. The manipulability enables photonic crystals composed of such rods to assume different functions in the same electromagnetic device, such as remaining transparent for the impinging wave or totally reflecting it. The fast switching time of magnetic systems might make it particularly useful for practical applications.

We demonstrate a robust frequency-modulated radio receiver using electron-spin-dependent photoluminescence of nitrogen-vacancy centers in diamond. The carrier frequency of the frequency-modulated signal is in the 2.8-GHz range, determined by the zero-field splitting in the nitrogen-vacancy electronic ground state. The radio can be tuned over 300 MHz by applying an external dc magnetic field. We show the transmission of high-fidelity audio signals over a bandwidth of 91 kHz using the diamond radio. We demonstrate operating temperature of the radio as high as $350°\mathrm{C}$.