Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. GaN or ZnO on sapphire are prominent examples for that. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. In this work, we study the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy as an example. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various ${\mathrm{Al}}_x{\mathrm{O}}_y{\mathrm{N}}_z$ phases of the pseudobinary ${\mathrm{Al}}_2{\mathrm{O}}_3$-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers.

Reciprocity relations based on the principle of detailed balance have been frequently used to analyze luminescence intensity and the spectrum of organic solar cells. These reciprocity relations were derived for cases where a linear extrapolation of equilibrium conditions to the nonequilibrium situations present during measurements is possible and therefore requires semiconductors with linear recombination mechanisms. Here, we discuss the impact of nonlinear recombination typically found in organic solar cells on the analysis of luminescence spectra and estimate criteria under which reciprocity relations can still be used to analyze the data. We find that depending on the exact application, only for low mobilities $\mu <{10}^{-4}{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ or very asymmetric mobilities do substantial disagreements between simulation and analytical equations occur.

We develop a predictive and mechanistic model for the intensity-modulated photocurrent spectroscopic (IMPS), intensity-modulated photovoltage spectroscopic (IMVS), and electrical impedance spectroscopic (EIS) responses of organic bulk heterojunction (BHJ) solar cells. Unlike the dominant analytical framework—equivalent circuit analysis—the model uses physical parameters that directly reflect the device’s fundamental electronic mechanisms, eliminating the ambiguity associated with interpreting phenomenological parameters. Formulated in the frequency domain, the model is a computationally efficient tool for extracting parameters from the measured spectra. With a set of physical parameters representing a device, we predict the device’s spectra (a) in techniques employing different methods of perturbing a device and (b) at different bias voltages and illumination intensities. The predicted spectra show good agreement with the measured ones. By quantifying the device’s internal electric field and charge carrier concentration and relating them to the spectra, we determine that the IMPS responses at the short-circuit condition and the IMVS responses at the open-circuit condition directly reflect the charge carrier extraction and recombination, respectively. Furthermore, the EIS response indicates the device’s recombination time scale at different bias voltages.

A large obstacle for realizing photonic logic is the weak optical nonlinearity of available materials, which results in large power consumption. In this paper, we present the theoretical design of all-optical logic with second-order (${\chi }^{(2)}$) nonlinear bimodal cavities and their networks. Using semiclassical models derived from the Wigner quasiprobability distribution function, we analyze the power consumption and signal-to-noise ratio (SNR) of networks implementing an optical and gate and an optical latch. A comparison between the second- and third-order $({\chi }^{(3)})$ optical logic reveals that, while the ${\chi }^{(3)}$ design outperforms the ${\chi }^{(2)}$ design in terms of the SNR for the same input power, employing the ${\chi }^{(3)}$ nonlinearity necessitates the use of cavities with ultrahigh-quality factors ($Q\sim {10}^6$) to achieve a gate power consumption comparable to that of the ${\chi }^{(2)}$ design at significantly smaller quality factors ($Q\sim {10}^4$). Using realistic estimates of the ${\chi }^{(2)}$ and ${\chi }^{(3)}$ nonlinear susceptibilities of available materials, we show that, at achievable quality factors ($Q\sim {10}^4$), the ${\chi }^{(2)}$ design is an order of magnitude more energy efficient than the corresponding ${\chi }^{(3)}$ design.

Experimental quantum information processing with superconducting circuits is rapidly advancing, driven by innovation in two classes of devices, one involving planar microfabricated (2D) resonators, and the other involving machined three-dimensional (3D) cavities. We demonstrate that circuit quantum electrodynamics can be implemented in a multilayer superconducting structure that combines 2D and 3D advantages. We employ standard microfabrication techniques to pattern each layer, and rely on a vacuum gap between the layers to store the electromagnetic energy. Planar qubits are lithographically defined as an aperture in a conducting boundary of the resonators. We demonstrate the aperture concept by implementing an integrated, two-cavity-mode, one-transmon-qubit system.

Compressing a porous material will decrease the volume of the pore space, driving fluid out. Similarly, injecting fluid into a porous material can expand the pore space, distorting the solid skeleton. This poromechanical coupling has applications ranging from cell and tissue mechanics to geomechanics and hydrogeology. The classical theory of linear poroelasticity captures this coupling by combining Darcy’s law with Terzaghi’s effective stress and linear elasticity in a linearized kinematic framework. Linear poroelasticity is a good model for very small deformations, but it becomes increasingly inappropriate for moderate to large deformations, which are common in the context of phenomena such as swelling and damage, and for soft materials such as gels and tissues. The well-known theory of large-deformation poroelasticity combines Darcy’s law with Terzaghi’s effective stress and nonlinear elasticity in a rigorous kinematic framework. This theory has been used extensively in biomechanics to model large elastic deformations in soft tissues and in geomechanics to model large elastoplastic deformations in soils. Here, we first provide an overview and discussion of this theory with an emphasis on the physics of poromechanical coupling. We present the large-deformation theory in an Eulerian framework to minimize the mathematical complexity, and we show how this nonlinear theory simplifies to linear poroelasticity under the assumption of small strain. We then compare the predictions of linear poroelasticity with those of large-deformation poroelasticity in the context of two uniaxial model problems: fluid outflow driven by an applied mechanical load (the consolidation problem) and compression driven by a steady fluid throughflow. We explore the steady and dynamical errors associated with the linear model in both situations, as well as the impact of introducing a deformation-dependent permeability. We show that the error in linear poroelasticity is due primarily to kinematic nonlinearity and that this error (i) plays a surprisingly important role in the dynamics of the deformation and (ii) is amplified by nonlinear constitutive behavior, such as deformation-dependent permeability.

Quench of superconducting radio-frequency cavities frequently leads to the lowered quality factor $Q_0$, which had been attributed to the additional trapped magnetic flux. Here we demonstrate that the origin of this magnetic flux is purely extrinsic to the cavity by showing no extra dissipation (unchanged $Q_0$) after quenching in zero magnetic field, which allows us to rule out intrinsic mechanisms of flux trapping such as generation of thermal currents or trapping of the rf field. We also show the clear relation of dissipation introduced by quenching to the orientation of the applied magnetic field and the possibility to fully recover the quality factor by requenching in the compensated field. We discover that for larger values of the ambient field, the $Q$-factor degradation may become irreversible by this technique, likely due to the outward flux migration beyond the normal zone opening during quench. Our findings are of special practical importance for accelerators based on low- and medium-$\beta$ accelerating structures residing close to focusing magnets, as well as for all high-$Q$ cavity-based accelerators.

Measuring the structure-function relation in photovoltaic materials has been a major drive for atomic force microscopy (AFM) and Kelvin-probe force microscopy (KPFM). The local surface photovoltage (SPV) is measured as the change in contact potential difference (CPD) between the tip and sample upon illumination. The quantities of interest that one will like to correlate with the structure are the decay times of SPV and/or its wavelength dependence. KPFM depends on the tip and sample potential; therefore, SPV is prone to tip changes, rendering an accurate measurement of SPV challenging. We present a measurement technique which allows us to directly measure the difference in the CPD between illuminated and dark states and, thus, SPV as well as the capacitance derivative by using pulsed illumination. The variation of the measured SPV can be minimized due to the time-domain measurement, allowing accurate measurements of the SPV. The increased accuracy enables the systematic comparison of SPV across different measurement setups and excitation conditions (e.g., wavelength dependence and decay time of SPV).

We explore broadband and omnidirectional midinfrared rectification based on nanopatterned hyperbolic metamaterials, composed of two dissimilar metals separated by an ultrathin dielectric layer. The exotic slow-light modes supported by such periodically trenched hyperbolic metamaterials efficiently trap incident radiation in massively parallel metal-insulator-metal tunnel junctions producing ultrafast infrared rectification via photon-assisted tunneling. This leads to a highly efficient photon-to-electron transduction with obtained photocurrent orders of magnitude larger than conventional lumped-element devices, such as nanorectennas and point-contact rectifiers. Our results promise an impact on infrared energy harvesters and plasmonic photodetectors.

We use 2D coupled sine-Gordon equations combined with 3D heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a $250\times 70\mu {\mathrm{m}}^2$ intrinsic Josephson junction stack. The 700 junctions are grouped to 20 segments; we assume that in a segment all junctions behave identically. At large input power, a hot spot forms in the stack. Resonant electromagnetic modes oscillating either along the length [(0, $n$) modes] or the width [($m$, 0) modes] of the stack or having a more complex structure can be excited both with and without a hot spot. At fixed bath temperature and bias current, several cavity modes can coexist in the absence of a magnetic field. The (1, 0) mode considered to be the most favorable mode for terahertz emission can be stabilized by applying a small magnetic field along the length of the stack. A strong field-induced enhancement of the emission power is also found in experiment for an applied field around 5.9 mT.

We present a scheme of positioning a single-electron spin with subnanometer resolution through multiple nitrogen-vacancy centers in diamond. With unwanted noise suppressed by dynamical decoupling, the spin coherence of each center develops characteristic oscillations due to a single-electron spin located 4–20 nm away from the centers. We can extract the position information from the characteristic electron spin-coherence oscillations of each center. This scheme is useful for high-resolution nanoscale magnetometry.

We study 95 split gates of different size on a single chip using a multiplexing technique. Each split gate defines a one-dimensional channel on a modulation-doped $\mathrm{GaAs}/\mathrm{AlGaAs}$ heterostructure, through which the conductance is quantized. The yield of devices showing good quantization decreases rapidly as the length of the split gates increases. However, for the subset of devices showing good quantization, there is no correlation between the electrostatic length of the one-dimensional channel (estimated using a saddle-point model) and the gate length. The variation in electrostatic length and the one-dimensional subband spacing for devices of the same gate length exceeds the variation in the average values between devices of different lengths. There is a clear correlation between the curvature of the potential barrier in the transport direction and the strength of the “0.7 anomaly”: the conductance value of the 0.7 anomaly reduces as the barrier curvature becomes shallower. These results highlight the key role of the electrostatic environment in one-dimensional systems. Even in devices with clean conductance plateaus, random fluctuations in the background potential are crucial in determining the potential landscape in the active device area such that nominally identical gate structures have different characteristics.

We demonstrate computerized tomography (CT) reconstructions from absorption, phase, and dark-field signals obtained from scans acquired when the x-ray probe light is modulated with speckle. Two different interlaced schemes are proposed to reduce the number of sample exposures. First, the already demonstrated x-ray speckle-vector tracking (XSVT) concept for projection imaging allows the three signal CT reconstructions from multiple images per projection. Second, a modified XSVT approach is shown to provide absorption and phase reconstructions, this time from a single image per angular projection. Reconstructions from data obtained at a synchrotron facility emphasize the potential of the approaches for the imaging of complex samples.

Sharp corners are an inherent component of most planar microfluidic systems, and thus their influence on flow within the microchannel is of significant interest. Here, we demonstrate that in electrokinetically driven devices, the presence of a sharp corner may result in localized vortices due to nonlinear induced-charge electro-osmosis (ICEO) and/or electrothermal forces. Application of an alternating-current electric field enables quantification of the nonlinear ICEO ejection-flow effect by isolating it from linear electro-osmotic background flow which is present under dc forcing. The hydrodynamic flow in the vicinity of a sharp channel corner is analyzed using experimental micro-particle-image-velocimetry and numerical simulations for different buffer concentrations, frequencies, and applied voltages. Divergence from the purely ICEO flow with increasing buffer conductivity is shown to be a result of increasing electrothermal effects due to Joule heating.

Quantum amplification is essential for various quantum technologies such as communication and weak-signal detection. However, its practical use is still limited due to inevitable device fragility that brings about distortion in the output signal or state. This paper presents a general theory that solves this critical issue. The key idea is simple and easy to implement: just a passive feedback of the amplifier’s auxiliary mode, which is usually thrown away. In fact, this scheme makes the controlled amplifier significantly robust, and furthermore it realizes the minimum-noise amplification even under realistic imperfections. Hence, the presented theory enables the quantum amplification to be implemented at a practical level. Also, a nondegenerate parametric amplifier subjected to a special detuning is proposed to show that, additionally, it has a broadband nature.

We report on a discrete layer-by-layer magnetic switching in $\mathrm{Fe}/\mathrm{MgO}$ superlattices driven by an antiferromagnetic interlayer exchange coupling. The strong interlayer coupling is mediated by tunneling through MgO layers with thicknesses up to at least 1.8 nm, and the coupling strength varies with MgO thickness. Furthermore, the competition between the interlayer coupling and magnetocrystalline anisotropy stabilizes both 90° and 180° periodic alignment of adjacent layers throughout the entire superlattice. The tunable layer-by-layer switching, coupled with the giant tunneling magnetoresistance of $\mathrm{Fe}/\mathrm{MgO}/\mathrm{Fe}$ junctions, is an appealing combination for three-dimensional spintronic memories and logic devices.

The negatively charged silicon-vacancy (${\mathrm{SiV}}^{-}$) center in diamond is a bright source of indistinguishable single photons and a useful resource in quantum-information protocols. Until now, ${\mathrm{SiV}}^{-}$ centers with narrow optical linewidths and small inhomogeneous distributions of ${\mathrm{SiV}}^{-}$ transition frequencies have only been reported in samples doped with silicon during diamond growth. We present a technique for producing implanted ${\mathrm{SiV}}^{-}$ centers with nearly lifetime-limited optical linewidths and a small inhomogeneous distribution. These properties persist after nanofabrication, paving the way for the incorporation of high-quality ${\mathrm{SiV}}^{-}$ centers into nanophotonic devices.

Orbital angular momentum (OAM) has been extensively studied to date and has become topical in the last few years due to its potential for increasing bandwidth in optical communications. The so-called Dammann vortex grating (DVG) can generate a series of equal-energy OAM beams and satisfactorily realize OAM parallel detection, an important challenge in this field. However, previously reported DVGs suffer from polarization sensitivity, low efficiency, or the lack of tunability and mode variety. Here, a design of liquid-crystal DVGs is proposed and demonstrated for the generation of various equal-energy OAM beams. The DVGs, featured by alternative orthogonally planar-aligned regions, are carried out via photopatterning technology. Beam arrays composed of arbitrary OAM modes, as well as two-dimensional ones, are generated in good quality and high efficiency. The liquid-crystal DVGs exhibit merits of excellent polarization independency, electrical switchability, and tunability. This supplies a promising approach towards OAM generation, manipulation, and detection.

Electromagnetic stimulation of energetic materials provides a noninvasive and nondestructive tool for detecting and identifying explosives. We combine structural information based on x-ray computed tomography, experimental dielectric data, and electromagnetic full-wave simulations to study microscale electromagnetic heating of realistic three-dimensional heterogeneous explosives. We analyze the formation of electromagnetic hot spots and thermal gradients in the explosive-binder mesostructures and compare the heating rate for various binder systems.

We demonstrate a centimeter-scale optomechanical magnetometer based on a crystalline whispering-gallery-mode resonator. The large size of the resonator, with a magnetic-field integration volume of $0.45{\mathrm{cm}}^3$, allows high magnetic-field sensitivity to be achieved in the hertz-to-kilohertz frequency range. A peak sensitivity of $131\mathrm{pT}{\mathrm{Hz}}^{-1/2}$ is reported, in a magnetically unshielded noncryogenic environment using optical power levels beneath $100\mu \mathrm{W}$. Femtotesla-range sensitivity may be possible in future devices with the further optimization of laser noise and the physical structure of the resonator, allowing applications in high-performance magnetometry.