Group III–group V nanowires are comprehensively studied because of their suitability for optoelectronic quantum technology applications. However, their small dimensions and the spatial separation of carriers from the wire surface render electrical contacting difficult. Systematically studying ohmic contact formation by diffusion to $n$-doped $\mathrm{Ga}\mathrm{As}$ nanowires, we provide a set of optimal annealing parameters for $\mathrm{Pd}/\mathrm{Ge}/\mathrm{Au}$ ohmic contacts. We reproducibly achieve low specific contact resistances of approximately $2\times {10}^{-7}\mathrm{\Omega }{\mathrm{cm}}^2$ at room temperature, becoming an order of magnitude higher at $T\simeq 4.2$ K. We provide a phenomenological model to describe contact resistances as a function of diffusion parameters. Implementing a transfer-matrix method, we numerically study the influence of the Schottky barrier on the contact resistance. Our results indicate that contact resistances can be predicted with various barrier shapes, but further insights into structural properties would require a full microscopic understanding of the complex diffusion processes.

We present a theoretical method, together with its experimental confirmation, to obtain structures of light by connecting diffraction-resistant cylindrical beams of finite lengths and different radii. The resulting “LEGO-like beams” can assume, on demand, various unprecedent spatial configurations. We also experimentally generate some of them using a computational holographic technique and a spatial light modulator. Our method of linking together various pieces of light can find applications in all fields where structured light beams are needed, in particular, optical tweezers (e.g., for biological manipulations), optical guiding of atoms, control of orbital angular momentum of light, holography, lithography, nonlinear optics, and interaction of electromagnetic radiation with Bose-Einstein condensates, besides, in general, the field of localized waves (nondiffracting beams and pulses).

Intersubband (ISB) absorption in quantum wells (QW) at normal incidence is forbidden by the selection rules for light polarization. To be useful under this convenient geometry, QW structures require postgrowth texturing of the surfaces. Here, we demonstrate polarization-sensitive ISB absorption of light at normal incidence using as-grown, nonpolar m-plane $\mathrm{Zn}\mathrm{O}/(\mathrm{Mg},\mathrm{Zn})\mathrm{O}$ multiple QWs without any postprocessing. This breakage of the selection rules is possible due to the formation of a self-assembled V-groove grating QW profile in the direction perpendicular to the c axis, which lies in the growth plane. A geometrical model is developed that predicts the dependence of the ISB absorption strength with the angle of incidence and the QW V-groove angle and is verified by high-resolution transmission electron microscope analysis. As a result of the in-plane anisotropy of the QW grating geometry, a strong light polarization sensitivity is demonstrated under normal incidence, with a predicted polarization sensitivity contrast above 80:1 for a ±5° cone of angles.

The multiple scattering of light presents major challenges in realizing useful in vivo imaging at tissue depths of more than about one millimeter, where many answers to health questions lie. Visible through near-infrared photons can be readily and safely detected through centimeters of tissue; however, limited information is available for image formation. One strategy for obtaining images is to model the photon transport and a simple incoherent model is the diffusion equation approximation to the Boltzmann transport equation. Such an approach provides a prediction of the mean intensity of heavily scattered light and hence provides a forward model for optimization-based computational imaging. While diffuse optical imaging methods have received substantial attention, they remain restricted in terms of resolution because of the loss of high-spatial-frequency information that is associated with the multiple scattering of photons. Consequently, only relatively large inhomogeneities, such as tumors or organs in small animals, can be effectively resolved. Here, we introduce a superresolution imaging approach based on point localization in a diffusion framework that enables over two orders of magnitude improvement in the spatial resolution of diffuse optical imaging. The method is demonstrated experimentally by localizing a fluorescent inhomogeneity in a highly scattering slab and characterizing the localization uncertainty. The approach allows imaging through centimeters of tissue with a resolution of tens of microns, thereby enabling cells or cell clusters to be resolved. More generally, this high-resolution imaging approach could be applied with any physical transport or wave model and hence to a broad class of physical problems. Paired with a suitable optical contrast mechanism, as can be realized with targeted fluorescent molecules or genetically modified animals, superresolution diffuse imaging should open alternative dimensions for in vivo applications.

We investigate the effect of the nature of the substrate and the bottom interface on the out-of-plane polarization orientation of ultrathin (10-nm) lead zirconate titanate (PZT) thin films of (001) orientation by photoelectron spectroscopy of samples without surface contamination. The substrate nature is varied between insulator (strontium titanate, STO) and semiconductor ($\mathrm{Nb}$-doped STO, STON) and finally to a metal with a work function lower than that of PZT (strontium ruthenate, SRO). Outward polarization is obtained for PZT/STON(001) and inward polarization is obtained for PZT/STO(001) and PZT/SRO(001). Explanations are given for all these typical cases, the main elements being charge accumulation for compensation of the depolarization field, self-doping of PZT films, and the interface electric field driving the orientation of the polarization of the ferroelectric films. We find p-type self-doping is correlated with the inward polarization, and the driving field is formed between a negatively charged region with negatively ionized acceptors near the interface with the substrate and the p-type degenerate region with holes accumulated inside, toward the surface. This mechanism may be reversed under the assumption of n-type self-doping, positively ionized donors near the interface, and accumulated electrons toward the surface in the case of an interface with a substrate with a higher work function, being in line with recent data (PZT/$\mathrm{Pt}$ or $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3/\mathrm{SRO}$).

Conventional transparent materials at optical frequencies have refractive-index values (n) greater than unity—most commonly between about 1 and 4. This paper explores optical phenomena made possible by using materials with refractive indices less than unity. We focus primarily on fused silica ($\mathrm{Si}{\mathrm{O}}_2$), a well-studied dielectric with strong optical-phonon resonances in the mid infrared that result in a spectral region in which n < 1 with modest optical loss. Using this ubiquitous easy-to-deposit material, we demonstrate infrared-frequency external reflection, frustrated external reflection, and direct coupling to surface-plasmon polaritons from free space. Our work suggests that materials with refractive indices below unity can bring significant new functionalities to optical devices.

Recent experimental reports on in-plane proton conduction in reduced graphene oxide (rGO) films open a new way for the design of a proton-exchange membrane essential for fuel cells and chemical filters. In humid conditions, water molecules attached to the rGO sheet are expected to play a critical role in this membrane, but theoretical studies of their involvement are scarcely found in the literature. In this study, we investigate proton migration on water-adsorbed monolayer and bilayer rGO sheets using first-principles calculations to reveal the mechanism. We devise a series of models for water-adsorbed rGO films by systematically varying the reduction degree and water content, and we optimize their atomic structures in reasonable agreement with experiments using a density functional that accounts for van der Waals correction. After suggesting two different transport mechanisms, epoxy-mediated hopping and water-mediated hopping, we determine the kinetic activation barriers for these in-plane proton transports on the rGO sheets. Our calculations indicate that water-mediated transport is more likely to occur due to its much lower activation energy than epoxy-mediated transport and reveal new prospects for developing efficient solid proton conductors.

In bulk proper uniaxial ferroelectrics, double antiferroelectriclike hysteresis loops are observed in the case of ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystal. The quantum-anharmonic-oscillator model is proposed for the description of such a polarization-switching process. This phenomenon is related to the three-well local potential of the spontaneous-polarization fluctuations at a distinctive negative ratio of coupling constants that correspond to intersite interaction in the given sublattice and interaction between two sublattices of the modeled ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystal structure. The data obtained can be used for the development of multilevel-cell-type memory technology.

Throughout biology, geometric hierarchy is a recurrent theme in structures where strength is achieved with efficient use of material. Acting over vast timescales, evolution has brought about beautiful solutions to problems in mechanics that are only now being understood and incorporated into engineering designs. One particular example of structural hierarchy is found in the junction between stiff keratinized material and the soft biological matter within the hooves of ungulates. Using this biological interface as a design motif, we investigate the role of hierarchy in the creation of a stiff, robust interface between two materials. We show that through hierarchical design, we can manipulate the scaling laws relating constituent-material stiffness and overall interface stiffness under loading. Furthermore, we demonstrate that through use of a hierarchical geometry, we can reduce the maximum stress the materials experience for a given loading and tailor the ratio of maximum stresses in the constituent materials. We demonstrate that when two materials of different stiffness are joined, hierarchical geometries are linked with beneficial mechanical properties and enhanced tailorability of mechanical response.

We demonstrate persistent high degrees of spin polarization (SPD) up to 70% during light emission in $({\mathrm{In}}_{1-x}{\mathrm{Ga}}_x)\mathrm{As}$ quantum-dot–well (QD–QW) hybrid nanosystems, where QD excited states are laterally tunnel coupled through the adjacent two-dimensional QW potential depending on the QW thickness. Spin-polarized electrons are photo-excited by using circularly polarized light pulses. The decay time of spin relaxation, obtained by that of the SPD, is 70 times larger than the photoluminescence decay time. The temporally constant SPD is sustained by a selective transfer of minority spins among QDs after flipping from the majority spins, which is promoted by a moderate state filling of lower-energy spin sublevels in surrounding QDs. The spin transfer times are deduced as functions of the QW thickness and excited-spin density. These results can provide a precise control of lateral interdot spin-transfer dynamics and resultant suppression of spin relaxation in QD ensembles.

Formation of moiré superlattices is common in van der Waals heterostructures as a result of the mismatch between lattice constants and misalignment of crystallographic directions of the constituent two-dimensional crystals. Here we discuss theoretically electron transport in a van der Waals tunneling transistor in which one or both of the electrodes are made of two crystals forming a moiré superlattice at their interface. As a proof of concept, we investigate structures containing either an aligned graphene/hexagonal boron nitride heterostructure or twisted-bilayer graphene and show that negative differential resistance is possible in such transistors and that this arises as a consequence of the superlattice-induced changes in the electronic density of states and without the need for momentum-conserving tunneling present in high-quality exfoliated devices. We extend this concept to a device with electrodes consisting of aligned graphene on $\alpha {\text{-In}}_2{\mathrm{Te}}_2$ and demonstrate negative-differential-resistance peak-to-valley ratios of approximately $10$.

We report the fabrication and testing, at 4.2 K, of four-terminal $SI{S}^{\mathrm{\prime }}{F}_{1}I{F}_{2}S$ devices, where $S(S^{\mathrm{\prime }})$ denotes a superconductor ($\mathrm{Nb}$), $F_{1,2}$ denotes a ferromagnetic material ($\mathrm{Ni}$ and permalloy ($\mathrm{Py}$), respectively), and I denotes an insulator ($\mathrm{Al}{\mathrm{O}}_x$). The $F_{1}I{F}_2$ junction plays the role of a pseudo-spin valve, in which the magnetization vector of the $\mathrm{Py}$ layer can be reversed either by an externally applied magnetic field, or, potentially, by the electric current properly supplied to the device. The total magnetic moment of the $F_{1}I{F}_2$ junction, determined by two different magnetization orientations in the $F_1$ and $F_2$ layers, can be sensed by an adjacent $SI{S}^{\mathrm{\prime }}$ junction, resulting in two distinct maximum Josephson critical current values. Such controlled manipulation of the Josephson critical current offers the possibility of building a cryogenic memory cell based on the four-terminal hybrid S/F device. One of the advantages of this memory device is its compatibility with single-flux quantum circuit elements.

We simultaneously observe the Larmor precession of rubidium atoms in both the temporal and spectral domains using an electro-optically generated frequency comb. Rubidium vapor is optically pumped on the $F=2\to F^{\prime }=1$ hyperfine transition of the ${}^{87}\mathrm{Rb}{\mathrm{D}}_1$ manifold, while its response is observed with a frequency comb that spans 8 GHz of the spectrum with a temporal resolution of $9.78\mu \mathrm{s}$. The frequency comb modes experience optical rotation by interacting with the $F=2\to F^{\prime }=1,2$ transitions. The spectral and temporal resolution of the comb allows us to observe that there are two separate channels for polarized atoms to appear in the probe beam: one pathway where atoms travel directly from the pump to the probe region, and a secondary pathway that involves interaction of the polarized atoms with the cell walls. The unique features of the comb allow a direct estimate of the relative density of polarized atoms in the vapor cell, as well as a measurement of the quality of the antirelaxation coating on the cell walls. We show that rotation measurements with the comb approach the limits set by photon shot noise.

Time-frequency duality, which enables the control of optical waveforms by manipulating the amplitudes and phases of electromagnetic fields, plays a pivotal role in a wide range of modern optics. The conventional one-dimensional (1D) time-frequency duality has been successfully applied to characterize the behavior of classical light, such as ultrafast optical pulses from a laser. However, the 1D treatment is not enough to characterize quantum-mechanical correlations in the time-frequency behavior of multiple photons, such as the biphotons from parametric down-conversion. Two-dimensional (2D) treatment is essentially required, but has not been fully demonstrated yet due to technical problems. Here, we study the 2D time-frequency duality of biphotons, by measuring two-photon distributions in both the frequency and the time domain. It is discovered that the generated biphotons satisfy the Fourier-transform-limited conditions in a two-photon spectral distribution, but do not satisfy those conditions in the single-photon marginal distribution. Our study may pave the way for the quantum-optical synthesizer, the next-generation technology beyond the traditional optical synthesis in the time-frequency domain.

We report on the experimental evidence of the guiding of the lowest-order antisymmetric Lamb mode ($A_0$) through a compact waveguide consisting of aligned hollow pillars in between a square-latticed array of solid pillars on a thin slab. We analyze both numerically and experimentally the guiding achievements and we highlight the role played by the whispering gallery mode of the hollow pillars. In particular, we highlight the evolution of the displacement field pattern behind the waveguide that we relate to the emission of a quadrupolar wave interacting with the surrounding solid pillars. We also show the asymmetric transmission of a compact waveguide based on the same mechanism.

Temperature sensing with nitrogen-vacancy (NV) centers using quantum techniques is very promising, and further development is expected. Recently, the optically detected magnetic-resonance (ODMR) spectrum of a high-density ensemble of NV centers was reproduced with noise parameters [inhomogeneous magnetic field, inhomogeneous strain (electric field) distribution, and homogeneous broadening] of the NV center ensemble. In this study, we use ODMR to estimate the noise parameters of the NV centers in several diamonds. These parameters strongly depend on the spin concentration. This knowledge is then applied to theoretically predict the temperature sensitivity. Using the diffraction-limited volume of $0.1\mu {\mathrm{m}}^3$, which is the typical limit in confocal microscopy, we estimate the optimal sensitivity to be around $0.76\mathrm{mK}/\sqrt{\mathrm{Hz}}$ with an NV center concentration of $5.0\times {10}^{17}/\mathrm{c}{\mathrm{m}}^3$. This sensitivity is much higher than previously reported sensitivities, demonstrating the excellent potential of temperature sensing with NV centers.

The mixed spin chain compound, $\mathrm{Li}\mathrm{Cu}{\mathrm{Fe}}_2({\mathrm{VO}}_4{)}_3$, reaches a magnetically ordered state in two steps at $T_{\mathrm{N}2}=9.8\mathrm{K}$ and $T_{\mathrm{N}1}=8.2\mathrm{K}$ detected in magnetic-susceptibility χ and specific-heat $C_p$ measurements. Dielectric permittivity ε evidences these transitions by a sharp peak at $T_{\mathrm{N}2}$ and a steplike anomaly at $T_{\mathrm{N}1}$, both easily suppressed by an external magnetic field. These features in permittivity are preceded by a frequency-dependent relaxation-type anomaly at $T^{\ast }$ insensitive to the magnetic field. Mössbauer spectroscopy reveals a wide distribution of a hyperfine field between $T_{\mathrm{N}2}$ and $T_{\mathrm{N}1}$ while the first-principles calculations provide values of magnetic-exchange-interaction parameters. For $\mathrm{Cu}$, the orbital moment aligned in the same direction as the spin moment is substantial. This large orbital moment is important in driving the polarization through an inverse Dzyaloshinksii-Moriya interaction in a situation where the spatial symmetry gets broken in the magnetic structure. The data obtained suggest the spin-order-induced ferroelectricity is inherent for improper multiferroics.

We report very efficient spin-current generation by the spin Hall effect in the alloy ${\mathrm{Au}}_{0.25}{\mathrm{Pt}}_{0.75}$, which, as determined by two different direct spin-orbit torque measurements, exhibits a giant internal spin Hall ratio of ≥0.58 (antidamping spin-orbit torque efficiency of approximately 0.35 in bilayers with $\mathrm{Co}$), a relatively low resistivity of approximately 83 µΩ cm, an exceptionally large spin Hall conductivity of $\ge 7.0\times {10}^5{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$, and a spin diffusion length of 1.7 nm. This work establishes ${\mathrm{Au}}_{0.25}{\mathrm{Pt}}_{0.75}$ as a milestone spin-current generator that provides greater energy efficiency than that yet obtained with other heavy metals or with the topological insulators ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and $(\mathrm{Bi},\mathrm{Se}{)}_2{\mathrm{Te}}_3$. Our findings should advance spin-orbit torque-based fundamental research and benefit the development of fast, efficient spin-orbit torque-driven magnetic memories, skyrmion and chiral domain wall devices, and microwave and terahertz emitters.

A key ingredient for mechanical information processing with resonator networks is the possibility to tune the relevant mode frequencies independently and to operate the resonators in the strong coupling regime. We investigate a proof-of-principle realization of such a network based on two coupled highly tensile-stressed silicon nitride nanostring resonators. We demonstrate that the fundamental mode frequencies of both nanostrings can be tuned individually by a strong drive tone resonant with one of their higher-harmonic modes. Using this frequency tuning concept, we investigate the coherent dynamics of the two strongly coupled nanostring resonators acting as an effective classical two-level system. We show classical Rabi oscillations as well as Landau-Zener dynamics, enabling the controlled spacial transfer of mechanical excitation.

Scalable quantum computing is based on realizable accurate quantum gates. For neutral atoms, it is an outstanding challenge to design a high-fidelity two-qubit entangling gate without resorting to difficult techniques such as shaping laser pulses or cooling atoms to motional ground states. By using spin echo to suppress the blockade error, we propose an easily realizable controlled-phase Rydberg quantum gate of high intrinsic fidelity. In the context of spin echo, we show that the fundamental blockade error of the traditional Rydberg gate, of the order of $ϵ\sim {10}^{-3}$, actually results from two “clockwise” rotations of Rabi frequencies ${\overline{\mathrm{\Omega }}}_{\pm }=V\pm \sqrt{V^2+{\mathrm{\Omega }}^2}$. In our “echo” sequence, such an error can be suppressed to the order of ${ϵ}^2$ by adding two “anticlockwise” rotations with frequencies $-{\overline{\mathrm{\Omega }}}_{\pm }$. With the blockade error effectively removed, the error caused by Rydberg-state decay becomes the final fundamental limit to the gate accuracy, which, in principle, can be reduced beyond the level of ${10}^{-5}$. Furthermore, due to the small population involved in the “echo” process, the spin-echo gate is robust against the variation of Rydberg blockade caused by the drift of the qubits, so that it can still be much more accurate than that of a traditional Rydberg gate even for qubits cooled only to the submillikelvin regime.