The spin Hall effect is a major source of spin-orbit torques (SOTs), which allow efficient electrical manipulation of magnetization. So far, the spin Hall effect has been investigated using materials with strong spin-orbit coupling, such as 5d transition metals, and the spin Hall effect in light elements, which results in weak spin-orbit coupling, is often considered to be negligible. Here, we report an efficient spin Hall material that can be realized using 3d transition metals, namely, an amorphous $\mathrm{Co}$-$\mathrm{Ni}$-$\mathrm{B}$ alloy under a paramagnetic state. Despite no heavy elements being used in this material, we estimate the spin Hall angle of $\mathrm{Co}$-$\mathrm{Ni}$-$\mathrm{B}$ alloys to be about 0.10 via harmonic Hall measurements, which is comparable to that of 5d transition metals. A highly efficient spin Hall angle originates from the high-spin Hall conductivity of paramagnetic $\mathrm{Ni}$. We also demonstrate SOT-induced magnetization switching using the spin Hall effect on the $\mathrm{Co}$-$\mathrm{Ni}$-$\mathrm{B}$ alloy with comparable switching current density using a heavy-metal spin Hall material. Our findings provide an approach to realize the coexistence between highly efficient and heavy-element-free SOT-based devices.

This paper presents a framework for analyzing and designing cylindrical $\mathrm{omega}$-bianisotropic metasurfaces, inspired by mode matching and digital signal-processing techniques. Using the discrete Fourier transform, we decompose the electromagnetic field distributions into orthogonal cylindrical modes and convert the azimuthally varying metasurface constituent parameters into their respective spectra. Then, by invoking appropriate boundary conditions, we set up systems of algebraic equations, which can be rearranged to either predict the scattered fields of prespecified metasurfaces, or to synthesize metasurfaces, which support arbitrarily stipulated field transformations. The proposed framework facilitates the efficient evaluation of field distributions that satisfy local power conservation, which is one of the key difficulties involved with the design of passive and lossless scalar metasurfaces. It represents a promising solution to circumvent the need for active components, controlled power dissipation, or tensorial surface polarizabilities in many state-of-the-art conformal metasurface-based devices. To demonstrate the robustness and the versatility of the proposed technique, we design several devices intended for different applications and numerically verify them using finite-element simulations.

We present a detailed experimental optical study supported by theoretical modeling of $\mathrm{In}\mathrm{As}$ quantum dots (QDs) embedded in an $(\mathrm{In},\mathrm{Al},\mathrm{Ga})\mathrm{As}$ barrier lattice matched to $\mathrm{In}\mathrm{P}$(001) grown with the use of a ripening step in molecular beam epitaxy. The method leads to the growth of in-plane symmetric QDs of low surface density, characterized by a multimodal size distribution resulting in a spectrally broad emission in the range of $1.4$–$2.0\mu \mathrm{m}$, essential for many near-infrared photonic applications. We find that, in contrast to the $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ system, the multimodal distribution results here from a two-monolayer difference in QD height between consecutive families of dots. This may stem from the long-range ordering in the quaternary barrier alloy that stabilizes QD nucleation. Measuring the photoluminescence (PL) lifetime of the spectrally broad emission, we find a nearly dispersionless value of $1.3\pm 0.3$ ns. Finally, we examine the temperature dependence of emission characteristics. We underline the impact of localized states in the wetting layer playing the role of carrier reservoir during thermal carrier redistribution. We determine the hole escape to the $(\mathrm{In},\mathrm{Al},\mathrm{Ga})\mathrm{As}$ barrier to be a primary PL quenching mechanism in these QDs.

The existence of nonlinearity in a spintronic device is indispensable for spin-based complex signal-processing operations. Here we measure the presence of electron-spin-dependent nonlinearity in a van der Waals heterostructure spintronic device. We further demonstrate its application for basic analog operations such as essential elements of amplitude modulation and frequency sum (heterodyne detection) on pure spin signals, by exploiting the second-harmonic generation of the spin signal due to nonlinear spin injection. Furthermore, we show that the presence of nonlinearity in the spin signal has an amplifying effect on the energy-dependent conductivity-induced nonlinear spin-to-charge conversion effect. The interaction of the two spin-dependent nonlinear effects in the spin-transport channel leads to a highly efficient modulation of the spin-to-charge conversion effect, which in principle can also be measured without using a ferromagnetic detector. These effects are measured both at room and low temperatures, and are suitable for their applications as nonlinear circuit elements in the fields of advanced spintronics and spin-based neuromorphic computing.

Optical fibers play a key role in many different fields of science and technology. In particular, fibers with a diameter of several micrometers are intensively used in photonics. For these applications, it is often important to precisely know and control the fiber radius. Here we describe a technique to determine the radius profile of an optical microstructure with ultrahigh precision from a single optical image. Using a basic microscopy setup, we demonstrate our method by measuring the axial radius variation along a 30-$\mu \mathrm{m}$-diameter silica fiber with precision less than 0.3 Å. The axial resolution is tens of micrometers, and the measurement range is more than 0.5 mm. Our method relies on imaging the fiber’s whispering-gallery modes in which the speed of light propagating along the fiber axis is strongly reduced. Imaging those whispering-gallery modes, we infer the local radius variations of the optical microstructure with ultrahigh precision. Because of the performance and simplicity of implementation, we are convinced that our scheme has high potential for precision metrology and optical sensing.

Short-wavelength-infrared (SWIR; 1.4–3.0 µm) photodetection is important for various applications. Inducing a low-cost silicon-compatible material, such as germanium, to detect SWIR light would be advantageous for SWIR applications compared with using conventional (III-V or II-VI) SWIR materials. Here, we present a scalable nonequilibrium method for hyperdoping germanium with gold for dopant-mediated SWIR photodetection. Using ion implantation followed by nanosecond pulsed laser melting, we obtain a single-crystal material with a peak gold concentration of 3 × ${10}^{19}{\mathrm{cm}}^{-3}$ $({10}^3$ times the solubility limit). This hyperdoped germanium has fundamentally different optoelectronic properties from those of intrinsic and conventionally doped germanium. This material exhibits sub-band-gap absorption of light up to wavelengths of at least 3 µm, with a sub-band-gap optical absorption coefficient comparable to that of commercial SWIR photodetection materials. We show that germanium hyperdoped with gold exhibits sub-band-gap SWIR photodetection at room temperature, in contrast with previous doped-germanium photodetector studies, which only show a low-temperature response. This material is a potential pathway to low-cost room-temperature silicon-compatible SWIR photodetection.

Ensembles of bismuth-donor spins in silicon are promising storage elements for microwave quantum memories due to their long coherence times, which exceed seconds. The operation of an efficient quantum memory requires the achievement of critical coupling between the spin ensemble and a suitable high-quality factor resonator—this in turn requires a thorough understanding of the line shapes for the relevant spin-resonance transitions, particularly considering the influence of the resonator itself on line broadening. Here, we present pulsed electron-spin-resonance measurements of ensembles of bismuth donors in natural silicon, above which niobium superconducting resonators have been patterned. By studying spin transitions across a range of frequencies and fields, we identify distinct line-broadening mechanisms and, in particular, those that can be suppressed by operating at magnetic-field-insensitive “clock transitions.” Given the donor concentrations and resonator used here, we measure a cooperativity $C\sim 0.2$ and based on our findings we discuss a route to achieve unit cooperativity, as required for a quantum memory.

Energetic photoinduced hot electrons have been attracting increased scientific attention owing to their potential applicability in numerous photoelectrical and photochemical processes. Normally, the energy of electrons quickly converts into heat by ultrafast cooling, which is considered as the bottleneck for high-efficiency utilization of hot electrons. In this work, we explore intentionally heavily $p$-doped graphene stacks by degenerate femtosecond pump-probe spectroscopy, and observe an excitation enhancement of hot electrons at weak pump fluence. The time scale of hot-electron excitation is of the same order as that of fast decay via electron-electron and electron-optical-phonon scattering in our experiments. Physically, both Auger processes and population inversion are suppressed in this system, yet it becomes possible for the conduction bands to be effectively evacuated within the pulse duration through the ultrafast cooling of hot electrons, which may lead to an enhanced excitation of hot electrons. This excitation enhancement can be further strengthened by multiple layer-stacking processes or a thermal annealing pretreatment. The optical absorption of graphene stacks increases correspondingly, exhibiting a value larger than the linear limit at low pump fluence. Furthermore, the absorption modulation depth can reach approximately 0.79% in monolayer graphene for a small pump-fluence change ($<20\mu {\mathrm{J/cm}}^2$), further increasing to approximately 1.2% and approximately 2.5% in double- and triple-stacking graphene layers, respectively, indicating that the absorption can be sufficiently altered by an extremely small variation of pump fluence. These outcomes can be applied for low-cost pulse operations. We suggest that this effect can have potential applications to harvesting energy from excited hot electrons, and may provide a unique way to achieve high-speed modulators, photodetectors, solar cells, and photocatalysts.

We perform a comparative study on the transfer velocity of charge carriers across two-dimensional and (2D) three-dimensional (3D) graphene-silicon and 3D-3D $\mathrm{Au}$-silicon Schottky junctions. Within the framework of classical thermionic emission theory, the transfer velocity of carriers across the $\mathrm{Au}$-$\mathrm{Si}$ interface is determined as $\overline{v}\sim {10}^7$ cm/s that is thoroughly consistent with the average thermal velocity of carriers in silicon. In contrast, the graphene-$\mathrm{Si}$ interface discloses a much lower transfer velocity of $\overline{v}\sim {10}^1$ cm/s, i.e., nearly 6 orders of magnitude smaller than the thermal velocity of carriers in silicon. Utilizing the $T^1$ universal scaling law for vertical 2D-3D Schottky junctions, we determine the effective out-of-plane velocity of charge carriers in graphene as $v_{\perp }^{\ast }\sim {10}^0$ cm/s. This indicates current flow across the graphene-$\mathrm{Si}$ contact is substantially limited by $v_{\perp }^{\ast }$. Moreover, by analyzing experimental activation-energy data, we uncover a general correlation between the kinetics of charge carriers and the potential barrier at the graphene-$\mathrm{Si}$ interface. It is found that the transfer velocity increases almost exponentially with the Schottky barrier height, which cannot be explained in the framework of classical theory. Considering nonconserving ${\mathbf{k}}_{\parallel }$ scattering strength as an exponential function of Schottky barrier height, $T^1$ scaling theory leads to a comprehensive interpretation of experimental data. This study sheds light on the electronic processes across 2D-3D interfaces.

Self-injection locking to a high-quality-factor microresonator is a key component of various up-to-date photonic applications, including compact narrow-linewidth lasers and microcomb sources. For optimal construction of such devices, it is necessary to know the parameters of the locking mode, which is a challenging task. We experimentally develop and verify an original technique based on the fundamentals of the self-injection locking effect, which allows determining the key parameters of the mode of the microresonator (quality factor, vertical index) as well as the laser diode parameters. We demonstrate that this method can be used in the spectral ranges where conventional methods are not applicable, for example, in the mid-IR. We stabilize a $2.64\mu \mathrm{m}$ laser diode by a high-$Q$ whispering gallery mode microresonator made of crystalline silicon. Using the elaborated technique, the quality factor of the microresonator is determined to be $5\times {10}^8$.

Photoisomeric molecules rearrange their structure when exposed to light, which alters their chemical, electronic, mechanical, as well as vibrational properties. The present study explores the possibilities to tune the thermal transport across molecular junctions by using photoisomeric molecules. The effect of isomeric switching on phonon transport through single-molecule junctions linking two macroscopic reservoirs is investigated using density-functional-theory-based tight-binding calculations and Green-function formalism. The junctions are built using azobenzene and its derivatives (azobiphenyl and azotriphenyl) that display photoisomeric behavior. Effects of system setup on the heat current and the switching coefficient are studied systematically. Dependence on the molecular species, the choice of reservoir, as well as the type of linkers that bind the molecules to the reservoir are investigated with calculating the phonon-transmission spectra and temperature-dependent thermal conductance values. The results show that thermal conductance can be altered significantly by switching the molecule from trans- to cis-configuration since all molecules yield higher conductances in trans-configurations than their cis-configurations at temperatures higher than 50 K. In the low-temperature range, results reveal considerable switching coefficients exceeding $50\mathrm{\%}$. At room temperature, the switching coefficient can be as high as $20\mathrm{\%}$. It is shown that the effect is robust under the variation of both the molecular species and the linkers.

Layered structures are widely used in optics to control wave reflection and propagation at interfaces but their thickness relative to the wavelength hinders their application in the radio-frequency regime. Here, we design and experimentally demonstrate cascaded metasurfaces with deeply subwavelength thickness that provide both antireflection and wavefront manipulation. We describe the systematic synthesis of the metasurface layers from a prescribed scattering response and demonstrate designs capable of providing near-perfect microwave transmission through glass as well as antireflection focusing of wireless signals from air into water.

Based on the metal-insulator transition of two phase-change materials (PCMs) with asymmetric thermal conductivities and emissivities sensitive to temperature, a radiative thermostat capable of maintaining a constant temperature without consuming energy is proposed. This is done by deriving explicit expressions for the heat flux and temperature profiles of the thermostat surrounded by these PCMs, and demonstrating that the thermostat can be maintained at a temperature nearly equal to the common transition temperature of both PCMs as the environmental temperature changes by more than 20 K. I show that the combined operation of two PCMs is much better than two non-PCMs to develop energy-free thermostats, especially those supporting poor thermal conduction or dominant heat radiation. The obtained results thus establish a fundamental theoretical method that exploits the nonlinear heat transport driven by electrons, phonons, and photons to maximize temperature preservation and thermal insulation.

The perpendicular magnetic anisotropy at the $\mathrm{Co}$-oxide interface in $\mathrm{Pt}/\mathrm{Co}/{\mathrm{MO}}_x$ (${\mathrm{MO}}_x={\mathrm{Mg}\mathrm{O}}_x$, $\mathrm{Al}{\mathrm{O}}_x$, $\mathrm{Tb}{\mathrm{O}}_x$) is modified by an electric field using a 10-nm-thick ${\mathrm{Zr}\mathrm{O}}_2$ as a solid electrolyte. The large voltage-driven modification of the interfacial magnetic anisotropy and the nonvolatility of the effect is explained in terms of the migration of oxygen ions toward or away from the $\mathrm{Co}/{\mathrm{MO}}_x$ interface. While the effect is reversible in ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Al}\mathrm{O}}_x$ and ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Tb}\mathrm{O}}_x$, where the $\mathrm{Co}$ layer can be oxidized or reduced, in ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Mg}\mathrm{O}}_x$ the effect is found to be irreversible. We propose that these differences may be related to the different nature of the ionic conduction within the ${\mathrm{MO}}_x$ layers.

Topological states of light open up exciting possibilities in quantum photonics, promising the topological protection of quantum entanglement. Here, we put forward an approach to realizing the topological states of photon pairs mediated by the effective density-dependent coupling, which is manifested as the dependence of the tunneling amplitude on the number of photons. As a specific platform, we investigate the arrays of nearest-neighbor coupled transmon qubits, where the effective density-dependent coupling is engineered by inserting auxiliary frequency-detuned resonators. We prove the topological origin of the designed model by the direct evaluation of the Zak phase, highlighting the feasibility of our proposal for state-of-the-art fabrication technologies.

The molten-pool flow, particularly the keyhole effect, plays a critical role in the formation of defects in additive-manufacturing and welding processes. In this study, we derive an evaporation model for metal alloys considering the gas-flow structure and material composition and implement it in a multiphysics thermal-fluid flow model, which utilizes the volume of fluid (VOF) in the finite volume method (FVM) to capture free surfaces and the ray-tracing method to track multireflections of laser within the keyhole. The current derived evaporation model is validated against in situ x-ray imaging results via multiple cases: (1) stationary laser melting of a $\mathrm{Ti}\text{−}6\mathrm{Al}\text{−}4\mathrm{V}$ base plate under 1-atm ambient pressure, (2) stationary laser melting of a 304L stainless-steel base plate under 0.0002-atm ambient pressure, (3) laser scanning of a $\mathrm{Ti}\text{−}6\mathrm{Al}\text{−}4\mathrm{V}$ base plate under 1-atm ambient pressure. The simulation results indicate that our evaporation model is applicable for both common and near-vacuum environment, while Anisimov’s evaporation model, which is widely used in keyhole simulation, is unsuitable in near-vacuum keyhole simulation. Moreover, the absorbed energy distribution, recoil pressure, $z$-direction recoil force and keyhole growth are analyzed in the simulations.

We propose a postselection technique, based on quantum error detection, for quantum key distribution (QKD) systems that run over quantum repeaters with encoding. In such repeaters, quantum error-correction techniques are used for entanglement distillation. By developing an analytical approach to study such quantum repeaters, we show that, in the context of QKD, it is often more efficient to use the error-detection, rather than the error-correction, capability of the underlying code to sift out cases where an error has been detected. We implement our technique for three-qubit repetition codes by modeling different sources of error in crucial components of the system. We then investigate in detail the impact of such imperfections on the secret key generation rate of the QKD system, and how one can use the information obtained during entanglement swapping and decoding stages to maximize the rate. For benchmarking purposes, we specify the maximum allowed error rates in different components of the setup below which positive key rates can be obtained.