Two-dimensional (2D) materials are becoming increasingly popular as a platform for studies of quantum phenomena and for the production of prototype quantum technologies. Quantum emitters in 2D materials can host two-level systems that can act as qubits for quantum information processing. Here, we characterize the behavior of position-controlled quantum emitters in hexagonal boron nitride at cryogenic temperatures. Over two dozen sites, we observe an ultranarrow distribution of the zero phonon line at approximately 436 nm, together with strong linearly polarized emission. We employ resonant excitation to characterize the emission lineshape and find spectral diffusion and phonon broadening contribute to linewidths in the range 1–2 GHz. Rabi oscillations are observed at a range of resonant excitation powers, and under 1-µW excitation a coherent superposition is maintained up to 0.90 ns. Our results are promising for future employment of quantum emitters in h-BN for scalable quantum technologies.

Insufficient operational lifetime continues to be one of the most challenging problems for organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF). Here, we study the degradation of single-layer OLEDs based on the TADF emitter 9,10-bis[4-(9H-carbazol-9-yl)-2,6-dimethylphenyl]-9,10-diboraanthracene. After quantitatively describing the current-voltage (J-V) and efficiency curves after electrical driving, we use a numerical model to reproduce the voltage increase and light decrease during degradation. We demonstrate that the fundamental mechanism behind OLED degradation is electron- and hole-trap formation due to triplet-polaron annihilation, the same as that reported for polymer light-emitting diodes. Other mechanisms, such as triplet-triplet annihilation and singlet-triplet annihilation, do not match experimental data. Understanding of the degradation mechanism allows us to predict how a broader emission zone in a thicker single-layer TADF OLED affects its lifetime, resulting in an improved stability with a $T_{80}$ lifetime of 1050 h at an initial luminance of 1000 cd ${\mathrm{m}}^{-2}$.

Accurate modeling of shallow impurities in semiconductors through first-principles density-functional-theory calculations is challenging due to the delocalized nature of the impurity wave function. The situation could be more complicated for shallow impurity complexes where the interactions between impurities lead to a large perturbation to the host. In this work, the shallow acceptor levels of the group-IIIA acceptor-carbon ($A$-C, with $A$=B, Al, $\mathrm{Ga}$, $\mathrm{In}$, $\mathrm{Tl}$) complexes in silicon are studied using a potential-patching method combined with a hybrid-functional correction. The potential-patching method removes the artificial interaction between periodic images and allows the calculations of large supercells containing over ${10}^4$ atoms to obtain converged acceptor levels. The correction based on hybrid-functional calculations overcomes the underestimation of the ionization energies predicted by semilocal exchange-correlation functionals, resulting in good agreements with experiments. The $A$-C complexes are found to have smaller acceptor ionization energies than the corresponding single-$A$ substitutional acceptors. The origin of the ionization energy reduction is further analyzed, and the roles of the chemical electronic effect and the strain-field effect are clarified. Our results indicate that the combination of the potential-patching method and the hybrid-functional correction could be a feasible approach for accurate simulations of shallow impurities and their complexes.

We present a black hole effect by strategically leveraging a conformal mapping in elastic continuum with a curved-space framework, which is less stringent compared to a Schwarzschild model transformed to isotropic refractive-index profiles. In the conformal map approach, the two-dimensional point singularity associated to the black hole effect is accomplished by physical plates with near-to-zero thickness. The analog gravity around the singularity results in highly confined energy and lagged timings within a branch cut of the conformal map. These effects are quantified both numerically and experimentally in reference to control trials in which the thickness is not modulated. The findings would deepen our understanding of the elastic analog in mimicking gravitational phenomena, as well as establish the elastic continuum framework for developing a generic design recipe in the presence of the index singularity. Geometric landscapes with elastically curved surfaces would be applicable in a variety of applications such as sensing, imaging, vibration isolation, and energy harvesting.

We present a compact high-resolution absolute-gravity gradiometer based on dual ${}^{85}$$\mathrm{Rb}$ atom interferometers using stimulated Raman transitions. A baseline of $L=44.5$ cm and an interrogation time of $T=130$ ms are realized in a sensor head with a volume of only 95 l. Many experimental parameters are optimized to improve the short-term sensitivity, whereas interleaving measurements by reversing the direction of the Raman wave vector are implemented to improve the long-term stability. After an averaging time of 17 000 s, a phase resolution of $104\mu \mathrm{rad}$ is achieved, which corresponds to a gravity-gradient resolution of 0.86 E (1 E = $1\times {10}^{-9}$/${\mathrm{s}}^2$). After the evaluation and correction of systematic errors induced by light shift, the residual Zeeman shift, the Coriolis effect, the self-attraction effect, etc., the instrument serves as an absolute-gravity gradiometer and with it the local gravity gradient is measured to be 3114 (53) E. This instrument combines the merits of high-resolution, high-compactness and high-accuracy, and has good application prospects in many fields.

The interlayer exchange coupling (IEC) in synthetic antiferromagnets (SAFs) is attracting considerable attention in the spintronics community. While exciting physics is revealed, systematic control of the IEC via tuning the thickness of the magnetic layer is yet to be adequately addressed in SAFs with perpendicular magnetic anisotropy (PMA). In this work, we experimentally investigate this aspect in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ru}/{\mathrm{Fe}}_{0.55}{\mathrm{Tb}}_{0.45}$ [ferromagnet (FM)/$\mathrm{Ru}/$ferrimagnet (FIM)] multilayers with PMA, which possess a broader material-parameter space beyond what can be achieved with traditional SAFs. More importantly, the incorporation of the $\mathrm{Fe}\text{−}\mathrm{Tb}$ FIM layer into the SAF multilayers allows us to directly probe the layer-resolved magnetization reversal by utilizing the anomalous Hall effect and standard magnetometry measurements. Our findings are further verified by x-ray magnetic circular dichroism measurements by providing element-resolved magnetic behaviors. Moreover, through systematic control of the IEC, IEC-modulated SAFs with different magnetization reversal behaviors can be acquired, which is further studied by performing magnetic simulations. Our results suggest that systematic control of the IEC through tuning the thickness of FIM layers can be achieved in FM/$\mathrm{Ru}/$FIM multilayers, which could be useful for designing SAF-based magnetic memory and sensing devices.

Mechanical metamaterials could exhibit tremendous design potential through synergistic material distribution and structural design. However, the existing designs mainly focus on the structures and neglect the material distributions. We propose a digital design and optimization method for lattice metamaterials using machine learning and an evolutionary algorithm taking into account both material distributions and structural design. The machine learning model is developed to predict mechanical behaviors accurately, based on the database generated by python-assisted finite-element simulations with random material and structural parameters. The optimal design is realized by the evolutionary algorithm method, which can quickly find the target mechanical responses in a broad design space. A digital voxel model is established to three-dimensionally print the designed multimaterial lattice metamaterials. The developed framework can be used to match multiple targets, such as reproducing the stress-strain and Poisson's ratio curves of biological tissues simultaneously. Lattice metamaterials with customized lateral profiles are also demonstrated. This work expands the design space of lattice metamaterials and is of great interest in flexible electronics and soft robotics.

Asymmetric spin-wave propagation in magnetic nanostructures has received significant attention due to the potential applications of magnon-based devices. In curved nanostructures and planar multilayers, the classical dipole-dipole interaction induces a significant frequency nonreciprocity in which two counterpropagating waves excited at the same frequency exhibit different wavelengths. This work proposes a cylindrical synthetic antiferromagnet as a potential three-dimensional waveguide design to generate nonreciprocal spin waves. The magnetochiral properties emerge from two mechanisms: the asymmetric interlayer dipolar coupling and the asymmetric dipolar coupling of the curved membrane. It is demonstrated that the cylindrical bilayer presents a notable spin-wave asymmetry induced by the combined action of antiparallel magnetic vortices and the curvature of the inner and outer surfaces. A substantial frequency range with waves having only a negative phase velocity is predicted, where unidirectional wave propagation is allowed. It is also found that the nonreciprocity reaches a constant value as the curvature decreases, which is an essential advantage over isolated nanotubes, where the frequency shift vanishes at a large radius. Besides, analytical expressions are proposed to predict the frequency shift in the case of coupled cylindrical shells. These results are relevant from fundamental and practical points of view since magnetochirality is a crucial ingredient in visualizing future spin-wave-based logic devices.

We report the measurement of efficient charge-to-spin conversion at room temperature in Weyl semimetal-ferromagnet heterostructures with both oxidized and pristine interfaces. Polycrystalline films of the Weyl semimetal, $\mathrm{Ta}\mathrm{As}$, are grown by molecular beam epitaxy on (001) $\mathrm{Ga}\mathrm{As}$ and interfaced with a metallic ferromagnet (${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$). Spin-torque ferromagnetic resonance (ST FMR) measurements yield a spin-torque ratio as large as ${\xi }_{\mathrm{FMR}}=0.45\pm 0.25$ in samples with an oxidized interface. By studying ST FMR in these samples with varying ${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$ layer thickness, we find that the dampinglike-torque efficiency is ${\xi }_{\mathrm{DL}}=1.36\pm 0.66$. In samples with a pristine (unoxidized) interface, the spin-torque ratio is ${\xi }_{\mathrm{FMR}}=-0.27\pm 0.14$ and has opposite sign to that observed in oxidized samples. We also find a lower bound on the spin Hall conductivity ($424\pm 110\hslash /e$ S/cm), which is surprisingly consistent with theoretical predictions for the single-crystal Weyl semimetal state of $\mathrm{Ta}\mathrm{As}$.

Wireless power transfer from a single transmitter to multiple receivers has broad applications in consumer devices and industrial systems. However, current approaches for achieving selective wireless power transfer among multiple receivers rely on complex tuning schemes, which suffer from reduced efficiency when there are slight deviations from the optimal operating point. Here, we generalize prior analyses of $PT$-symmetric wireless power transfer to the case of multiple receivers and show that the asymmetrical energy distribution arising from operation in a bistable region can solve such problems. Using a system with three equally distant receivers, we experimentally demonstrate wireless power transfer to a selected receiver with 65% efficiency and 99% selectivity without any tuning of the transmitter, achieving localization of 83% of the total energy at the target.

Hybrid photonic-plasmonic cavities can be tailored to display high $Q$ factors and extremely small mode volumes simultaneously, which ultimately results in large values of the Purcell factor, $F_P$. Amongst the different hybrid configurations, those based on a nanoparticle-on-a-mirror plasmonic cavity provide one of the lowest mode volumes, though so far their operation has been constrained to wavelengths below $1\mu \mathrm{m}$. Here, we propose a hybrid configuration consisting of a silicon photonic crystal cavity with a slot at its center in which a gold nanoparticle is introduced. This hybrid system operates at telecom wavelengths and provides high $Q$-factor values ($Q\approx {10}^5$) and small normalized mode volumes ($V_m\approx {10}^{-4}$), leading to extremely large Purcell-factor values, $F_P\approx {10}^7-{10}^8$. The proposed cavity could be used in different applications such as molecular optomechanics, bio- and chemosensing, efficient quantum emitters, or enhanced Raman spectroscopy in the relevant telecom-wavelength regime.

High-performance quantum memories are an essential component for regulating temporal events in quantum networks. As a component in quantum repeaters, they have the potential to support the distribution of entanglement beyond the physical limitations of fiber loss. This will enable key applications such as quantum key distribution, network-enhanced quantum sensing, and distributed quantum computing. Here, we present a quantum memory engineered to meet real-world deployment and scaling challenges. The memory technology utilizes a warm rubidium vapor as the storage medium, and operates at room temperature, without the need for vacuum and/or cryogenic support. We demonstrate performance specifications of high-fidelity retrieval (95%) and low operation error $({10}^{-2})$ at a storage time of $160\mu \mathrm{s}$ for single-photon level quantum memory operations. We further show a substantially improved storage time (with classical-level light) of up to 1 ms by suppressing atomic diffusions. The device is housed in an enclosure with a standard 2U rackmount form factor, and can robustly operate on a day scale in a noisy environment. This result marks a step toward implementing quantum networks in the field.

We present designs for silicon-on-insulator nanowaveguide sources capable of generating polarization-entangled photon pairs with high concurrence (approximately $0.98$) across the S, C, and L bands (approximately $1475–1625$ nm) in the telecommunications spectrum via degenerate-pump spontaneous four-wave mixing. Silica-clad waveguides with $\mathrm{Si}$ film thickness 280 nm, width in the range 700–800 nm, and length up to 1 cm are shown to be optimum, generating near-perfect entangled pairs over a bandwidth >100 nm around 1550 nm. Further, the use of relatively narrow pump bandwidths (<10 GHz) can still yield a concurrence $>0.95$ in waveguides up to 7 mm long despite the presence of large polarization-mode dispersion in these high-index-contrast structures. Such broadband integrated sources can be employed in multichannel wavelength-division multiplexed entanglement-based quantum key distribution protocols over standard optical-fiber networks.

Acoustic metasurfaces have the potential to be a promising route for the development of compact sound-absorptive devices with ventilation capability. However, the dissipation mechanism of limited plane wave fronts and elaborate geometry configured by extensive parametric sweepings severely restricts the available designs. Here, via establishing a cylindrical transfer matrix method combined with an inverse-design strategy of a genetic algorithm, we construct an optimized ultrasparse (with filling ratio of framework at 50%) dissipated-sound metacage (DSM), which theoretically (experimentally) demonstrates 99.1% (98.2%) absorptance for omnidirectionally radiated cylindrical sound at a prescribed frequency of 260 Hz in deep-subwavelength thickness. The perfect absorption is ascribed to the mode hybridization between two resonant meta-atoms in which one acts as a dissipated mode and the other as an acoustically soft boundary. Moreover, the balance between thickness and sparsity is investigated by demonstrating DSMs that show different filling ratios of air channels. We finally extend the paradigm into a broadband regime for exploring more potential practicability.

We experimentally demonstrate the difference between a ridge polariton laser and a conventional edge-emitting ridge laser operating under electron-hole population inversion. The horizontal laser cavities are $20$–$60$-$\mu \mathrm{m}$-long $\mathrm{Ga}\mathrm{N}$ etched-ridge structures with vertical Bragg reflectors. We investigate the laser threshold under optical pumping and assess quantitatively the effect of a varying optically pumped length. The laser effect is achieved for an exciton reservoir length of just 15% of the cavity length, which would not be possible in a conventional ridge laser, with an inversion-less polaritonic gain about 10 times larger than in equivalent $\mathrm{Ga}\mathrm{N}$ lasers. This combination of a very short injection section and a strong gain paves the way for compact microlasers with nonlinear functionalities for integrated photonics.

The visualization of transparent phase objects such as living cells and tissues plays an important role in biological research. Transparency and invisibility caused by weak scattering and absorption make it challenging to image phase objects. Here, we propose differential microscopy based on the photonic spin Hall effect at a simple glass interface. We find that the photonic spin Hall effect can perform a spatial differentiation operation on the phase distribution, and therefore is extremely sensitive to the structures of phase objects. By incorporating this into a bright-field microscope, high-contrast images of pure phase objects can be achieved. We further reconstruct the phase distribution by biased imaging, which provides potential possibilities for high-resolution reconstruction and quantitative analysis of phase images. Compared with the conventional differential-interference-contrast microscope, which requires the introduction of complicated phase-contrast devices into a bright-field microscope, our proposed photonic spin-Hall differential microscopy requires only a simple glass interface. The combination of the photonic spin Hall effect and a bright-field microscope enables compact and low-cost differential microscopy, which may lead to important applications in versatile and high-contrast biological imaging.

A common scientific inverse problem is the placement of magnets that produce a desired magnetic field inside a prescribed volume. This is a key component of stellarator design and recently permanent magnets have been proposed as a potentially useful tool for magnetic field shaping. Here, we take a closer look at possible objective functions for permanent-magnet optimization, reformulate the problem as sparse regression, and propose an algorithm that can efficiently solve many convex and nonconvex variants. The algorithm generates sparse solutions that are independent of the initial guess, explicitly enforces maximum strengths for the permanent magnets, and accurately produces the desired magnetic field. The algorithm is flexible, and our implementation is open source and computationally fast. We conclude with two permanent-magnet configurations for the NCSX and MUSE stellarators. Our methodology can be additionally applied for effectively solving permanent-magnet optimizations in other scientific fields, as well as for solving quite general high-dimensional constrained sparse-regression problems, even if a binary solution is required.

Nanophotonic devices based on two-dimensional crystals enable various technological applications, ranging from biosensing to quantum communication. In those devices, plasmonic antennas have been extensively explored in the photon-polariton conversion, as they allow field confinement within subdiffraction volumes. Despite the wide-reaching potential of polaritonics, essential rules for engineering polariton launchers are still to be developed, as the influence of the antenna geometry and source parameters on the polariton directivity is unknown. Here, we address this issue by combining concepts of radio-frequency antenna design with established polariton modeling. As an input for the model, we simulate hyperbolic phonon polariton waves in hexagonal boron nitride launched by metallic antennas. By adapting a Fresnel and Fraunhofer field regions formalism to polaritonics, we optimize the model accuracy and graphically represent several launching parameters as radiation patterns. Furthermore, we demonstrate how our framework can be applied to real antennas by employing it to experimental near-field images of polaritons reported in the literature. Our results show that the antenna geometry, its resonance order, and the angle of incidence of the light can strongly influence the polariton-wave pattern in the crystal. We foresee that our framework can add to further studies approaching optimized polariton launching and help the engineering of nanophotonic chips.

This paper is a contribution to the Physical Review Applied collection titled Photovoltaic Energy Conversion.

The hitherto subdued power conversion efficiencies of $\mathrm{Sn}$-based hybrid perovskite solar cells are generally attributed to severe nonradiative recombination; however, the responsible deep-level defects are still unclear. Here, we report an important nonradiative energy loss mechanism in the prototypical FA${\mathrm{Sn}\mathrm{I}}_3$ [FA = $\mathrm{HC}({\mathrm{NH}}_2{)}_2$, formamidinium]. High-density tin vacancies $(V_{\mathrm{Sn}})$ can effectively capture hydrogen to form $V_{\mathrm{Sn}}-{\mathrm{H}}_2$ complexes that act as highly detrimental nonradiative recombination centers. We quantitatively show that they can give rise to strong carrier recombination and thus energy loss due to a high nonradiative recombination rate constant. These key findings identify a hidden yet critical origin for the low performance of FA${\mathrm{Sn}\mathrm{I}}_3$-based devices and highlight the significance of controlling the hydrogen environment in the development of broad high-efficiency nontoxic halide perovskite device applications.

Conventional circulators are made of magnetic ferrites and suffer from a cumbersome architecture, incompatibility with integrated circuit technology, and inability for high-frequency applications. To overcome these limitations, here we propose a low-noise, lightweight, low-profile, and linear nonmagnetic circulator comprising a nonreciprocal time-varying phase shifter. This circulator is composing a nonreciprocal temporal phase shifter and two reciprocal delay-line-based phase shifters. The proposed nonreciprocal temporal phase shifter is based on the generation of time-harmonic signals, enforcing destructive interference for undesired time harmonics and constructive interference for desired time harmonics at different locations of the structure. Such a unique task is accomplished through two phase-engineered temporal loops. The phase and frequency of these two loops are governed by external signals with different phases, imparting an effective electronic angular momentum to the system. We observe a large isolation level of greater than 32.3 dB, a low noise figure of less than 2.5 dB, a P1dB of +30.2 dBm, and IIP3 of +43.5 dBm. Furthermore, this circulator is endowed with a reconfigurable architecture and can be directly embedded in a conventional integrated circuit (IC) technology to realize a class of high-power handling and linear IC circulators.