We propose a two-dimensional material, δ-CS, and show that it is a direct-band-gap semiconductor with strong absorption of solar radiation. Power conversion efficiencies of 7.0% to 20.1% are predicted for solar cells with δ-CS as a donor and a transition metal dichalcogenide as an acceptor. δ-CS also excels in terms of having an exceptionally large negative in-plane Poisson’s ratio. Its ferroelasticity with moderate switching barrier is promising for applications in shape memory devices.

Natural hyperbolic two-dimensional systems are a fascinating class of materials that could open alternative pathways to the manipulation of plasmon propagation and light-matter interactions. Here, we present a comprehensive study of the optical response in ${\mathrm{T}}_d$-${\mathrm{W}\mathrm{Te}}_2$ by means of density-functional and many-body perturbation theories. We show how monolayer ${\mathrm{W}\mathrm{Te}}_2$ with in-plane anisotropy sustains hyperbolic plasmon polaritons, which can be tuned via chemical doping and strain. The latter is able to extend the hyperbolic regime toward the near infrared with low losses. Moreover, with a moderate strain, ${\mathrm{W}\mathrm{Te}}_2$ can even be switched between elliptic and hyperbolic regimes. In addition, plasmons in ${\mathrm{W}\mathrm{Te}}_2$ are characterized by low losses owing to electron-phonon scattering, which is responsible for the temperature dependence of the plasmon line width. Interestingly, the temperature can also be utilized to tune the in-plane anisotropy of the ${\mathrm{W}\mathrm{Te}}_2$ optical response.

We present a magneto-optical trap (MOT) design based on millimeter ball lenses, contained within a metal cube of 19-mm side length. The ball lenses create highly divergent conical MOT trapping beams. In our experiment and in trajectory simulations, we study the effect of beam divergence and light-intensity gradients on MOT performance. We trap approximately $4.2\times {10}^5{}^{85}\mathrm{Rb}$ atoms in our ball-lens MOT at a density of $3.2\times {10}^9{\mathrm{cm}}^{-3}$ and a loading time of 1.3 s. Experimental measurements and the trajectory model are used to calibrate the trapping efficiency in the highly divergent light fields of the ball lenses. The advantages of the design include small lens and chamber sizes favorable to miniaturization, the absence of large-diameter optical-beam pathways, and a low laser-power requirement. These features are conducive to atom-trap geometries with well-defined electromagnetic boundary conditions, as required for ion and plasma traps, and for efficient shielding of black-body and dc stray fields.

Improved electron-photon spin conversion efficiency is a key component of technological platforms for optospintronics integration in information processing; this concept is based on optical devices transmitting and receiving spin information superimposed on light. Semiconductor quantum dots (QDs) are the most promising materials for optospintronic devices; however, in addition to their weak room-temperature luminescence, their electron-photon spin conversion efficiencies are lower than 50%. Here, we present semiconductor QDs embedded in quantum wells (QWs) containing quasi-QDs. The proposed semiconductor hybrid nanosystem with gradual quantum dimensionality reduction demonstrates luminescence one order of magnitude stronger than that of conventional QDs and an electron-photon spin conversion efficiency of almost 80% at room temperature. Optical characterization reveals that efficient carrier capture, suppressed depolarized-spin reinjection, and quasi-three-dimensional quantum confinements in the QWs facilitate the highly efficient electron-photon spin conversion. This study constitutes a significant advance towards the realization of QD-based spin-functional optical devices for electron-spin-based quantum information platforms.

Non-Hermitian systems exploiting the synergy between the properties of closed-conservative systems and open-dissipative (gain-loss) systems have recently become the playground to uncover unusual physical phenomena. Indeed, the spatial symmetry breaking in such systems allows tailoring of the wave propagation at will. Inspired by such a property, we propose a feasible approach based on local Hilbert transform to control the field flows in two- or higher-dimensional non-Hermitian systems. Moreover, we invent an iterative procedure to reduce the dimensionality of complex refractive-index parameter space to two, one, or zero dimensions, restricting the complex refractive index within practical limits. The proposal provides a flexible way to systematically design locally non-Hermitian systems realizable with a limited collection of realistic materials.

Fano resonance has attracted great attention in nanophotonics attributed to its unique properties. In this work, we study the imaging function of a Fano-resonant silicon metasurface governed by quasi--bound states in the continuum (quasi-BICs). First, by breaking the in-plane symmetry of nanodisks, a symmetry-protected quasi-BIC is excited with the emergence of a sharp Fano resonance. The near-field distributions, multipole contributions, and radiation patterns of the metasurface are investigated to uncover the mechanism and characteristics of this resonance. In addition, we investigate the imaging function of this Fano-resonant metasurface assisted by phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST). Through selective modification of different units from $a$-GST to $c$-GST, the produced transmitted image well reconstructs the target letter. Our findings may provide a route to achieve efficient metasurface-based imaging and fast spatial modulations.

Magnetic skyrmions are topologically protected spin structures, offering great promise as information carriers for future spintronic devices. However, a series of challenges, such as unreliable skyrmion motion, pinning effects, and a weak readout signal, prevent the development of skyrmionic applications. The recently demonstrated capability to engineer the local exchange-bias field (LEBF) in an exchange-biased composite, provides solutions to these challenges. By exploiting LEBF, we design and analyze a robust skyrmion shift device. LEBF-induced magnetic domain walls form the boundary of the device channel, and LEBF potential wells are implemented in the channel for positioning skyrmions. Based on simulations with comprehensive models, we demonstrate the proposed device has the advantages of (i) localizing the skyrmions in the absence of current, (ii) suppressing skyrmion annihilation during transport, (iii) promoting steady skyrmion motion, (iv) enhancing the skyrmion velocity, and (v) magnifying the readout signal. Our results form the basis for the design of robust skyrmion devices through LEBF engineering.

The last decade was marked by the advent of the concepts of phase gradient and Huygens’ metasurfaces, and simple theoretical models have been leading the research on metasurfaces ever since. Meanwhile, theoretical modeling of nonplanar metasurfaces has appeared to be exceptionally challenging where it demands accurate analysis of the metasurface geometry. The present work addresses this challenge with a radically different theoretical approach demonstrating how numerical calculation of Green’s function can be employed to design conformal metasurfaces of arbitrary geometries within the same framework and without any accommodation. Against the classical concepts, the alternative approach permits building a metasurface of meta-atoms with an electric only response and significantly simplifying its design and fabrication. The theory is endorsed experimentally by testing several metasurface prototypes for different beam-forming functionalities: radiation with single or multiple beams in any desired direction.

Aligned densely packed carbon-nanotube metamaterials prepared using vacuum filtration are an emerging infrared nanophotonic material. We report multiple hyperbolic plasmon resonances, together spanning the mid-infrared, in individual resonators made from aligned and densely packed carbon nanotubes. In a near-field scanning optical microscopy (NSOM) imaging study of nanotube metamaterial resonators, we observe distinct deeply subwavelength field profiles at the fundamental and higher-order resonant frequencies. The wafer-scale area of the nanotube metamaterials allows us to combine this near-field imaging with a systematic far-field spectroscopic study of the scaling properties of many resonator arrays. Thorough theoretical modeling agrees with these measurements and identifies the resonances as higher-order Fabry-Perot (FP) resonances of hyperbolic waveguide modes. Nanotube resonator arrays show broadband extinction from 1.5–10 µm and reversibly switchable extinction in the 3–5 µm atmospheric transparency window through the coexistence of multiple modes in individual ribbons. Broadband carbon-nanotube metamaterials supporting multiple resonant modes are a promising candidate for ultracompact absorbers, tunable thermal emitters, and broadband sensors in the mid-infrared.

Microwave-to-optical conversion via ferromagnetic magnons has so far been limited by the optical coupling rates achieved in millimeter-scale whispering-gallery-mode resonators. Toward overcoming this limitation, we propose and demonstrate an open magneto-optical cavity containing a thin film of yttrium iron garnet (YIG). We achieve a 0.1 pL ($100\mu {\mathrm{m}}^3$) optical mode volume, approximately $50$ times smaller than previous devices. From this, we estimate that the magnon single-photon coupling rate is $G\approx 50$ Hz. This open-cavity design offers the prospect of wavelength-scale mode volumes, small polarization splittings, and good magneto-optical mode overlap. With achievable further improvements and optimization, efficient microwave-optical conversion and magnon cooling devices become a realistic possibility.

The development of size-efficient and cost-effective micropumps is an important problem given their wide applications. However, efficient designs are difficult to realise practically due to the complex machining required at small length scales. Here, we use Taylor’s swimming sheet as an exemplar to show that the collective behavior of simple constructs like rotating particles can capture much of the rich behavior exhibited by microorganisms, and propose a compact and easy-to-make micropump based on the swimming sheet. We use analytical techniques and dissipative particle dynamics simulations to show that a staggered arrangement of rotating particles can emulate the flow characteristics of the swimming sheet and work as a micropump. Our analytical calculations, based on two approximate approaches, predict the dependence of the flow rate on control parameters.

A controllable and steerable transport of acoustic wave signals along preferred paths with unitary efficiency is highly desired for underwater acoustic applications such as underwater communication, navigation, and tracking. Here we present a systematical study of a class of simply structured acoustic metagratings containing iron cylinders only. Through diffraction analysis and a genetic-algorithm-assisted optimization process, we propose a powerful inverse-design approach that can faithfully and promptly produce desired wave-front manipulation effects. Various interesting diffraction-engineering functionalities such as perfect beam splitting, anomalous reflection and transmission, retroreflection, and highly asymmetric transmission response are unambiguously demonstrated with unitary efficiency. These simply structured metagrating configurations, as well as the efficient inverse-design methodology behind them, are expected to be useful in the design of multifunctional acoustic planar devices for diverse underwater applications.

Bound states in superconductor-nanowire hybrid devices play a central role, carrying information on ground-state properties (Shiba or Andreev states) or on the topological properties of the system (Majorana states). The spectroscopy of such bound states relies on the formation of well-defined tunnel barriers, usually defined by gate electrodes, which results in smooth tunnel barriers. Here we used thin $\mathrm{In}\mathrm{P}$ segments embedded into $\mathrm{In}\mathrm{As}$ nanowire during the growth process to form a sharp built-in tunnel barrier. Gate dependence and thermal-activation measurements are used to confirm the presence and estimate the height of this barrier. By coupling these wires to superconducting electrodes we investigate the gate-voltage dependence of the induced gap in the nanowire segment, which we can understand using a simple model based on Andreev bound states. Our results show that these built-in barriers are promising as future spectroscopic tools.

Vector vortex beams are a class of optical beams with singularities in their space-variant polarization. Vector vortex beam lasers have applications in many areas including imaging and communication, where vertical-cavity lasers emitting Gaussian beams have been most widely used so far. Generation of vector vortex beams from vertical-cavity lasers has required external control or modulation. Here, by utilizing a polarization-selective subwavelength grating as one of the reflectors in a vertical semiconductor microcavity, we design the spin textures of the polariton mode and demonstrate polariton lasing in a single-mode, radially polarized vector vortex beam. Polarization and phase distributions of the emission are characterized by polarization-resolved imaging and interferometry. This method of vector vortex laser beam generation allows low threshold power, stable single-mode operation, scalability, and on-chip integration, all of which are important for applications in imaging and communication.

We predict that it is possible to achieve a purely voltage-driven switching of a ferromagnet using spin-dependent resonant tunneling. In a configuration of two exchange-coupled magnets through a resonant-tunneling barrier, application of a voltage leads to a resonant enhancement and an oscillatory nature of the exchange coupling. The peak equivalent exchange field is strong enough to switch typical ferromagnets used in scaled magnetic memory devices. The switched configuration is retained once the electric field is removed since the equilibrium exchange coupling is negligible, suppressed by large barriers. Bidirectional switching is possible with the same polarity of the voltage, unlike conventional magnetic memory devices where a bidirectional current or a magnetic field is necessary. Further, the threshold of switching is decoupled from the speed, due to the conservative nature of the exerted torque. This is very different from the conventional spin-torque devices that exhibit a trade-off due to the nonconservative nature of the switching torque. We further show that the structure shows an oscillation in the magnetoresistance (MR) stemming from the resonant tunneling. Interestingly, the MR is higher for smaller voltages while the exchange field is higher for larger voltages—this is promising for efficient read and write operations in potential memory applications.

The superposition of a Gaussian mode and a Laguerre-Gauss mode with $\ell =0,p\ne 0$ generates the so-called bottle beam: a dark focus surrounded by a bright region. In this paper, we theoretically explore the use of bottle beams as an optical trap for dielectric spheres with a refractive index smaller than that of their surrounding medium. The forces acting on a small particle are derived within the dipole approximation and used to simulate the Brownian motion of the particle in the trap. The intermediate regime of particle size is studied numerically and it is found that stable trapping of larger dielectric particles is also possible. Based on the results of the intermediate-regime analysis, an experiment aimed at trapping living organisms in the dark focus of a bottle beam is proposed.

With excellent folding-induced deformability and shape reconfigurability, origami-based designs have shown great potential in developing deployable structures. Noting that origami deployment is essentially a dynamic process, while its dynamic behavior remains largely unexplored, owing to challenges in modeling. This research aims to advance the state of the art of origami deployable structures by exploring the transient dynamics under free deployment, with the Miura-origami tube being selected as the object of study because it possesses a relatively simple geometry, exceptional kinematic properties, and a wide range of potential applications. In detail, a preliminary free-deployment test is performed, which indicates that transient oscillations in the transverse direction are non-negligible, and tube deployment is no longer a single degree of freedom mechanism. Based on experimental observations, four assumptions are made for modeling purposes, and a 2N degree of freedom dynamic model is established for an N-cell Miura-origami tube to predict the transient oscillations in both deploying and transverse directions. Employing the settling times and the overshoot values as transient dynamic indexes, a comprehensive parameter study is then carried out. It reveals that both the physical and geometrical parameters will significantly affect the transient deploying dynamics, with some of the parameter dependence relationships being counterintuitive. The results show that the relationships between the transient dynamic behaviors and examined parameters are sometimes contradictory in the deploying and transverse directions, suggesting the necessity of a compromise in design. Overall, this research proposes a systematic method for constructing an origami dynamic model. Although only the Miura-origami tube is exemplified in detail, the methodologies are promising for application to other origami deployable systems because the established model is believed to be able to capture the essential characteristics of origami deployment, and meanwhile, hold good processability. In addition, comprehensive parameter analysis results could provide useful guidance for the design and optimization of Miura-origami deployable tubes with robust dynamic performance.

The mark of 18% power conversion efficiency (PCE) was recently overcome by laboratory-scale organic solar cells (OSCs) thanks to the development of nonfullerene acceptors (NFAs). NFA-based solar cells show improved performance and stability compared with those of their fullerene-acceptor-based counterparts. However, only a few studies focus on scalable deposition techniques or roll-to-roll compatible processing, which is of paramount importance for the commercialization of the technology. Here, we report a simple and fast fabrication of slot-die-coated poly(3-hexylthiophene-2,5-diyl):(5Z,5'Z)-5,5'-{7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)]bis(methanylylidene)}bis(3-ethyl-2-thioxothiazolidin-4-one) (P3HT:O-IDTBR) OSCs using a roll platform on flexible ITO-free substrates under ambient conditions. We show that the optical band gap of the active layer increases when an isopropanol-diluted poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) hole-transport layer is coated on top of it, changing the device properties. Optimization of the coating conditions leads to the achievement of up to 3.6% PCE for single cells of $1{\mathrm{cm}}^2$ fabricated under ambient conditions with flexographic printed $\mathrm{Ag}$ back electrodes, compared with solar cells with evaporated Ag (3.8% PCE), $\mathrm{Au}$ (2.1% PCE), or $\mathrm{Cu}$ (3.0% PCE) back contacts. OSCs with larger areas of $4{\mathrm{cm}}^2$ with 2.3% PCE are also fabricated, where the fast increase of the series resistance with the area is the main PCE-limiting factor. The efficiencies herein reported for NFAs obtained by roll processing show the excellent potential of the P3HT:O-IDTBR blend for large-scale fabrication.

This work presents a superresolution imaging approach by using subwavelength hole resonances. We employ a subwavelength structure in which an array of tiny holes are etched in a metallic slab with the neighboring distance $\ell$ that is smaller than half of the wavelength. By tuning the frequencies of the incident wave at resonant frequencies, the subwavelength structure generates strong illumination patterns that are able to probe both low and high spatial frequency components of the imaging sample sitting above the structure. The image of the sample is obtained by performing stable numerical reconstruction from the far-field measurement of the diffracted wave. It is demonstrated that a resolution of $\ell /2$ can be obtained for reconstructed images, thus one can achieve superresolution by arranging multiple holes within one wavelength. The proposed approach may find applications in microscopies as well as wave-based imaging such as electromagnetic and ultrasound imaging. It attains two advantages that are important for practical realization. It avoids the difficulty of controlling the distance between the probe and the sample surface with high precision. In addition, the numerical reconstructed images are very stable against noise by only using the low-frequency band of the far-field data in the numerical reconstruction.