The recent realization of all-dielectric valley photonic crystals (VPCs) in nanoscale not only provides a standard platform to explore many topological phases of light with protected edge modes, but also shows promising applications for designing high-performance nanophotonic devices. However, the widely reported all-dielectric VPCs are limited to one single polarization, and it is challenging to manipulate the polarization degree of freedom of light and design polarizing devices using VPCs. Here, we design a dual-polarization all-dielectric VPC and propose a topological polarization beam splitter. The phase vortex distributions of bulk modes at different valleys characterize the nonzero valley Chern numbers for both TE and TM polarizations, indicating the implementation of topologically nontrivial dual-polarization band gaps. This leads to dual-polarization valley-dependent edge modes located at interfaces with different shapes. The topological valley transport around sharp-bends is also demonstrated for both TE and TM polarizations. Finally, a harpoon-shaped polarization beam splitter is proposed, and the simulated results confirm the good functionality of polarization separation. Our work shows the flexible control of light in topological photonic systems with the polarization degree of freedom, and has promising applications in polarization multiplexing photonic devices owing to the enlarged the optical information-processing capacity.

In a flywheel thermoacoustic engine, the rotating motion of the flywheel is sustained by the reciprocal motion of the working gas confined in an externally heated thermoacoustic engine. In this study, an equation of motion for the flywheel in the steady rotational state is derived and the key parameters governing the rotation dynamics of the looped-tube thermoacoustic engine loaded with a piston-flywheel assembly are investigated. We demonstrate that the relationship between the steady rotation frequency and the heating temperature is determined by the real part of the acoustic impedance of the engine subsystem. This result is in contrast to that of a conventional thermoacoustic engine with a linear load, where both the real and imaginary parts of the acoustic impedance impact the operating condition. When the flywheel is connected, the imaginary part of the acoustic impedance only affects the temporal fluctuation of the rotation frequency. The attainable shaft power that can be extracted from the engine is also discussed based on experimental results.

Optical metasurfaces enable us to engineer the electromagnetic space and control light propagation at an unprecedented level, offering a powerful tool to achieve modulation of light over multiple physical dimensions. Here, we demonstrate a $\mathrm{Sb}$${}_2$$\mathrm{S}$${}_3$ phase-change metasurface platform that allows active manipulation of both the amplitude and the phase. In particular, we implement dynamic nanoprinting and holographic image display through the tuning of the crystallization levels of this phase-change material. The $\mathrm{Sb}$${}_2$$\mathrm{S}$${}_3$ nanobricks tailored to perform the amplitude, geometric, and propagation-phase modulation constitute the dynamic meta-atoms in the multiplexed metasurfaces. Using the incident polarizations as decoding keys, the encoded information can be reproduced in the form of a nanoprinted grayscale image in the near field and two holographic images in the far field. These images can be switched on and off by taking advantage of the reversible tunability of the $\mathrm{Sb}$${}_2$$\mathrm{S}$${}_3$ nanostructure between the amorphous and crystalline states. The proposed phase-change metasurfaces featuring manifold information and multifold encryption promise ultracompact data storage with high capacity and high security, which suggests an exciting direction for modern cryptography and security applications.

Optical waveguide structures can make the state-of-the-art micro- and nanofabricated devices faster and less energy consuming. However, on-chip optical components must be placed at relatively large distances from each other, on the order of the wavelength $\lambda$, to eliminate the crosstalk between them. This makes on-chip optical structures too large to compete with their electronic counterparts. In this work, we explore the possibility of suppressing the crosstalk between closely spaced dielectric waveguides by making use of higher-order modes. We show that the crosstalk between two waveguides can be essentially eliminated, even if the waveguides are separated by a distance of $\lambda /10$ only. We also study arrays of more than two waveguides and find the conditions for the efficient crosstalk reduction in them. Our results can lead to further miniaturization of photonic integrated circuits.

Distributing entanglement through standard telecommunication fiber is particularly important for quantum key distribution protocols with low vulnerability over metropolitan distances. However, entanglement distribution over long distance through optical fiber so far could only be accomplished with moderate photon pair rates. In this work, we present entanglement distribution over 50 km of standard telecommunication fiber with a pair rate of more than 10 000 ${\mathrm{s}}^{-1}$ using a bright nondegenerate photon pair source. Signal and idler wavelengths of this source are optimized for low dispersion in optical fiber and high efficiency for single-photon avalanche diode detectors, respectively. The resulting modest hardware requirement and high rate of detected entangled photon pairs could significantly enhance practical entanglement-based quantum key distribution in existing metropolitan fiber networks.

In the past few years, the emerging photon-magnon dissipative coupling offers us an effective way to design and exploit losses to fulfill quantum tasks in hybrid cavity magnonics systems. In this paper, we theoretically investigate the dissipative-coupling-induced transparency (DCIT) and the enhancement of the high-order sidebands in a dissipative photon-magnon coupling system with Kerr nonlinearity. We find that the DCIT can be observed with both a weak and strong driving field. Due to the existence of the Kerr nonlinearity and introduction of the strong driving field of magnon, the sidebands can be enhanced highly by choosing the optimized parameters. In order to find the optimized parameters to enhance sidebands, we utilize the eigenvalues of the effective Hamiltonian to find the critical point of the stability region, near which sidebands can be enhanced in maximal degree.

The spin-orbit interaction (SOI) offers a nonferromagnetic scheme to realize spin polarization through utilizing an electric field. Electrically tunable SOIs through electrostatic gates have been investigated; however, the relatively weak and volatile tunability limits their practical applications in spintronics. Here, we demonstrate the nonvolatile electric field control of the SOI via constructing ferroelectric Rashba architectures, i.e., two-dimensional ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}/\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3$-${\mathrm{PbTiO}}_3$ ferroelectric field-effect transistors. The experimentally observed weak antilocalization (WAL) cusp in ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ films implies the Rashba-type SOI that arises from the asymmetric confinement potential. Significantly, taking advantage of the switchable ferroelectric polarization, the WAL-to-weak-localization-transition trend reveals the competition between spin relaxation and the dephasing process, and the variation of carrier density leads to a reversible and nonvolatile modulation of the spin-relaxation time and the spin-splitting energy of ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ films by this ferroelectric gating. Our work provides a scheme to achieve nonvolatile control of the Rashba SOI with the utilization of ferroelectric remanent polarization.

Conductive sulfur/carbon copolymers with a high reversible capacity are promising alternative cathode materials for lithium-sulfur batteries. Here, the focus is set on nitrogen-terminated zigzag graphene nanoribbons (N-GNRs) resembling intermediate product of electrospun polyacrylonitrile (PAN)-based carbon nanofibers. In particular, the possibility of combining strong nitrogen-sulfur interactions, which has been recently shown to lead to a shuttle-free discharge mechanism, and superior electrical conductivity of graphene nanoribbons is explored in S/N-GNR copolymers. Structural and electronic properties of ${\mathrm{S}}_x/$N-GNR structures, $x=1,\dots ,8$, prior and during the discharge are studied using density-functional theory calculations along with ab initio molecular dynamics simulations. It is found that the GNR backbone assumes a rippled structures in all S/N-GNR structures considered here. The most favorable sulfur structures in S/N-GNR copolymers are found to consist of short sulfur chains with $x\sim 4,5$. It is also observed that, similar to N-GNRs, S/N-GNR copolymers show a metallic behavior which is brought about by the conductive backbone. In addition, consecutive lithiation reactions are studied and product structures are obtained. It is demonstrated that a shuttle-free, solid-solid transformation could also be expected during the discharge of S/N-GNR copolymers, whereas the lithiated structures remain electrically conductive, irrespective of the discharge state. Therefore, the current study promotes the use of S/N-GNR copolymers as an alternative to other poorly conductive PAN-based S/C copolymer cathodes for lithium-sulfur batteries (such as S/cPAN), while largely mitigating lithium polysulfide formation.

Doped graphene nanostructures are a promising platform for photonics due to their exceptionally strong and tunable plasmonic resonances. When placed in a periodic array configuration, the plasmons supported by the individual nanostructures interact with each other and, under the appropriate conditions, can give rise to a collective mode known as a lattice resonance. Here, we perform a comprehensive analysis of the response of periodic arrays of graphene nanodisks and identify the conditions under which the system is able to support lattice resonances. We find that the ratio between the period of the array and the wavelength of the plasmon completely determines the behavior of the system. As a consequence, strong lattice resonances are achieved for micron-size nanodisks in the terahertz regime. We develop a theoretical model valid beyond the electrostatic approximation and use it to derive closed analytical expressions for the strength, the wavelength, and the width of the optical resonance of the arrays. The theoretical framework developed in this work paves the way for facile design and discovery of emerging properties of periodic arrays of graphene nanostructures that could enable applications in photonics and plasmonics.

Current-induced spin-orbit torques provide a versatile tool for switching magnetic devices. In perpendicular magnets, the dampinglike component of the torque is the main driver of magnetization reversal. The degree to which the fieldlike torque assists the switching is a matter of debate. Here we study the switching of magnetic tunnel junctions with a Co-Fe-B free layer and either $\mathrm{W}$ or $\mathrm{Ta}$ underlayers, which have a ratio of fieldlike to dampinglike torque of 0.3 and 1, respectively. We show that the fieldlike torque can either assist or hinder the switching of Co-Fe-B when the static in-plane magnetic field required to define the polarity of spin-orbit torque switching has a component transverse to the current. In particular, the noncollinear alignment of the field and current can be exploited to increase the switching efficiency and reliability compared with the standard collinear alignment. By probing individual switching events in real time, we also show that the combination of transverse magnetic field and fieldlike torque can accelerate or decelerate the reversal onset. We validate our observations using micromagnetic simulations and extrapolate the results to materials with different torque ratios. Finally, we propose device geometries that leverage the fieldlike torque for density increase in memory applications and synaptic weight generation.

One of the most prominent techniques to enhance the performance of practical quantum key distribution (QKD) systems with laser sources is the decoy-state method. Current decoy-state QKD setups operate at gigahertz repetition rates, a regime where memory effects in the modulators and electronics that control them create correlations between the intensities of the emitted pulses. This translates into information leakage about the selected intensities, which cripples a crucial premise of the decoy-state method, thus invalidating the use of standard security analyzes. To overcome this problem, a security proof that exploits the Cauchy-Schwarz constraint has been introduced recently. Its main drawback is, however, that the achievable key rate is significantly lower than that of the ideal scenario without intensity correlations. Here, we improve this security proof technique by combining it with a fine-grained decoy-state analysis, which can deliver a tight estimation of the relevant parameters that determine the secret key rate. This results in a notable performance enhancement, being now the attainable distance double that of previous analyzes for certain parameter regimes. Also, we show that when the probability density function of the intensity fluctuations, conditioned on the current and previous intensity choices, is known, our approach provides a key rate very similar to the ideal scenario, which highlights the importance of an accurate experimental characterization of the correlations.

One of the major challenges in the design of nonreciprocal devices is the application of magnetic biasing. This problem has motivated a flurry of work for the development of magnetless nonreciprocal devices, with spatiotemporal modulation being one of the most promising approaches to this end. The most fundamental nonreciprocal component is the gyrator, which can be the building block of other nonreciprocal components, like ciculators and isolators. Spatiotemporally modulated (STM) gyrators are typically based on a pair of frequency converters with different modulation phases, connected through a linear time-invariant network. Such gyrators are usually designed to operate with a single modulation sideband. In this paper we argue that this approach is suboptimal and show how adding just one more sideband can increase the bandwidth by $40\mathrm{\%}$ with a lower modulation frequency and the same modulation amplitude. We present an implementation of the proposed approach based on a double-balanced circuit with Wheatstone bridges of varactors, which provides large isolation between the input signal and the sidebands. We provide a theoretical and an experimental analysis of the circuit with excellent agreement to each other. In addition to introducing a topology of STM gyrators with improved bandwidth compared with existing designs, our results highlight a potential connection between the bandwidth of STM devices and their harmonic order, which might also find applications in other STM systems, like isolators, circulators, and photonic topological insulators.

Achromatic lenses are crucial to light focusing and imaging. However, conventional achromatic lenses are restricted by the Abbe diffraction limit. Although superresolution optical lenses have been demonstrated to overcome the limit, the correction of chromatic aberration in continuous broadband for superresolution lenses still remains challenging. To address this issue, we propose a generalized strategy of constructing a frequency-dependent phase profile of linear dispersion for achromatic superresolution focusing in a continuous broad bandwidth. Specifically, based on an arbitrary wavelength-independent discrete phase profile of a monochromatic superresolution lens optimized at a given single optical frequency, a frequency-dependent phase profile of an achromatic superresolution lens can be constructed as the sum of the given discrete phase profile and a linear dispersion phase term. Based on the proposed approach, broadband achromatic superresolution metalenses are designed and demonstrated in the terahertz regime. The proposed method can be applied to visible and infrared optical spectra, which could have promising potential in various superresolution-related applications.

The concept of photonic frequency-momentum ($\omega$-$q$) dispersion has been extensively studied in artificial dielectric structures such as photonic crystals and metamaterials. However, the $\omega$-$q$ dispersion of electrodynamic waves hosted in natural materials at the atomistic level is far less explored. Here, we develop a Maxwell Hamiltonian theory of matter combined with the quantum theory of atomistic polarization to obtain the electrodynamic dispersion of natural materials interacting with the photon field. We apply this theory to silicon and discover the existence of anomalous atomistic waves. These waves occur in the spectral region where propagating waves are conventionally forbidden in a macroscopic theory. Our findings demonstrate that natural media can host a variety of yet to be discovered waves with subnanometer effective wavelengths in the picophotonics regime.

Any architecture for practical quantum computing must be scalable. An attractive approach is to create multiple cores, computing regions of fixed size that are well spaced but interlinked with communication channels. This exploded architecture can relax the demands associated with a single monolithic device: the complexity of control, cooling and power infrastructure, as well as the difficulties of crosstalk suppression and near-perfect component yield. Here we explore interlinked multicore architectures through analytic and numerical modeling. While elements of our analysis are relevant to diverse platforms, our focus is on semiconductor electron spin systems in which numerous cores may exist on a single chip within a single fridge. We model shuttling and microwave-based interlinks and estimate the achievable fidelities, finding values that are encouraging but markedly inferior to intracore operations. We therefore introduce optimized entanglement purification to enable high-fidelity communication, finding that $99.5\mathrm{\%}$ is a very realistic goal. We then assess the prospects for quantum advantage using such devices in the noisy intermediate-scale quantum era and beyond: we simulate recently proposed exponentially powerful error mitigation schemes in the multicore environment and conclude that these techniques impressively suppress imperfections in both the inter- and intracore operations.

Nonlinear Cherenkov radiation, an intriguing noncollinear optical parametric process with versatile longitudinal phase matching, is commonly realized in engineered nonlinear photonic structures. Here, we report on the observation of ultrabroadband rainbow Cherenkov second-harmonic generation (UBR CSHG) in a single periodically poled lithium niobate (PPLN) nonlinear crystal driven by a near-infrared high-peak-power ultrashort femtosecond pump laser. Multiple colored nonlinear radiation bands along the direction of Cherenkov conical angles are simultaneously exhibited, leading to a beautifully colorful rainbow picture never reported before. We systematically analyze the multicolor spectral characteristics of the UBR CSHG in the whole conical direction and explicitly clarify their phase-matching conditions. We show that this UBR CSHG interaction is subject to remarkable spectral broadening owing to both the third-order nonlinearity effect induced by the high-peak-power ultrashort pump laser and the reciprocal lattice vector of the PPLN crystal, which eventually enables the generation of three bright dominating colors—red, orange-yellow, and green bands—and their automatic dispersion in space. The experimental results suggest rich physics of nonlinear optical interactions and indicate the possibility to engineer broadband nonlinear frequency conversion for applications in compact colored laser sources and others.

We present results of in situ optical detection of ultrasonic waves generated in stainless steel and graphite samples mounted in a flat plate model during hypersonic flow in the 31-Inch Mach 10 wind tunnel at NASA Langley Research Center. Longitudinal waves are excited in the stainless steel and graphite sample inserts by using a contact piezoelectric transducer and the normal displacement on the surface of the sample, exposed to hypersonic fluid flow, is measured optically using a Sagnac interferometer. Measurements are consistent at different Reynolds numbers both with and without a turbulent trip strip present. Additionally, optical detection of laser-generated surface acoustic waves in a stainless-steel sample during flow is presented. These results are a demonstration of laser-based ultrasonics as an in situ material characterization technique in hypersonic flow, enabling potential applications of in situ monitoring of defect initialization and morphology in hypersonic environments.

Soft materials that respond to external stimuli, such as moisture, heat, and light, can convert the environmental energy into mechanical motions, and thus can be considered engines. The energy-conversion process within the soft-materials-based engines is fundamentally different from that of conventional heat engines utilizing gaseous media, but has been rarely studied to date. Here we construct a theoretical framework to analyze the thermodynamic performance of humidity-responsive soft actuators, as a canonical model of soft engines, which operate in a similar manner to such motile plants as wild wheat seeds and pine cones. Considering the free-energy change and the work generation through a four-process cycle, we define a work ratio as the work of actual engines relative to that of a so-called hygro-Carnot cycle going through reversible changes of volume and chemical energy. We propose an explanation why natural motile plants exhibit higher work ratios than common artificial hygroscopic actuators based on the time scales of soft actuation and the environmental humidity change. Our thermodynamic model can be extended to a range of soft engines driven by diffusion of stimulus, e.g., solvent, heat, and ions, into soft media.