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Twistronics, the manipulation of Moir\'e superlattices via the twisting of two layers of two-dimensional (2D) materials to control diverse and nontrivial properties, has recently revolutionized the condensed matter and materials physics. Here, we introduce the principles of twistronics to spin photonics, coining this emerging field spintwistronics. In spintwistronics, instead of 2D materials, the two layers consist of photonic topological spin lattices on a surface plasmonic polariton (SPP) platform. Each 2D SPP wave supports the construction of topological lattices formed by photonic spins with stable skyrmion topology governed by rotational symmetry. By introducing spintwistronics into plasmonics, we demonstrate theoretically and experimentally that two layers of photonic spin lattices can produce Moir\'e spin superlattices at specific magic angles. These superlattices, modulated periodically by the quantum number of total angular momentum, exhibit novel properties-including new quasiparticle topologies, multiple fractal patterns, extremely slow-light control, and more-that cannot be achieved in conventional plasmonic systems. As a result, they open up multiple degrees of freedom for practical applications in quantum information, optical data storage and chiral light-matter interactions., Comment: 4 figures

Inspired by neural network algorithms in deep learning, diffractive optical networks have arisen as new platforms for manipulating light-matter interactions. Seemly inherited from the deep learning black box nature, clear physical meanings have never been given for these complex diffractive networks at the layer level, even though the systems are visible in physical space. Using exemplified mode conversion systems, this work showed how various physical transformation rules within diffractive networks can be unveiled with properly defined input/output relations. Surprising physical transformation division phenomenon and an optical analogy of gradient-vanishing-effect have been observed and discussed for high-dimensional mode sorting tasks. The use of physical transparency for the effective design of parameter-varying networks is also demonstrated. These physically transparent optical networks resolve the contradiction between rigorous physical theorem and operationally vague network structure, and pave the way for transparentizing other physical neural networks.

Skyrmions are important topologically non-trivial fields characteristic of models spanning scales from the microscopic to the cosmological. However, the Skyrmion number can only be defined for fields with specific boundary conditions, limiting its use in broader contexts. Here, we address this issue through a generalized notion of the Skyrmion derived from the De Rham cohomology of compactly supported forms. This allows for the definition of an entirely new $\coprod_{i=1}^\infty \mathbb{Z}^i$-valued topological number that assigns a tuple of integers $(a_1, \ldots, a_k)\in \mathbb{Z}^k$ to a field instead of a single number, with no restrictions to its boundary. The notion of the generalized Skyrmion presented in this paper is completely abstract and can be applied to vector fields in any discipline, not unlike index theory within dynamical systems. To demonstrate the power of our new formalism, we focus on the propagation of optical polarization fields and show that our newly defined generalized Skyrmion number significantly increases the dimension of data that can be stored within the field while also demonstrating strong robustness. Our work represents a fundamental paradigm shift away from the study of fields with natural topological character to engineered fields that can be artificially embedded with topological structures.

Quasicrystals,characterized by long-range order without translational symmetry,have catalyzed transformative advances in various fields,including optics in terms of field quasicrystals.Here,we present the first demonstration of photonic quasicrystals formed by spin angular momentum, unveiling novel spin-orbit coupling effects absent in traditional field quasicrystals.A de Bruijn tiling like theoretical framework was built elucidating the formation mechanism of spin quasicrystals for diverse symmetries.Moreover,the configurations of these spin textures can be manipulated through the adjustments of the wavefronts,among which phason-like discontinuous dynamics is observed and quantitatively measured. Unlike optical quasicrystals shaped by electromagnetic fields,these spin-governed quasicrystals exhibit quasi-periodic properties of kinematic parameters,extending their potential applications to other physical systems. These findings hold promise for novel advancements in optical trapping,quasicrystal fabrication,and optical encryption systems.

The widely adopted optical information detection strategies nowadays can be roughly classified into spatial-resolve-based and time-sequence-based methods. The former is highly reliant on detectors with spatial resolution that works in a limited spectrum range to capture the intensity distribution of the object, whereas the latter employs single-pixel detectors (SPDs) with improved reliability and stability in detection at the expense of time consumption due to sequential intensity recording. However, there is a scarcity of research on using high-performance SPDs for single-shot object information detection. To capture and extract features of objects efficiently and economically, in this work, we propose a single-shot optical information-detecting method, tentatively named spatially multiplexed point detection (SMPD) technology, in which some SPDs are randomly distributed at different locations to acquire the information of the object. To validate the validity of the proposed method, we demonstrate high-fidelity recognition of orbital angular momentum (OAM) modes from speckle patterns generated by the transmission of two orthogonally polarized vortex beams through a multimode fiber. It is feasible to apply this approach in wide applications, such as image transmission based on multiplexed-OAM decoding and recognizing hand-written digits. Compared with traditional image recognition methods, the new approach yields a recognition accuracy of over 98% with an effective detection area that is only 0.02% of its counterparts. With further engineering, the proposed method may spur many exciting developments in OAM-based optical communication systems, image classification, object detections, and other optical information detections.

Electromagnetic waves characterized by intensity, phase, and polarization degrees of freedom are widely applied in data storage, encryption, and communications. However, these properties can be substantially affected by phase disorders and disturbances, whereas high-dimensional degrees of freedom including momentum and angular momentum of electromagnetic waves can offer new insights into their features and phenomena, for example topological characteristics and structures that are robust to these disturbances. Here, we discover and demonstrate theoretically and experimentally spin-momentum locking and topological defects in unpolarized light. The coherent spin is locked to the kinetic momentum except for a small coupling spin term, due to the simultaneous presence of transverse magnetic and electric components in unpolarized light. To cancel the coupling term, we employ a metal film acting as a polarizer to form some skyrmion-like spin textures at the metal/air interface. Using an in-house scanning optical microscopic system to image the out-of-plane spin density of the focused unpolarized vortex light, we obtained experimental results that coincide well with our theoretical predictions. The theory and technique promote the applications of topological defects in optical data storage, encryption, and decryption, and communications., Comment: 9 pages, 3 figures, 47 references

The recent emergence of electromagnetic topological defects has attracted wide interest in fields from topological photonics to deep-subwavelength light-mater interactions. Previously, much of the research has focused on constructing specific topological defects but the fundamental theory describing the physical mechanisms underlying their formation and transitions is lacking. Here, we present a spin-orbit coupling based theory describing such mechanisms for various configurations of spin topological defects in confined electromagnetic fields. The results reveal that their formation originates from the conservation of total angular momentum and that their transitions are determined by anisotropic spin-orbit couplings. By engineering the spin-orbit couplings, we observe the formation and transitions of Neel-type, twisted-type, and Bloch-type spin topological defects in confined electromagnetic fields. A stable Block-type spin topological defect is reported for the first time. Our theory can also describe the transitions of field topological defects. The findings enrich the portfolio of electromagnetic topological defects, deepen our understanding of conserved laws, spin-orbit couplings and transitions of topological defects in confined electromagnetic systems, and predict applications in high-density optical data transmissions and chiral quantum optics., Comment: 12 pages, 3 figures and 64 references in main text; 28 pages, 15 figures, 5 tables and 4 tables in supplementary materials

Spin angular momentum, an elementary dynamical property of classical electromagnetic fields, plays an important role in spin-orbit and light-matter interactions, especially in near-field optics. The research on optical spins has led to the discovery of phenomena such as optical spin-momentum locking and photonic topological quasiparticles, as well as applications in high-precision detection and nanometrology. Here, we investigate spin-momentum relations in paraxial optical systems and show that the optical spin angular momentum contains transverse and longitudinal spin components simultaneously. The transverse spin originates from inhomogeneities of field and governed by the vorticity of the kinetic momentum density, whereas the longitudinal spin parallel to the local canonical momentum is proportional to the polarization ellipticity of light. Moreover, the skyrmionlike spin textures arise from the optical transverse spin can be observed in paraxial beams, and their topologies are maintained free from the influence of the Gouy phase during propagation. Interestingly, the optical singularities, including both phase and polarization singularities, can also affect the spin-momentum properties significantly. Our findings describe the intrinsic spin-momentum properties in paraxial optical systems and apply in the analysis of the properties of spin-momentum in optical focusing, imaging, and scattering systems., Comment: 20 pages; 6 figures, 151 references

Skyrmions are topologically stable quasiparticles that have been predicted and demonstrated in quantum fields, solid-state physics, and magnetic materials, but only recently observed in electromagnetic fields, triggering fast expanding research across different spectral ranges and applications. Here we review the recent advances in optical skyrmions within a unified framework. Starting from fundamental theories, including classification of skyrmionic states, we describe generation and topological control of different kinds of optical skyrmions in structured and time-dependent optical fields. We further highlight generalized classes of optical topological quasiparticles beyond skyrmions and outline the emerging applications, future trends, and open challenges. A complex vectorial field structure of optical quasiparticles with versatile topological characteristics emerges as an important feature in modern spin-optics, imaging and metrology, optical forces, structured light and topological and quantum technologies.

Spin-momentum locking is an intrinsic property of surface electromagnetic fields and its study has led to the discovery of photonic spin lattices and diverse applications. Previously, dispersion was ignored in the spin-momentum locking, giving rise to abnormal phenomena contradictory to the physical realities. Here, we formulate four dispersive spin-momentum equations for surface waves, revealing universally that the transverse spin vector is locked with the momentum. The locking property obeys the right-hand rule in the dielectric but the left-hand rule in the dispersive metal/magnetic materials. In addition to the dispersion, the structural features can affect the spin-momentum locking significantly. Remarkably, an extraordinary longitudinal spin originating from the coupling polarization ellipticity is uncovered even for the purely polarized state. We further demonstrate the spin-momentum locking properties with diverse photonic topological lattices by engineering the rotating symmetry. The findings open up opportunities for designing robust nanodevices with practical importance in chiral quantum optics., Comment: 6 pages, 4 figures in main text 36 pages, 15 figures and 81 equations in supplemental materials

Conventional biomedical imaging modalities, including endoscopy, X-rays, and magnetic resonance, are invasive and cannot provide sufficient spatial and temporal resolutions for regular imaging of gastrointestinal (GI) tract to guide prognosis and therapy of GI diseases. Here we report a non-invasive method for optical imaging of GI tract. It is based on a new type of lanthanide-doped nanocrystal with near-infrared (NIR) excitation at 980 nm and second NIR window (NIR-IIb) (1500~1700 nm) fluorescence emission at around 1530 nm. The rational design and controlled synthesis of nanocrystals with high brightness have led to an absolute quantum yield (QY) up to 48.6%. Further benefitting from the minimized scattering through the NIR-IIb window, we enhanced the spatial resolution by 3 times compared with the other NIR-IIa (1000~1500 nm) contract agents for GI tract imaging. The approach also led to a high temporal resolution of 8 frames per second, so that the moment of mice intestinal peristalsis happened in one minute can be captured. Furthermore, with a light-sheet imaging system, we demonstrated a three-dimensional (3D) imaging of the stereoscopic structure of the GI tract. Moreover, we successfully translate these advances to diagnose inflammatory bowel disease (IBD) in a pre-clinical model of mice colitis.

Since the discovery of graphene, its excellent physical properties has greatly improved the performance of many optoelectronic devices and brought important technological revolution to optical research and application. Here, we introduce graphene into the field of optical tweezers technology and demonstrate a new thermoelectric optical tweezers technology based on graphene. This technology can not only reduce the incident light energy by 2 orders of magnitude (compared with traditional optical tweezers), but also bring new advantages such as much broader working bandwidth and larger working area than the thermoelectric optical tweezers based on gold film widely studied before. Compared with gold film, graphene has more novel characteristics like high thermal conductivity, high uniformity and easy process. Thus, we found even monolayer graphene can achieve stable trapping for particles in a broad band, and the performance is enhanced with more graphene layers. Furthermore, structured graphene patterns can be easily generated to holographically trap multiple particles as desired shapes. This work verifies the great application potential of two-dimensional materials in op-tical tweezers technology, and it will promote more promising applications in cell trapping, tapping or concentration of biomolecules, microfluidics and biosensors.

Electromagnetic spins, including longitudinal and transverse ones, have been playing important roles in light-matter interactions. Here, we formulate a unified equation to uncover the physical origins and topological properties of longitudinal and transverse spins in a generic electromagnetic field. The equation reveals universally that the transverse spin is locked with the kinetic momentum and originated from the transverse inhomogeneities of field, whereas the helix-dependent longitudinal spin orients parallel to the local wavevector. Remarkably, a hidden extraordinary helix-dependent transverse spin possessing helix-dependent spin-momentum locking is discovered and the number of locking states consistent with the nontrivial topological spin Chern number. Furthermore, this spin which determines the inverted helical components is related to the Berry curvature closely. The findings, which are demonstrated experimentally by measuring the three-dimensional spin components in the focusing configuration, will deepen the understanding the underlying physics of spins and open an avenue for chiral quantum optical applications., Comment: 15 pages, 3 figures, 2 table, 7 equations, 56 references in main text; 39 pages, 16 figures, 100 equations, 16 references in supplemental materials

Chiral spin textures are researched widely in condensed matter systems and show potential for spintronics and storage applications. Along with extensive condensed-matter studies of chiral spin textures, photonic counterparts of these textures have been observed in various optical systems with broken inversion symmetry. Unfortunately, the resemblances are only phenomenological. This work proposes a theoretical framework based on the variational theorem to show that the formation of photonic chiral spin textures in an optical interface is derived from the system's symmetry and relativity. Analysis of the optical system's rotational symmetry indicates that conservation of the total angular momentum is preserved from the local variations of spin vectors. Specifically, although the integral spin momentum does not carry net energy, the local spin momentum distribution, which determines the local subluminal energy transport and minimization variation of the square of total angular momentum, results in the chiral twisting of the spin vectors. The findings here deepen the understanding of the symmetries, conservative laws and energy transportation in optical system, construct the comparability in the formation mechanisms and geometries of photonic and condensed-matter chiral spin textures, and suggest applications to optical manipulation and chiral photonics., Comment: 20 pages, 4 figures

Symmetry governs many electronic and photonic phenomena in optics and condensed matter physics. Skyrmions and merons are prominent topological structures in magnetic materials, with the topological features determined by the interplay between anisotropy of a material and its magnetization. Here we theoretically show and experimentally demonstrate that the symmetry of the electromagnetic field determines the spin topological properties of the guided modes via spin-orbit coupling and may only result in either hexagonal spin-skyrmion or square spin-meron lattices. We also show that in the absence of spin-orbit coupling these spin topologies are degenerated in dynamic field-skyrmions, unifying description of electromagnetic field topologies. The results provide new understanding of electromagnetic field topology and its transformations as well as new opportunities for applications in quantum optics, spin-optics and topological photonics.

Magnetic skyrmions are topological quasiparticles in magnetic field. Until recently, as one of their photonic counterparts, N\'eel-type photonic skyrmion is discovered in surface plasmon polaritons. The deep-subwavelength features of the photonic skyrmions suggest their potentials in quantum technologies and data storage. So far, the Bloch-type photonic skyrmion has yet to be demonstrated in this brand new research field. Here, by exploiting the quantum spin Hall effect of a plasmonic optical vortex in multilayered structure, we predict the existence of photonic twisted-N\'eel- and Bloch-type skyrmions in chiral materials. Their chirality-dependent features can be considered as additional degrees-of-freedom for future chiral sensing, information processing and storage technologies. In particular, our findings enlarge the family of photonic skyrmions and reveal a remarkable resemblance of the feature of chiral materials in two seemingly distant fields: photonic skyrmions and magnetic skyrmions., Comment: 6 pages, 5 figures

Multilayer hyperbolic metamaterials consisting of alternating metal and dielectric layers have important applications in spontaneous emission enhancement. In contrast to the conventional choice of at least dozens of layers in multilayer structures to achieve tunable Purcell effect on quantum emitters, our numerical calculations reveal that multilayers with fewer layers and thinner layers would outperform in Purcell effect. These discoveries are attributed to the negative contributions by an increasing layer number to the imaginary part of the reflection coefficient, and the stronger coupling between surface plasmon polariton modes on a thinner metal layer. This work could provide fundamental insights and practical guide for optimizing the local density of optical states enhancement functionality of ultrathin and even two-dimensional photon sources., Comment: 4 pages, 5 figures

Optical spin angular momenta in a confined electromagnetic field exhibit remarkable difference with their free space counterparts, in particular, the optical transverse spin that is locked with the energy propagating direction lays the foundation for many intriguing physical effects such as unidirectional transportation, quantum spin Hall effect, photonic Skyrmion, etc. In order to investigate the underlying physics behind the spin-orbit interactions as well as to develop the optical spin-based applications, it is crucial to uncover the spin texture in a confined field, yet it faces challenge due to their chiral and near-field vectorial features. Here, we propose a scanning imaging technique which can map the near-field distributions of the optical spin angular momenta with an achiral dielectric nanosphere. The spin angular momentum component normal to the interface can be uncovered experimentally by employing the proposed scanning imaging technique and the three-dimensional spin vector can be reconstructed theoretically with the experimental results. The experiment is demonstrated on the example of surface plasmon polaritons excited by various vector vortex beams under a tight-focusing configuration, where the spin-orbit interaction emerges clearly. The proposed method, which can be utilized to reconstruct the photonic Skyrmion and other photonic topological structures, is straightforward and of high precision, and hence it is expected to be valuable for the study of near-field spin optics and topological photonics.

Two novel visual cryptography (VC) schemes are proposed by combining VC with single-pixel imaging (SPI) for the first time. It is pointed out that the overlapping of visual key images in VC is similar to the superposition of pixel intensities by a single-pixel detector in SPI. In the first scheme, QR-code VC is designed by using opaque sheets instead of transparent sheets. The secret image can be recovered when identical illumination patterns are projected onto multiple visual key images and a single detector is used to record the total light intensities. In the second scheme, the secret image is shared by multiple illumination pattern sequences and it can be recovered when the visual key patterns are projected onto identical items. The application of VC can be extended to more diversified scenarios by our proposed schemes.

Deep learning has been extensively applied in many optical imaging applications in recent years. Despite the success, the limitations and drawbacks of deep learning in optical imaging have been seldom investigated. In this work, we show that conventional linear-regression-based methods can outperform the previously proposed deep learning approaches for two black-box optical imaging problems in some extent. Deep learning demonstrates its weakness especially when the number of training samples is small. The advantages and disadvantages of linear-regression-based methods and deep learning are analyzed and compared. Since many optical systems are essentially linear, a deep learning network containing many nonlinearity functions sometimes may not be the most suitable option.

A digital micromirror device (DMD) is an amplitude-type spatial light modulator. However, a complex-amplitude light modulation with a DMD can be achieved using the superpixel scheme. In the superpixel scheme, we notice that multiple different DMD local block patterns may correspond to the same complex superpixel value. Based on this inherent encoding redundancy, a large amount of external data can be embedded into the DMD pattern without extra cost. Meanwhile, the original complex light field information carried by the DMD pattern is fully preserved. This proposed scheme is favorable for applications such as secure information transmission and copyright protection.

Quantum spin-Hall effect, a manifestation of topological properties that govern the behavior of surface states, was studied intensively in condensed matter physics resulting in the discovery of topological insulators. The quantum spin-Hall effect of light was introduced for surface plane-waves which intrinsically carry transverse optical spin, leading to many intriguing phenomena and applications in unidirectional waveguiding, metrology and quantum technologies. In addition to spin, optical waves can exhibit complex topological properties of vectorial electromagnetic fields, associated with orbital angular momentum or nonuniform intensity variations. Here, by considering both spin and angular momentum, we demonstrate a generalized spin-momentum relationship that governs vectorial properties of guided electromagnetic waves, extending optical quantum spin-Hall effect to a two-dimensional vector field of structured guided wave. The effect results in the appearance of the out-of-plane transverse optical spins, which vary progressively from the 'up' state to the 'down' state around the energy flow, and their variation is uniquely locked to the energy propagation direction. The related spin-momentum locking in a chiral spin swirl is demonstrated with four kinds of surface structured waves and proven both theoretically and experimentally. The results provide understanding of the spin dynamics in electromagnetic guided waves and show great importance in spin optics, topological photonics and optical spin-based devices and techniques., Comment: 30 pages; 13 figures; 44 equations; 1 table; 41 references for main text and 45 references for Supplementary Materials

In many previous works, a single-pixel imaging (SPI) system is constructed as an optical image encryption system. Unauthorized users are not able to reconstruct the plaintext image from the ciphertext intensity sequence without knowing the illumination pattern key. However, little cryptanalysis about encrypted SPI has been investigated in the past. In this work, we propose a known-plaintext attack scheme and a ciphertext-only attack scheme to an encrypted SPI system for the first time. The known-plaintext attack is implemented by interchanging the roles of illumination patterns and object images in the SPI model. The ciphertext-only attack is implemented based on the statistical features of single-pixel intensity values. The two schemes can crack encrypted SPI systems and successfully recover the key containing correct illumination patterns.

An optical diffractive neural network (DNN) can be implemented with a cascaded phase mask architecture. Like an optical computer, the system can perform machine learning tasks such as number digit recognition in an all-optical manner. However, the system can only work under coherent light illumination and the precision requirement in practical experiments is quite high. This paper proposes an optical machine learning framework based on single-pixel imaging (MLSPI). The MLSPI system can perform the same linear pattern recognition task as DNN. Furthermore, it can work under incoherent lighting conditions, has lower experimental complexity and can be easily programmable.

A cascaded phase-only mask architecture (or an optical diffractive neural network) can be employed for different optical information processing tasks such as pattern recognition, orbital angular momentum (OAM) mode conversion, image salience detection and image encryption. However, for optical encryption and watermarking applications, such a system usually cannot process multiple pairs of input images and output images simultaneously. In our proposed scheme, multiple input images can be simultaneously fed to an optical diffractive neural network (DNN) system and each corresponding output image will be displayed in a non-overlap sub-region in the output imaging plane. Each input image undergoes a different optical transform in an independent channel within the same system. The multiple cascaded phase masks in the system can be effectively optimized by a wavefront matching algorithm. Similar to recent optical pattern recognition and mode conversion works, the orthogonality property is employed to design a multiplexed DNN.

As one fundamental property of light, the orbital angular momentum (OAM) of photon has elicited widespread interest. Here, we theoretically demonstrate that the OAM conversion of light without any spin state can occur in homogeneous and isotropic medium when a specially tailored locally linearly polarized (STLLP) beam is strongly focused by a high numerical aperture (NA) objective lens. Through a high NA objective lens, the STLLP beams can generate identical twin foci with tunable distance between them controlled by input state of polarization. Such process admits partial OAM conversion from linear state to conjugate OAM states, giving rise to helical phases with opposite directions for each focus of the longitudinal component in the focal field.

In magnetic materials, skyrmions are nanoscale regions where the orientation of electron spin changes in a vortex-type manner. Here we show that spin-orbit coupling in a focused vector beam results in a skyrmion-like photonic spin distribution of the excited waveguided fields. While diffraction limits the spatial size of intensity distributions, the direction of the field, defining photonic spin, is not subject to this limitation. We demonstrate that the skyrmion spin structure varies on the deep-subwavelength scales down to 1/60 of light wavelength, which corresponds to about 10 nanometre lengthscale. The application of photonic skyrmions may range from high-resolution imaging and precision metrology to quantum technologies and data storage where the spin structure of the field, not its intensity, can be applied to achieve deep-subwavelength optical patterns.

Single-pixel imaging (SPI) has a major drawback that many sequential illuminations are required for capturing one single image with long acquisition time. Basis illumination patterns such as Fourier patterns and Hadamard patterns can achieve much better imaging efficiency than random patterns. But the performance is still sub-optimal since the basis patterns are fixed and non-adaptive for varying object images. This Letter proposes a novel scheme for designing and optimizing the illumination patterns adaptively from an image dictionary by extracting the common image features using principal component analysis (PCA). Simulation and experimental results reveal that our proposed scheme outperforms conventional Fourier SPI in terms of imaging efficiency.