We demonstrate that a moiré superlattice nanocavity constructed in a photonic crystal slab promises strongly enhanced nonlinear optics, which is beneficial from the high-quality factor and high coupling efficiency of the flat-band mode with zero group velocity. From a silicon moiré superlattice nanocavity integrated with few-layer gallium selenide (GaSe), the second-harmonic generation (SHG) of GaSe is enhanced by over 10,000 times, and the third-harmonic generation (THG) is enhanced by 8500 times. Our results suggest the moiré superlattice nanocavity could be considered as a promising platform for developing high-efficiency nonlinear photonic devices.

Inhomogeneous uniaxial strain-induced lattice deformations result in the Dirac point shift, leading to a strong synthetic pseudomagnetic field. The chiral edge state in the Haldane model and the antichiral edge state in the modified Haldane model can be realized in gyromagnetic photonic crystals, immersed in external real magnetic fields. Here, the interplay of the real- and pseudo-magnetic fields is investigated based on the onsite magnetization modulation and the uniaxial strain within gyromagnetic photonic crystals, thereby resulting in photonic band deformations including the shift of the chiral edge states and the lift of the degenerate antichiral edge states. The experiment is further performed to observe the imbalanced transport of these edge states on two opposite sides. Our findings can help to deeply explore rich and significant physics of synthetic gauge fields, and facilitate designs of photonic functional devices, such as the proposed unidirectional multichannel waveguide.

Symmetry plays a fundamental role in topological photonic crystals, and topological phase transitions induced by disorder have also been extensively explored in recent years. However, in this work, we find anisotropy can be induced by reducing symmetry in a C2v symmetry triangular photonic crystal. We investigate that anisotropy-induced interfaces profoundly affect edge states and enable the realization of slow light dispersion. Numerical simulations reveal a transition from gapless chiral edge modes to gapped flat band dispersion. Furthermore, we observe higher-order corner states in corner structures constructed by anisotropic interfaces. The corner states can be induced and localized at different lattice positions, thereby realizing multiple types of higher-order topological states. We demonstrate the significance of anisotropic geometry in topological photonics. These findings open new possibilities for steering wave transport in multiple dimensions and offer, to our knowledge, a novel research perspective on the transformation of topological states induced by anisotropic lattices.

Two-dimensional second-order spatial differentiation metasurfaces with different numerical apertures (NAs) were designed by the spatial-frequency Trust-Region algorithm, which can be directly embedded into existing optical imaging systems to efficiently extract edge information of the observed targets. The spatial-frequency Trust-Region algorithm was implemented by integrating the Fourier modal method (FMM) with the Trust-Region algorithm to perform inverse optimization of the metasurface nanostructure. The fabricated metasurface with high-resolution functionality achieved a resolution of 1.2 μm and numerical aperture of 0.87, while the high-contrast one obtained a root-mean-square (RMS) contrast higher than that of the first with a numerical aperture of 0.26. Embedded in an optical microscope, the high-resolution differentiation metasurface, with more high-spatial-frequency components in the transfer function, was utilized to extract fine structures of unstained, even transparent, cell images, providing a new avenue for image segmentation, such as in magnetic resonance imaging. The high-contrast counterpart, due to its high transmission efficiency, was employed to detect edges in dynamic images of paramecia and Brachionus without motion smear, offering potential for application in microsurgical procedures involving real-time image analysis.

Circular dichroism (CD) spectroscopy, widely used for chiral sensing, has been limited by the detection sensitivity. Enhancing optical chirality in the light fields interacting with chiral molecules is crucial for achieving ultrasensitive chiral detection. Here, we present a new paradigm for ultrasensitive chiral detection by creating accessible chiral hotspots using a toroidal dipole Fabry–Perot bound state in the continuum (TD FP-BIC) metasurface. BIC resonance is achieved by controlling the coupling between the TD resonance and its multilayer reflector-induced perfect mirror image. This method enables unprecedented local maximum and average optical chirality enhancements of up to 6×104-fold and 2×103-fold, respectively, within non-structured regions, resulting in an 866-fold increase in CD signals compared to chiral molecules alone without nanostructures. Our results pave the way for enhanced light–matter interactions and ultrasensitive enantiomeric operation.

Polarization detection is essential for various applications, ranging from biological diagnostics to quantum optics. Although various metasurface-based polarimeters have emerged, these platforms are commonly realized through spatial-division designs, which restrict detection accuracy due to inherent factors such as crosstalk. Here, we propose, to our knowledge, a novel strategy for high-accuracy, broadband full-Stokes polarization detection based on the analysis of a single vector beam, whose polarization profile varies sensitively and exhibits a one-to-one correspondence with the incident polarization. Based on this, the incident polarization is completely encoded into the field profile of the vector beam, which avoids crosstalk in principle, and results in high-accuracy polarization detection without any calibration process. As a proof of concept, a geometric-phase metasurface-based grafted perfect vector vortex beam (GPVVB) generator was designed and fabricated. Experimental results demonstrate that our method achieves polarization detection with an average relative error of 2.25%. Benefiting from the broadband high transmittance exceeding 95% of the metasurface due to the femtosecond laser-induced birefringence process, our method operates across a wavelength range of 450–1100 nm. Furthermore, the detection capability of the vector beam polarization profile was validated using a GPVVB-generating array. These results highlight the potential of our approach for transformative applications in polarization detection, including optical communication and machine vision.

Chip-based optical microresonators with ultra-high Q-factors are becoming increasingly important to a variety of applications. However, the losses of on-chip microresonators with the highest Q-factor reported in the past are still far from their material absorption limits. Here, we demonstrate an on-chip silica microresonator that has approached the absorption limit of the state-of-the-art material on chip, realizing, to our knowledge, record intrinsic Q-factors exceeding 3 billion at both 1560 nm and 1064 nm. This fact is corroborated by photo-thermal spectroscopy measurements. Especially, compared with the standard optical fibers, its corresponding optical losses are only 38.4 times and 7.7 times higher at the wavelengths of 1560 nm and 1064 nm, respectively. To exhibit the performance of such fabricated microresonator, we achieve a record-low optical parametric oscillation threshold (31.9 μW) for millimeter-sized microresonators and generate a single-soliton microcomb with a record-low pump power of 220.2 μW for all soliton microcombs realized thus far.

The introduction of non-Hermiticity provides photonic systems with more design degrees of freedom, along with unique properties, which have aroused widespread interest. On the other hand, the concept of synthetic dimensions has also been introduced into non-Hermitian topological physics. In this work, we theoretically investigate the two-dimensional (2D) band structure of a 1D non-Hermitian photonic crystal (PC) by introducing globally a translation deformation as a synthetic dimension. The resulting two-dimensional photonic crystal is a Chern insulator, which is numerically verified by calculated Chern numbers and edge dispersions. We find that this property stems from the inherent topology of synthetic space (kx,Δx), which does not depend on the crystal’s structural and material parameters. It guarantees robust edge states traversing the gap along the synthetic dimension. To provide deeper insight, we derive the reflection phase of a 1D crystal using the plane wave expansion method and give a clear physical picture of the topological edge states generated by translation deformation. These findings may pave the way for translation-based photonic devices, including topological filters and lasers.

Identifying optical modes in chaotic cavities is crucial for exploring and understanding the physical mechanisms inside them. Compared with free spectral range estimation, the direct imaging technique has the capability of providing more precise mode information, but it is extremely time-consuming and susceptible to environmental perturbations. Here we report a high-speed imaging technique for visualizing field distributions in chaotic microcavities. When a silicon microdisk is excited by a femtosecond laser, free carriers are locally generated, thereby reducing the refractive index. Under a constant laser power, the spatial distribution of mode inside the silicon microdisk is proportional to its wavelength shift and can be precisely identified by comparing it with numerical simulation. With the assistance of a galvanometer, imaging a mode profile only takes a few hundred milliseconds to a few seconds, orders of magnitude faster than previous reports. The impacts of slight fabrication deviations on spectra have also been identified.

The dynamic tunability of vector beams (VBs) with metasurfaces plays an important role in the discovery of exotic optical phenomena and development of classic and quantum applications. Using the tunability with longitudinal propagation distance and multifunctional capability of the quarter-waveplate (QWP) meta-atoms, dielectric metasurfaces were designed to generate high-order Poincaré (HOP) beams with tunable elliptical polarization states at arbitrary latitudes. The metasurface contained two interleaved sub-metasurfaces of QWP meta-atoms, each configured with helical, hyperbolic, and primary and secondary axicon-phase profiles to generate a vortex, beam focus, and beam deflection, respectively. Importantly, the axicons were suitably designed by combining the propagation and geometric phases to introduce differences in the z-component wavevectors, and the amplitudes of the co- and cross-polarized vortices were tuned by the longitudinal distance. The method broke through the limitation of previously generating only the linear polarization states on the equator of the HOP sphere, and it also circumvented the traditional tunability using the troublesome waveplate-polarizer combination. This study is of great significance for the miniaturization and integration of optical systems for applications such as optical communications, micromanipulation, and high-precision detection.

The rapid development of topological photonics has significantly facilitated the development of novel microwave and optical devices with richer electromagnetic properties. A stable and efficient guided wave is a necessary condition for optical information transmission and processing. However, most topological waveguides are confined at a domain wall around the interfaces and usually operate in a single-type topological mode, leading to low-throughput energy transmission over a single frequency band. Here, we propose, design, and experimentally demonstrate a novel planar microstrip heterostructure system based on topological LC circuits that supports a dual-type topological large-area waveguide state, and the system showcases tunable mode widths with different operating bandwidths. Inheriting from the pseudospin and valley topology, the topological large-area waveguides exhibit the pseudospin- and valley-locked properties at different frequency windows and have strong robustness against defects. Moreover, the large-area topological waveguide states of high-energy capacity channel intersections and beam expanders with topological pseudospin and valley mode width degrees of freedom are verified numerically and experimentally. We also show the distinct topological origins of large-area topological waveguide states that provide versatile signal routing paths by their intrinsic coupling properties. Our system provides an efficient scheme to realize the tunable width and the multi-mode bandwidth of topological waveguides, which can further promote the applications of multi-functional high-performance topological photonic integrated circuit systems in on-chip communication and signal processing.

Topological photonics provides a strategy that makes light transmission immune to structural-defects-induced backward scattering. Leveraging this, topological negative refraction enables robust, reflectionless light deflection, but directly controlling the refraction direction remains challenging. We demonstrate continuously tunable topological negative refraction at the interface between a one-way waveguide state and a free-space beam, overcoming the limitations of fixed refraction angles in conventional systems. The key insight is the ability to adjust the wavevector of the incident one-way waveguide state. Through manipulating the Bloch wavevector of the waveguide states in momentum space, we achieve a transition from negative to positive refraction. The unidirectional nature of these states prevents backscattering from defects, ensuring immunity to imperfections. As a prototypical demonstration, we achieve dynamic steering of refraction beams from -38° to +12° through active magnetic bias control. Our findings provide an exotic pathway for photon manipulation and a promising route toward topological photonics applications.

Unidirectional guided resonances (UGRs) in planar photonic lattices are distinctive resonant eigenstates that emit light in a single direction. A recent study has demonstrated that UGRs can be utilized to implement ultralow-loss grating couplers for integrated photonic applications. In this study, we investigate the formation and radiation of UGRs in two types of L-shaped gratings, type I and type II, which exhibit both broken up-down mirror symmetry and broken in-plane C2 symmetry. In type I gratings, quasi-UGRs are readily identified in the lower band, whereas in type II gratings they appear in the upper band. We demonstrate that, as the relevant grating parameters are gradually varied, these quasi-UGRs evolve into genuine UGRs in the lower band for type I gratings and in the upper band for type II gratings. In type I gratings, UGRs produce negative-angle emission because their Poynting vectors are oriented antiparallel to their wavevectors, while in type II gratings, UGRs yield positive-angle emission due to the parallel alignment of their Poynting vectors and wavevectors. Moreover, the position and emission angle of UGRs can be systematically controlled by varying the lattice parameters. Our findings offer valuable insights for developing high-efficiency optical interconnects that leverage UGRs.

The integrated quantum interferometer has provided a promising route for manipulating and measuring quantum states of light with high precision, requiring negligible optical loss, broad bandwidth, robust fabrication tolerance, and scalability. In this paper, a rapid adiabatic coupler (RAC) is presented as a compelling solution for implementing the integrated quantum interferometer on a thin-film lithium niobate (TFLN)-based platform, enabling a compact, broadband, and low-loss optical coupler. The TFLN-based RACs are carefully designed by manipulating a curvature along the structures with consideration of inherent birefringence as well as fabrication-induced slanted sidewalls. The high extinction ratio over 20 dB of the RAC-based Mach–Zehnder interferometer (MZI) is achieved in the wavelength range from 1500 to 1600 nm. The beam splitter (BS) with the balanced splitting ratio is exploited for observation of on-chip Hong–Ou–Mandel (HOM) interference with high visibility of 99.25%. We believe TFLN-based RACs hold great potential to be favorably utilized for integrated quantum interferometers, enabling widespread adoptions in myriad applications in integrated quantum optics.

Introducing topological lattice defects, such as dislocations, into topological photonic crystals enables the emergence of many interesting phenomena, including robust bound states and fractional charges. Previous studies have primarily focused on the realization of dislocation modes within a single band gap, which limits the number of dislocation modes and their applications. Here, we design a topological photonic crystal with two topologically non-trivial band gaps. By introducing a dislocation defect into this system, we observe the emergence of localized dislocation modes in both band gaps. Furthermore, we demonstrate a two-channel add-drop filter by coupling two dislocation modes with topological edge modes. These findings are rigorously validated through full-wave numerical simulations and experimental pump-probe transmission measurements. Our results provide a foundation for further exploration of dislocation modes and unlock the potential for harnessing other topological defect modes in dual-band-gap systems.

Spatiotemporal optical vortices (STOVs) exhibit characteristics of transverse orbital angular momentum (OAM) that is perpendicular to the direction of pulse propagation, indicating significant potential for diverse applications. In this study, we employ vanadium dioxide and photonic crystal plates to design tunable transreflective dual-channel terahertz (THz) spatiotemporal vortex generation devices that possess multipolarization adaptability. In the reflection channel, we achieve active tunability of the topological dark lines by utilizing circularly polarized light, based on the topological dark phenomenon, and observe variations in the number of singularities across the parameter space from different observational perspectives. In the transmission channel, we generate independent vortex singularities using linearly polarized light. This multifunctional terahertz device offers a novel approach for the generation and active tuning of spatiotemporal vortices.

Non-Hermitian topology provides an emergent research frontier for studying unconventional topological phenomena and developing new materials and applications. Here, we study the non-Hermitian Chern bands and the associated non-Hermitian skin effects in Floquet checkerboard lattices with synthetic gauge fluxes. Such lattices can be realized in a network of coupled resonator optical waveguides in two dimensions or in an array of evanescently coupled helical optical waveguides in three dimensions. Without invoking nonreciprocal couplings, the system exhibits versatile non-Hermitian topological phases that support various skin-topological effects. Remarkably, the non-Hermitian skin effect can be engineered by changing the symmetry, revealing rich non-Hermitian topological bulk-boundary correspondences. Our system offers excellent controllability and experimental feasibility, making it appealing for exploring diverse non-Hermitian topological phenomena in photonics.

Bound states in the continuum (BICs) open up a unique avenue of enhancing light–matter interactions due to their extreme field confinement and infinite quality (Q) factors. Although tremendous progress has been made in the past 10 years, the majority of previous works focused on either a single BIC or dual BICs. In this work, we present both theoretical investigation and experimental demonstration on multiple BICs in a photonic crystal slab with a hexagonal lattice. All of these BICs at Γ-point can be categorized as symmetry-protected (SP) BICs. Furthermore, two BICs belong to merging BICs with topological charges q=-2. Breaking the structural symmetry will split these BICs with q=-2 into two accidental BICs with q=-1. While the other two are different from the former two, the Q-factors of both modes at the Γ-point retain a stably ultrahigh value (Q>108) when the circular hole is transformed into a rotated elliptical hole with different size ratios of semi-long and semi-short axes. In addition, the Q-factors of the latter two BICs decrease rapidly with kx, indicating that the quasi-BICs become accessible at an ultra-small incident angle. We also show that the Q-factors of the former two BICs exhibit different dependence on the asymmetry parameters, suggesting a viable way of realizing high-Q resonances at multi-wavelengths. Finally, we presented experimental demonstration of four high-Q quasi-BICs at four different wavelengths in the near infrared by fabricating a series of photonic crystal slabs made of rotated elliptical holes and characterizing their reflection spectra. We showed that most of the measured Q-factors are above 1000 for four quasi-BICs, and the highest one can reach 16,764. Our results may find promising applications in sum-frequency generation, four-wave mixing, multiband sensing, lasing, etc.

Due to large anisotropy and tunable exciton transitions observed in visible light, transition metal dichalcogenides could become platform materials for on-chip next-generation photonics and nano-optics. For this to happen, one needs to be able to nanostructure transition metal dichalcogenides without losing their optical properties. However, both our understanding of the physics of such nanostructures and their technology are still at infancy and, therefore, experimental works on optics of transition metal dichalcogenides nanostructures are urgently required. Here, we study optical characteristics of bilayer MoS2 nanoribbons by measuring reflection and photoluminescence of nanostructured bilayer MoS2 flakes near exciton transitions. We show that there exist optically inactive “exciton-free” regions near the edges of nanoribbons with sizes of around 10 nm. We demonstrate that the “exciton-free” regions can be controlled by external electrical gating. These results are important for nanostructured optoelectronic devices made of MoS2 and other transition metal dichalcogenides.

Structural colors have always attracted much attention due to important applications in display devices, imaging security certification, optical data storage, and so on. The brightness of structure colors, as the carrier of chiaroscuro information, is the key to making images appear stronger in the spatial and three-dimensional sense. However, relatively little work has been done on the control of the color brightness, and the reported structures are complex and difficult to fabricate. Here, we demonstrate a low-aspect-ratio anisotropic metasurface consisting of a PMMA film patterned by arrays of elliptical-shaped holes clamped by two thin aluminum films. By utilizing localized surface plasmon resonances, we realize a three-dimensional (3D) HSB (hue, saturation, and brightness) structure color with independent brightness control and enhance the cross-polarization reflection, covering approximately 120% of the sRGB color gamut. It is shown that the ratio of the major and minor axes leads to the independent control of brightness of the structural colors. The nanoprinting of HSB images with smooth brightness transitions is demonstrated through elaborate design of the metasurface geometry parameters and CMOS-compatible micro–nano fabrication process. Our findings will facilitate the broad range of 3D nanoprinting and modern advanced display applications.

High-index dielectric nanoparticles supporting strong Mie resonances, such as silicon (Si) nanoparticles, provide a platform for manipulating optical fields at the subwavelength scale. However, in general, the quality factors of Mie resonances supported by an isolated nanoparticle are not sufficient for realizing strong light-matter interaction. Here, we propose the use of dielectric-metal hybrid nanocavities composed of Si nanoparticles and silicon nitride/silver (Si3N4/Ag) heterostructures to improve light-matter interaction. First, we demonstrate that the nonlinear optical absorption of the Si nanoparticle in a Si/Si3N4/Ag hybrid nanocavity can be greatly enhanced at the magnetic dipole resonance. The Si/Si3N4/Ag nanocavity exhibits luminescence burst at substantially lower excitation energy (∼20.5 pJ) compared to a Si nanoparticle placed on a silica substrate (∼51.3 pJ). The luminescence intensity is also enhanced by an order of magnitude. Second, we show that strong exciton-photon coupling can be realized when a tungsten disulfide (WS2) monolayer is inserted into a Si/Si3N4/Ag nanocavity. When such a system is excited by using s-polarized light, the optical resonance supported by the nanocavity can be continuously tuned to sweep across the two exciton resonances of the WS2 monolayer by simply varying the incident angle. As a result, Rabi splitting energies as large as ∼146.4 meV and ∼110 meV are observed at the A- and B-exciton resonances of the WS2 monolayer, satisfying the criterion for strong exciton-photon coupling. The proposed nanocavities provide, to our knowledge, a new platform for enhancing light-matter interaction in multiple scenarios and imply potential applications in constructing nanoscale photonic devices.

Point-of-care sensors are pivotal for early disease diagnosis, significantly advancing global health. Surface plasmons, the collective oscillations of free electrons under electromagnetic excitation, have been widely studied for biosensing due to their electromagnetic field enhancements at sub-wavelength scales. We introduce a plasmonic biosensor on a compact photonic integrated circuit (PIC) enhanced by exceptional points (EPs). EPs, singularities in non-Hermitian optical systems, provide extreme sensitivity to external perturbations. They emerge when two or more complex resonating modes merge into a single degenerate mode. We demonstrate an EP in a single coupled nanoantenna particle positioned in a uniquely designed silicon nitride slot-waveguide, which we call a junction-waveguide. By laterally shifting two optically coupled gold nanobars of different lengths, we achieve a single particle EP. The junction-waveguide enables efficient coupling of the plasmonic nanoantenna to the waveguide mode. The system integrates a four-port Mach–Zehnder interferometer (MZI), allowing for simultaneous measurements of the amplitude and phase of EP, facilitating highly accurate real-time eigenvalue extraction. For biosensing, we encapsulated the detection zone with a microchannel, enabling low-volume and simple sample handling. Our single particle integrated EP sensor demonstrates superior sensitivity compared to the corresponding linear diabolic point (DP) system under both local and bulk sensing schemes, even at large perturbations. Our studies revealed that the integrated EP sensor can detect a single molecule captured by the nanobars with the average size ranging from 10 to 100 nm. The proposed EP biosensor, with its extreme sensitivity, compact form, and real-time phase sensing capabilities, provides an approach for detecting and quantifying various biomarkers such as proteins and nucleic acids, offering a unique platform for early disease diagnosis.

Bound states in the continuum (BICs) have gained considerable attention for their ability to strengthen light–matter interactions, enabling applications in lasing, sensing, and imaging. These properties also show great promise for intensifying free-electron radiation. Recently, researchers realized momentum-mismatch-driven quasi-BICs in compound grating waveguides. This category of quasi-BICs exhibits high Q factors over a broad frequency spectrum. In this paper, we explore the possibility of achieving multi-frequency terahertz Smith–Purcell radiation empowered by momentum-mismatch-driven quasi-BICs in silicon compound grating waveguides. By leveraging the low-loss properties of silicon in the terahertz range, quasi-BICs are achieved through guided-mode resonance, delivering exceptionally high Q factors over a broad frequency spectrum. The broadband nature of these quasi-BICs enables efficient energy extraction from electron beams across varying voltages, while their multimode characteristics support simultaneous interactions with multiple modes, further boosting radiation intensity. The findings demonstrate significant enhancement of free-electron radiation at multiple frequencies, addressing the limitations of narrowband methods and high-loss metallic systems. By integrating broadband performance with the advantages of low-loss dielectric platforms, this work advances the development of compact, tunable terahertz free-electron radiation sources and provides valuable insights into optimizing quasi-BIC systems for practical applications.

Multiplexing techniques have always been one of the important components of optical communication research. These techniques can transmit multiple signals in a shared information channel and can greatly increase the maximum capacity of an information channel. The Dirac-vortex cavity is a type of photonic crystal surface emission system, and its characteristics of miniaturization and high stability make it very suitable for on-chip optical system. In this paper, we realized dual-channel emission of the Dirac-vortex cavity, which is achieved by modulating the size and phase of hexagonal holes in the hexagon lattice. The characteristics of dual-channel emission are investigated by numerical simulation, and the dual-channel emission rules are summarized. The double Dirac-vortex cavity model is not only explored for its multiplexing capability but also as an alternative scheme for the application of Dirac-vortex cavity in multiplex communication systems.

By introducing photonic crystals with Dirac point based on valley edge states, we design heterostructure waveguides on the silicon-on-insulator platform, promising waveguides with different widths to operate in the single-mode state. Benefiting from the unidirectional transmission and backscattering-immunity characteristics enabled by the topological property, there is no scattering loss induced by the mode-mismatch at the transition junction between the waveguides with different widths. Therefore, the valley-locked heterostructure waveguide possesses unique width degrees of freedom. We demonstrate it by designing and fabricating waveguides with expanding, shrinking, and Z-type configurations. Thanks to the free transition between waveguides with different widths, an interesting energy convergency is observed, which is represented from the imaging of the enhanced third-harmonic generation of the silicon slab. Consequently, these heterostructure waveguides can be more flexibly integrated with existing on-chip devices and have the potential for high-capacity energy transmission, energy concentration, and field enhancement.

Nanoscale light manipulation using plasmonic metasurfaces has emerged as a frontier in photonic research, offering strongly enhanced light–matter interactions with potential applications in sensing, communications, and quantum optics. Here, we unveil the realization and control of chiral quasi-bound states in the continuum (quasi-BICs) by judiciously rotating one of the paired plasmonic bricks and thereby influencing structural asymmetry. By precisely controlling the rotation angle, we enable continuous modulation of the radiation loss in quasi-BICs and transition from a perfect half-wave plate to a good absorber for the left-handed circularly polarized light. This transformation leverages the intrinsic chirality with moderately high circular dichroism of ∼0.35 in both simulation and experimental observations, manifesting unprecedented control over the chiral light within sub-wavelength scales. Theoretical modeling and numerical simulations complement our experimental findings, offering deep insights into underlying mechanisms and the role of symmetry breaking in realizing chiral quasi-BICs. The observed phenomena open new pathways for developing ultra-compact chiral photonic devices with tailored optical properties, including highly sensitive chiral biosensors, circular dichroism spectroscopy, and chiral flat optical components for information processing.

Accurate positioning of nanoantennas is critical for their efficient excitation and integration. However, since nanoantennas are subwavelength nanoparticles, normally smaller than the diffraction limit, measuring their positions presents a significant challenge. This is particularly true for locating the nanoantenna along the z-direction, for which no suitable method currently exists. Here, we have theoretically developed and experimentally validated a novel light field capable of measuring the 3D positions of nanoantennas accurately. This field’s polarization chirality transitions from right-handed to left-handed along a predefined 3D direction at a subwavelength scale. For a spherical single-element nanoantenna, the polarization components of the scattering field change significantly as the nanoantenna moves, due to the rapid polarization transformation in the excitation light field. By analyzing the polarization components of the scattering field, we can achieve positional accuracy of the nanoantenna along the specified direction close to 20 pm. This work improves the accuracy of transversely distinguishing nanoantennas from 100 pm in conventional methods to 20 pm. Moreover, the positioning of the nanoantenna along three dimensions is all available as polarization transitions can be predefined along arbitrary 3D direction, which is significant for precision measurement and nanoscale optics.

A fiber-based, self-aligned dual-beam laser direct writing system with a polarization-engineered depletion beam is designed, constructed, and tested. This system employs a vortex fiber to generate a donut-shaped, cylindrically polarized depletion beam while simultaneously allowing the fundamental mode excitation beam to pass through. This results in a co-axially self-aligned dual-beam source, enhancing stability and mitigating assembly complexities. The size of the central dark spot of the focused cylindrical vector depletion beam can be easily adjusted using a simple polarization rotation device. With a depletion wavelength of 532 nm and an excitation wavelength of 800 nm, the dual-beam laser direct writing system has demonstrated a single linewidth of 63 nm and a minimum line spacing of 173 nm. Further optimization of this system may pave the way for practical superresolution photolithography that surpasses the diffraction limit.

Realizing optical trapping enhancement is crucial in biomedicine, fundamental physics, and precision measurement. Taking the metamaterials with artificially engineered permittivity as photonic force probes in optical tweezers will offer unprecedented opportunities for optical trap enhancement. However, it usually involves multi-parameter optimization and requires lengthy calculations; thereby few studies remain despite decades of research on optical tweezers. Here, we introduce a deep learning (DL) model to attack this problem. The DL model can efficiently predict the maximum axial optical stiffness of Si/Si3N4 (SSN) multilayer metamaterial nanoparticles and reduce the design duration by about one order of magnitude. We experimentally demonstrate that the designed SSN nanoparticles show more than twofold and fivefold improvement in the lateral (kx and ky) and the axial (kz) optical trap stiffness on the high refractive index amorphous TiO2 microsphere. Incorporating the DL model in optical manipulation systems will expedite the design and optimization processes, providing a means for developing various photonic force probes with specialized functional behaviors.

Structured light provides unique opportunities to spatially tailor the electromagnetic field of laser beams. These include the possibility of a sub-wavelength spatial separation of their electric and magnetic fields, which would allow isolating interactions of matter with pure magnetic (or electric) fields. This could be particularly interesting in molecular spectroscopy, as excitations due to electric and—usually very weak—magnetic transition dipole moments can be disentangled. In this work, we show that the use of tailored metallic nanoantennas drastically enhances the strength of the longitudinal magnetic field carried by an ultrafast azimuthally polarized beam (by a factor of ∼65), which is spatially separated from the electric field by the beam’s symmetry. Such enhancement is due to favorable phase-matching of the magnetic field induced by the electric current loops created in the antennas. Our particle-in-cell simulation results demonstrate that the interactions of moderately intense (∼1011 W/cm2) and ultrafast azimuthally polarized laser beams with conical, parabolic, Gaussian, or logarithmic metallic nanoantennas provide spatially isolated magnetic field pulses of several tens of Tesla.

The room temperature strong coupling between the photonic modes of micro/nanocavities and quantum emitters (QEs) can bring about promising advantages for fundamental and applied physics. Improving the electric fields (EFs) by using plasmonic modes and reducing their losses by applying dielectric nanocavities are widely employed approaches to achieve room temperature strong coupling. However, ideal photonic modes with both large EFs and low loss have been lacking. Herein, we propose the abnormal anapole mode (AAM), showing both a strong EF enhancement of ∼70-fold (comparable to plasmonic modes) and a low loss of 34 meV, which is much smaller than previous records of isolated all-dielectric nanocavities. Besides realizing strong coupling, we further show that by replacing the normal anapole mode with the AAM, the lasing threshold of the AAM-coupled QEs can be reduced by one order of magnitude, implying a vital step toward on-chip integration of nanophotonic devices.

2D materials are promising candidates as nonlinear optical components for on-chip devices due to their ultrathin structure. In general, their nonlinear optical responses are inherently weak due to the short interaction thickness with light. Recently, there has been great interest in using quasi-bound states in the continuum (q-BICs) of dielectric metasurfaces, which are able to achieve remarkable optical near-field enhancement for elevating the second harmonic generation (SHG) emission from 2D materials. However, most studies focus on the design of combining bulk dielectric metasurfaces with unpatterned 2D materials, which suffer considerable radiation loss and limit near-field enhancement by high-quality q-BIC resonances. Here, we investigate the dielectric metasurface evolution from bulk silicon to monolayer molybdenum disulfide (MoS2), and discover the critical role of meta-atom thickness design on enhancing near-field effects of two q-BIC modes. We further introduce the strong-coupling of the two q-BIC modes by oblique incidence manipulation, and enhance the localized optical field on monolayer MoS2 dramatically. In the ultraviolet and visible regions, the MoS2 SHG enhancement factor of our design is 105 times higher than that of conventional bulk metasurfaces, leading to an extremely high nonlinear conversion efficiency of 5.8%. Our research will provide an important theoretical guide for the design of high-performance nonlinear devices based on 2D materials.

Bound states in the continuum (BICs) in artificial photonic structures have received considerable attention since they offer unique methods for the extreme field localization and enhancement of light-matter interactions. Usually, the symmetry-protected BICs are located at high symmetric points, while the positions of accidental BICs achieved by tuning the parameters will appear at some points in momentum space. Up to now, to accurately design the position of the accidental BIC in momentum space is still a challenge. Here, we theoretically and experimentally demonstrate an accurately designed accidental BIC in a two-coupled-oscillator system consisting of bilayer gratings, where the optical response of each grating can be described by a single resonator model. By changing the interlayer distance between the gratings to tune the propagation phase shift related to wave vectors, the position of the accidental BIC can be arbitrarily controlled in momentum space. Moreover, we present a general method and rigorous numerical analyses for extracting the polarization vector fields to observe the topological properties of BICs from the polarization-resolved transmission spectra. Finally, an application of the highly efficient second harmonic generation assisted by quasi-BIC is demonstrated. Our work provides a straightforward strategy for manipulating BICs and studying their topological properties in momentum space.

Second-order topological photonic crystals support localized corner modes that deviate from the conventional bulk-edge correspondence. However, the frequency shift of corner modes spanning the photonic band gap has not been experimentally reported. Here, we observe the gapless corner modes of photonic crystal slabs within a parameter space by considering translation as an additional synthetic dimension. These corner modes, protected by topological pumping in synthetic translation dimensions, are found to exist independently of the specific corner configuration. The gapless corner modes are experimentally imaged via the near-field scanning measurement and validated numerically by full-wave simulations. We propose a topological rainbow with gradient translation, demonstrating the ability to extract and separate specific frequency components of light into different spatial locations. Our work contributes to the advancement of topological photonics and provides valuable insights into the exploration of gapless corner modes in synthetic dimensions.

Nanoparticles made of different materials usually support optical resonances in the visible to near infrared spectral range, such as the localized surface plasmons observed in metallic nanoparticles and the Mie resonances observed in dielectric ones. Such optical resonances, which are important for practical applications, depend strongly on the morphologies of nanoparticles. Laser irradiation is a simple but effective way to modify such optical resonances through the change in the morphology of a nanoparticle. Although laser-induced shaping of metallic nanoparticles has been successfully demonstrated, it remains a big challenge for dielectric nanoparticles due to their larger Young’s modulus and smaller thermal conductivities. Here, we proposed and demonstrated a strategy for realizing controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force induced by femtosecond laser pulses. It was found that both Si and Ge nanoparticles can be lit up by resonantly exciting the optical resonances with femtosecond laser pulses, leading to the luminescence burst when the laser power exceeds a threshold. In addition, the morphologies of Si and Ge nanoparticles can be modified by utilizing the giant absorption force exerted on them and the reduced Young’s modulus at high temperatures. The shape transformation from sphere to ellipsoid can be realized by laser irradiation, leading to the blueshifts of the optical resonances. It was found that Si and Ge nanoparticles were generally elongated along the direction parallel to the polarization of the laser light. Controllable shaping of Si and Ge can be achieved by deliberately adjusting the excitation wavelength and the laser power. Our findings are helpful for understanding the giant absorption force of femtosecond laser light and are useful for designing nanoscale photonic devices based on shaped high-index nanoparticles.

Plasmonic sensing technology has attracted considerable attention for high sensitivity due to the ability to effectively localize and manipulate light. In this study, we demonstrate a refractive index (RI) sensing scheme based on open-loop twisted meta-molecule arrays using the versatile nano-kirigami principle. RI sensing has the features of a small footprint, flexible control, and simple preparation. By engineering the morphology of meta-molecules or the RI of the ambient medium, the chiral surface lattice resonances can be significantly enhanced, and the wavelength, intensity, and sign of circular dichroism (CD) can be flexibly tailored. Utilizing the relation between the wavelength of the CD peak and the RI of the superstrate, the RI sensor achieves a sensitivity of 1133 nm/RIU. Additionally, we analyze these chiroptical responses by performing electromagnetic multipolar decomposition and electric field distributions. Our study may serve as an ideal platform for applications of RI measurement and provide new insights into the manipulation of chiral light–matter interactions.

Bound states in the continuum (BICs), which are exotic localized eigenstates embedded in the continuum spectrum and exhibit topological polarization singularities in momentum space, have recently attracted great attention in both fundamental and applied physics. Here, based on a magneto-optical (MO) photonic crystal (PhC) slab placed in external magnetic fields with time-reversal symmetry (TRS) breaking, we theoretically propose magnetically tunable BICs with arbitrary polarization covering the entire Poincaré sphere and efficient off-Γ chiral emission of circularly polarized states (C point). More interestingly, by further breaking the in-plane inversion symmetry of the MO PhC slab to generate a pair of C points spawning from the eliminated BICs and tuning the external magnetic field strength to move one C point to the Γ point, an at-Γ intrinsic chiral BIC exhibits chiral characteristics on both sides of the PhC slab with near-unity circular dichroism exceeding 0.99 and a high-quality factor of 46,000 owing to the preserved out-of-plane mirror symmetry. Moreover, the chirality of the chiral BICs can be inverted by flipping the magnetic bias. Our work opens an unprecedented avenue to explore the unique topological photonics of BICs with broken TRS and promises multiple applications in chiral-optical effects, structured light, and tunable optical devices.

Dielectric nanostructures are widely embraced in the field of structural color design due to their low-cost characteristics, enabling sub-micron scale color printing. However, challenges still exist in the selection of structures and image encryption. In this study, we propose a method for printing dual patterns using tailored scattering structures based on two-photon polymerization. We extensively analyze the color performance of each structure in zeroth-order diffraction under cross-polarized transmission and bright-field transmission illumination. By selecting appropriate structures based on their characteristics, we prepared full-color panels and successfully utilized these panels to print both color patterns and dual patterns, achieving multi-level control of color and information. Based on the above study, a large-sale color pattern with a hidden message in an area of 3.2 cm×2.4 cm is printed, which can be directly observed. Our results demonstrate a sustainable and eco-friendly approach to color preparation, offering innovative strategies and methods for the fields of color science and steganography for information security.

Dispersion management in guided wave optics is of vital importance for various applications. Topological photonics opens new horizons for manipulating unidirectional guided waves utilizing edge states. However, the experimental observation of spatiotemporal dynamics for guided waves with on-demand dispersion in topological photonic crystal is an important issue awaiting exploitation. Herein, we experimentally investigate the spatiotemporal properties of topological surface states with on-demand dispersion, where they are supported by a truncated valley photonic crystal with surface modulation. We observe the electromagnetic dynamics of surface states with typical dispersions, where dynamical trapping of an electromagnetic pulse mediated by the unidirectional surface state with flat dispersion and backward beam routing using reversed dispersion properties are achieved in photonic crystal slabs. Both numerical and experimental results substantiate the ultimate dispersion management for topological surface states, which could pave new ways for the manipulation of electromagnetic waves on the surface of photonic devices.

The sensitivity of guided mode resonance (GMR) sensors is significantly enhanced under oblique incidence. Here in this work, we developed a simplified theoretical model to provide analytical solutions and reveal the mechanism of sensitivity enhancement. We found that the sensitivity under oblique incidence consists of two contributions, the grating sensitivity and waveguide sensitivity, while under normal incidence, only waveguide sensitivity exists. When the two contributions are constructively superposed, as in the case of positive first order diffraction of the grating, the total sensitivity is enhanced. On the other hand, when the two parts are destructively superposed, as in the case of negative first order diffraction, the total sensitivity decreases. The findings are further supported by FDTD numerical calculations and proof-of-concept experiments.

We investigate the chiral emission from non-chiral molecules coupled to metasurfaces with a unit cell formed by dimers of detuned and displaced Si nanodisks. The detuning and displacement lead to the formation of narrow modes, known as quasi-bound states in the continuum (Q-BICs), with different electric and magnetic characteristics. The dispersion and character of the modes are explained by using the guided-mode expansion method and finite-element simulations. The coupling between these modes leads to an extrinsic chiral response with large circular dichroism for defined energies and wavevectors. When the lattice constant of the metasurface is changed, the dispersion of the extrinsic chiral Q-BICs can be tuned and the emission properties of a thin film of dye molecules on top of the metasurface are modified. In particular, we observe strongly directional and circularly polarized emission from the achiral dye molecules with a degree of circular polarization reaching 0.8 at the wavelengths defined by the dispersion of the Q-BICs. These results could enable the realization of compact light sources with a large degree of circular polarization for applications in displays, optical recording, or optical communication.

Higher-order topological insulators, originally proposed in quantum condensed matters, have provided a new avenue for localizing and transmitting light in photonic devices. Nontrivial band topology in crystals with certain symmetries can host robust topological edge states and lower dimensional topological corner states (TCS), making them a promising platform for photonics applications. Here, we have designed several types of TCS with only two specific C6v-symmetric photonic crystals with various seamless splicing boundaries, where all the supposed TCS with diverse electromagnetic characteristics are visualized via numerical simulations and experimental measurements. More interestingly, we have observed that those TCS overlapping in spectral and spatial space tend to interweaved, inducing spectrum division. Meanwhile, the equivalent corners appear to have TCS with a phase difference, which is critical for directional activation of pseudospin dependence. Our findings demonstrate that coupled TCS with phase difference at different nanocavities can be selectively excited by a chiral source, which indicates that the TCS at this time have pseudospin-dependent properties. We further design a specific splicing structure to prevent coupling between adjacent TCS. This work provides a flexible approach for space- and frequency-division multiplexing in photonic devices.

Optical singularity is pivotal in nature and has attracted wide interest from many disciplines nowadays, including optical communication, quantum optics, and biomedical imaging. Visualizing vortex lines formed by phase singularities and fabricating chiral nanostructures using the evolution of vortex lines are of great significance. In this paper, we introduce a promising method based on two-photon polymerization direct laser writing (2PP-DLW) to record the morphology of vortex lines generated by tightly focused multi-vortex beams (MVBs) at the nanoscale. Due to Gouy phase, the singularities of the MVBs rotate around the optical axis and move towards each other when approaching the focal plane. The propagation dynamics of vortex lines are recorded by 2PP-DLW, which explicitly exhibits the evolution of the phase singularities. Additionally, the MVBs are employed to fabricate stable three-dimensional chiral nanostructures due to the spiral-forward property of the vortex line. Because of the obvious chiral features of the manufactured nanostructures, a strong vortical dichroism is observed when excited by the light carrying orbital angular momentum. A number of applications can be envisioned with these chiral nanostructures, such as optical sensing, chiral separation, and information storage.

An optical field with sub-nm confinement is essential for exploring atomic- or molecular-level light-matter interaction. While such fields demonstrated so far have typically point-like cross-sections, an optical field having a higher-dimensional cross-section may offer higher flexibility and/or efficiency in applications. Here, we propose generating a nanoscale blade-like optical field in a coupled nanofiber pair (CNP) with a 1-nm-width central slit. Based on a strong mode coupling-enabled slit waveguide mode, a sub-nm-thickness blade-like optical field can be generated with a cross-section down to ∼0.28 nm×38 nm at 1550 nm wavelength (i.e., a thickness of ∼λ0/5000) and a peak-to-background intensity ratio (PBR) higher than 20 dB. The slit waveguide mode of the CNP can be launched from one of the two nanofibers that are connected to a standard optical fiber via an adiabatical fiber taper, in which a fundamental waveguide mode of the fiber can be converted into a high-purity slit mode with high efficiency (>98%) within a CNP length of less than 10 μm at 1550 nm wavelength. The wavelength-dependent behaviors and group velocity dispersion in mode converting processes are also investigated, showing that such a CNP-based design is also suitable for broadband and ultrafast pulsed operation. Our results may open up new opportunities for studying light-matter interaction down to the sub-nm scale, as well as for exploring ultra-high-resolution optical technology ranging from super-resolution nanoscopy to chemical bond manipulation.

Epsilon-near-zero (ENZ) media were demonstrated to exhibit unprecedented strong nonlinear optical properties including giant second-harmonic generation (SHG) due to their field-enhancement effect. Here, on the contrary, we report the quenching of SHG by the ENZ media. We find that when a tiny nonlinear particle is placed very close to a subwavelength ENZ particle, the SHG from the nonlinear particle can be greatly suppressed. The SHG quenching effect originates from the extraordinary prohibition of electric fields occurring near the ENZ particle due to evanescent scattering waves, which is found to be universal in both isotropic and anisotropic ENZ particles, irrespective of their shapes. Based on this principle, we propose a kind of dynamically controllable optical metasurface exhibiting switchable SHG quenching effect. Our work enriches the understanding of optical nonlinearity with ENZ media and could find applications in optical switches and modulators.

The realization of pseudomagnetic fields for lightwaves has attained great attention in the field of nanophotonics. Like real magnetic fields, Landau quantization could be induced by pseudomagnetic fields in the strain-engineered graphene. We demonstrated that pseudomagnetic fields can also be introduced to photonic crystals by exerting a linear parabolic deformation onto the honeycomb lattices, giving rise to degenerate energy states and flat plateaus in the photonic band structures. We successfully inspire the photonic snake modes corresponding to the helical state in the synthetic magnetic heterostructure by adopting a microdisk for the unidirectional coupling. By integrating heat electrodes, we can further electrically manipulate the photonic density of states for the uniaxially strained photonic crystal. This offers an unprecedented opportunity to obtain on-chip robust optical transports under the electrical tunable pseudomagnetic fields, opening the possibility to design Si-based functional topological photonic devices.

We propose a pseudospin-field-dependent waveguide (PFDW) by constructing a sandwiched heterostructure consisting of three magneto-optical photonic crystals (MOPCs) with different geometric parameters. The upper expanded MOPC applied with an external magnetic field has broken time-reversal symmetry (TRS) and an analogous quantum spin Hall (QSH) effect, while the middle standard and the lower compressed ones are not magnetized and trivial. Attributed to the TRS-broken-QSH effect of the upper MOPC, the topological large-area one-way transmission that uniformly distributes over the middle domain is achieved and exhibits the characteristics of a pseudospin-field-momentum-locking; i.e., pseudospin-down (or pseudospin-up) leftward (or rightward) waveguide state when the positive (or negative) magnetic field is applied on the upper MOPC. We further demonstrate the strong robustness of the PFDW against backscattering from various kinds of defects. In addition, a topological beam modulator that can compress or expand the light beam, and a large-area pseudospin beam splitter have been designed. These results have potential in various applications such as sensing, signal processing, and optical communications.

Metasurfaces have provided unprecedented degrees of freedom in manipulating electromagnetic waves upon interfaces. In this work, we first explore the condition of wide operating bandwidth in the view of reflective scheme, which indicates the necessity of anomalous dispersion. To this end, the leaky cavity modes (LCMs) in the meta-atom are analyzed and can make effective permittivity inversely proportional to frequency. Here we employ the longitudinal Fabry–Perot (F-P) resonances and transverse plasmonic resonances to improve the LCMs efficiency. It is shown that the order of F-P resonance can be customized by the plasmonic modes, that is, the F-P cavity propagation phase should match the phase delay of surface currents excited on the meta-atom. The nth order F-P resonance will multiply the permittivity by a factor of n, allowing larger phase accumulation with increasing frequencies and forming nonlinear phase distribution which can be applied in weak chromatic-aberration focusing design. As a proof-of-concept, we demonstrate a planar weak chromatic-aberration focusing reflector with a thickness of λ0/9 at 16.0–21.0 GHz. This work paves a robust way to advanced functional materials with anomalous dispersion and can be extended to higher frequencies such as terahertz, infrared, and optical frequencies.

Optical nanofiber cavity research has mainly focused on the fundamental mode. Here, a Fabry–Pérot fiber cavity with an optical nanofiber supporting the higher-order modes (TE01, TM01, HE21o, and HE21e) is demonstrated. Using cavity spectroscopy, with mode imaging and analysis, we observed cavity resonances that exhibited complex, inhomogeneous states of polarization with topological features containing Stokes singularities such as C-points, Poincaré vortices, and L-lines. In situ tuning of the intracavity birefringence enabled the desired profile and polarization of the cavity mode to be obtained. We believe these findings open new research possibilities for cold atom manipulation and multimode cavity quantum electrodynamics using the evanescent fields of higher-order mode optical nanofibers.

We propose a design on integrated optical devices on-chip with an extra width degree of freedom by using a photonic crystal waveguide with Dirac points between two photonic crystals with opposite valley Chern numbers. With such an extra waveguide, we demonstrate numerically that the topologically protected photonic waveguide retains properties of valley-locking and immunity to defects. Due to the design flexibility of the width-tunable topologically protected photonic waveguide, many unique on-chip integrated devices have been proposed, such as energy concentrators with a concentration efficiency improvement of more than one order of magnitude, and a topological photonic power splitter with an arbitrary power splitting ratio. The topologically protected photonic waveguide with the width degree of freedom could be beneficial for scaling up photonic devices, and provides a flexible platform to implement integrated photonic networks on-chip.

The manipulation of polarization states beyond the optical limit presents advantages in various applications. Considerable progress has been made in the design of meta-waveplates for on-demand polarization transformation, realized by numerical simulations and parameter sweep methodologies. However, due to the limited freedom in these classical strategies, particular challenges arise from the emerging requirement for multiplex optical devices and multidimensional manipulation of light, which urge for a large number of different nanostructures with great polarization control capability. Here, we demonstrate a set of self-designed arbitrary wave plates with a high polarization conversion efficiency. We combine Bayesian optimization and deep neural networks to design perfect half- and quarter-waveplates based on metallic nanostructures, which experimentally demonstrate excellent polarization control functionalities with the conversion ratios of 85% and 90%. More broadly, we develop a comprehensive wave plate database consisting of various metallic nanostructures with high polarization conversion efficiency, accompanying a flexible tuning of phase shifts (0–2π) and group delays (0–10 fs), and construct an achromatic metalens based on this database. Owing to the versatility and excellent performance, our self-designed wave plates can promote the performance of multiplexed broadband metasurfaces and find potential applications in compact optical devices and polarization division multiplexing optical communications.

Optical skyrmions formed by photonic spin–orbit (SO) coupling are of significant interest in high-dimensional optical information processing. We report the formation mechanism and non-Hermitian properties of skyrmion-like states in a circular confinement potential with photonic SO coupling, which is preferably realized in a concave-planar microcavity system. We show that the effective photonic gauge field leads to two split manifolds of degenerate skyrmions whose spin textures can be controlled via the non-Hermitian properties by introducing circularly polarized gain and loss, exhibiting dramatically discrepant evolutions at the two sides of the exceptional point (EP). Furthermore, the lifetime degeneracy can be lifted by spatially inhomogeneous pumping according to the non-Hermitian mechanism, enabling the possibility for the skyrmion laser. By introducing shape asymmetry of the confinement potential, a double EP evolution can be achieved, which allows non-Hermitian control of the SO coupled states with higher degrees of freedom. These results open the way for the non-Hermitian control of photonic spin in confined systems, which would be of great significance for the fundamentals of advanced optical information processing.

Realizing a large-scale fully controllable quantum system is a challenging task in current physical research and has broad applications. In this work, we create a reconfigurable optically levitated nanoparticle array in vacuum. Our optically levitated nanoparticle array allows full control of individual nanoparticles to form an arbitrary pattern and detect their motion. As a concrete example, we choose two nanoparticles without rotation signals from an array to synthesize a nanodumbbell in situ by merging them into one trap. The nanodumbbell synthesized in situ can rotate beyond 1 GHz. Our work provides a platform for studying macroscopic many-body physics and quantum sensing.

Topological edge states have an important role in optical modulation with potential applications in wavelength division multiplexers (WDMs). In this paper, 2D photonic crystals (PCs) with different rotation angles are combined to generate topological edge states. We reveal the relationship between the edge states and the rotation parameters of PCs, and further propose a WDM to realize the application of adjustable beams. Our findings successfully reveal the channel selectivity for optical transmission and provide a flexible way to promote the development of topological photonic devices.

Recently, the concepts of parity–time (PT) symmetry and band topology have inspired many novel ideas for light manipulation in their respective directions. Here we propose and demonstrate a perfect light absorber with a PT phase transition via coupled topological interface states (TISs), which combines the two concepts in a one-dimensional photonic crystal heterostructure. By fine tuning the coupling between TISs, the PT phase transition is revealed by the evolution of absorption spectra in both ideal and non-ideal PT symmetry cases. Especially, in the ideal case, a perfect light absorber at an exceptional point with unidirectional invisibility is numerically obtained. In the non-ideal case, a perfect light absorber in a broken phase is experimentally realized, which verifies the possibility of tailoring non-Hermiticity by engineering the coupling. Our work paves the way for novel effects and functional devices from the exceptional point of coupled TISs, such as a unidirectional light absorber and exceptional-point sensor.

Faint light spectroscopy has many important applications such as fluorescence spectroscopy, lidar, and astronomical observations. However, the long measurement time limits its application to real-time measurement. In this work, a photon counting reconstructive spectrometer combining metasurfaces and superconducting nanowire single-photon detectors is proposed. A prototype device was fabricated on a silicon-on-insulator substrate, and its performance was characterized. Experiment results show that this device supports spectral reconstruction of mono-color lights with a resolution of 2 nm in the wavelength region of 1500–1600 nm. Its detection efficiency is 1.4%–3.2% in this wavelength region. The measurement time required by the photon counting reconstructive spectrometer was also investigated experimentally, showing its potential to be applied in scenarios requiring real-time measurement.

Strong coupling of mid-infrared (mid-IR) vibrational transitions to optical cavities provides a means to modify and control a material’s chemical reactivity and offers a foundation for novel chemical detection technology. Currently, the relatively large volumes of the mid-IR photonic cavities and weak oscillator strengths of vibrational transitions restrict vibrational strong coupling (VSC) studies and devices to large ensembles of molecules, thus representing a potential limitation of this nascent field. Here, we experimentally and theoretically investigate the mid-IR optical properties of 3D-printed multimode metal–insulator–metal (MIM) plasmonic nanoscale cavities for enabling strong light–matter interactions at a deep subwavelength regime. We observe strong vibration-plasmon coupling between the two dipolar modes of the L-shaped cavity and the carbonyl stretch vibrational transition of the polymer dielectric. The cavity mode volume is half the size of a typical square-shaped MIM geometry, thus enabling a reduction in the number of vibrational oscillators to achieve strong coupling. The resulting three polariton modes are well described by a fully coupled multimode oscillator model where all coupling potentials are non-zero. The 3D printing technique of the cavities is a highly accessible and versatile means of printing arbitrarily shaped submicron-sized mid-IR plasmonic cavities capable of producing strong light–matter interactions for a variety of photonic or photochemical applications. Specifically, similar MIM structures fabricated with nanoscopic voids within the insulator region could constitute a promising microfluidic plasmonic cavity device platform for applications in chemical sensing or photochemistry.

To enhance the strength of chiral light–matter interaction for practical applications, the chirality and quality factors (Q-factors) of current methods need to be strengthened simultaneously. Here, we propose a design of photonic crystal slabs (PhCs) supporting chiral bound states in the continuum (BICs) of transverse electric (TE) and transverse magnetic (TM) modes, exhibiting maximal chiroptical responses with high Q-factors and near-unity circular dichroism (CD=0.98). Different from the past, the PhCs we employed only have reduced in-plane symmetry and can support simultaneously chiral quasi-BICs (q-BICs) of TE and TM mode with two-dimensional ultra-strong external and internal chirality. Based on the temporal coupled-mode theory, two analytical expressions of CD of chiral q-BICs response are revealed, which are consistent with the simulation results. Furthermore, we elucidate these results within the charge-current multipole expansion framework and demonstrate that the co-excitation of higher-order multipole electric/magnetic modes is responsible for near-perfect CD. Our results may provide more flexible opportunities for various applications requiring high Q-factors and chirality control, such as chiral lasing, chiral sensing, and enantiomer separation.

Zero-index metamaterials (ZIMs) feature a uniform electromagnetic mode over a large area in arbitrary shapes, enabling many applications including high-transmission supercouplers with arbitrary shapes, direction-independent phase matching for nonlinear optics, and collective emission of many quantum emitters. However, most ZIMs reported to date are passive; active ZIMs that allow for dynamic modulation of their electromagnetic properties have rarely been reported. Here, we design and fabricate a magnetically tunable ZIM consisting of yttrium iron garnet (YIG) pillars sandwiched between two copper clad laminates in the microwave regime. By harnessing the Cotton–Mouton effect of YIG, the metamaterial was successfully toggled between gapless and bandgap states, leading to a “phase transition” between a zero-index phase and a single negative phase of the metamaterial. Using an S-shaped ZIM supercoupler, we experimentally demonstrated a tunable supercoupling state with a low intrinsic loss of 0.95 dB and a high extinction ratio of up to 30.63 dB at 9 GHz. We have also engineered a transition between the supercoupling state and the topological one-way transmission state at 10.6 GHz. Our work enables dynamic modulation of the electromagnetic characteristics of ZIMs, enabling various applications in tunable linear, nonlinear, quantum, and nonreciprocal electromagnetic devices.

Recently studied bound states in the continuum (BICs) enable perfect localization of light and enhance light–matter interactions although systems are optically open. They have found applications in numerous areas, including optical nonlinearity, light emitters, and nano-sensors. However, their unidirectional nature in nonreciprocal devices is still elusive because such trapping states are easily destroyed when the symmetry of an optical system is broken. Herein, we propose nonreciprocal and dynamically tunable BICs for unidirectional confinement of light and symmetry-protected BICs at Γ-point by introducing antiparallel magnetism into the optical system. We demonstrate that such BICs can be achieved by using topological magnetic Weyl semimetals near zero-index frequency without any structural asymmetry, and are largely tunable via modifying the Fermi level. Our results reveal a regime of extreme light manipulation and interaction with emerging quantum materials for various practical applications.

Tunable coupled mechanical resonators with nonequilibrium dynamic phenomena have attracted considerable attention in quantum simulations, quantum computations, and non-Hermitian systems. In this study, we propose tunable mechanical-mode coupling based on nanobeam-double optomechanical cavities. The excited optical mode interacts with both symmetric and antisymmetric mechanical supermodes and mediates coupling at a frequency of approximately 4.96 GHz. The mechanical-mode coupling is tuned through both optical spring and gain effects, and the reduced coupled frequency difference in non-Hermitian parameter space is observed. These results benefit research on the microscopic mechanical parity–time symmetry for topology and on-chip high-sensitivity sensors.

The light manipulation beyond the diffraction limit plays an invaluable role in modern physics and nanophotonics. In this work, we have demonstrated a strong coupling with a large Rabi splitting of 151 meV between bulk WS2 excitons and anapole modes in the WS2-Si nanodisk heterostructure array with nanoholes as small as 50 nm radius. This result is acquired by introducing anapole modes to suppress radiative losses to confine light into subwavelength volumes and large spatial overlapping between excitons and strong optical fields. Our work shows that anapole modes may serve as a powerful way to enhance the interaction between light and matter at nanoscales, and it should pave an avenue toward high-performance all-dielectric optoelectronic applications.

The phenomenon of bound state in the continuum (BIC) with an infinite quality factor and lifetime has emerged in recent years in photonics as a new tool to manipulate light–matter interactions. However, most of the investigated structures only support BIC resonances at very few discrete points in the ω∼k space. Even when the BIC is switched to a quasi-BIC (QBIC) resonance through perturbation, its frequency will still be located within a narrow spectral band close to that of the original BIC, restricting their applications in many fields where random or multiple input frequencies beyond the narrow band are required. In this work, we demonstrate that a new set of QBIC resonances can be supported by using a special binary grating consisting of two alternatingly aligned ridge arrays with the same period and zero-approaching ridge width difference on a slab waveguide. These QBIC resonances are distributed continuously over a broad band along a line in the ω∼k space and can thus be considered as 1D QBICs. With the Q factors generally affected by the ridge difference, it is now possible to arbitrarily choose any frequencies on the dispersion line to achieve significantly enhanced light–matter interactions, facilitating many applications where multiple input wavelengths are required; e.g., sum or difference frequency generations in nonlinear optics.

Traditional optical components are usually designed for a single functionality and narrow operation band, leading to the limited practical applications. To date, it is still quite challenging to efficiently achieve multifunctional performances within broadband operating bandwidth via a single planar optical element. Here, a broadband high-efficiency polarization-multiplexing method based on a geometric phase polymerized liquid crystal metasurface is proposed to yield the polarization-switchable functionalities in the visible. As proofs of the concept, two broadband high-efficiency polymerized liquid crystal metalenses are designed to obtain the spin-controlled behavior from diffraction-limited focusing to sub-diffraction focusing or focusing vortex beams. The experimental results within a broadband range indicate the stable and excellent optical performance of the planar liquid crystal metalenses. In addition, low-cost polymerized liquid crystal metasurfaces possess unique superiority in large-scale patterning due to the straightforward processing technique rather than the point-by-point nanopatterning method with high cost and low throughput. The high-efficiency liquid crystal metasurfaces also have unrivalled advantages benefiting from the characteristic with low waveguide absorption. The proposed strategy paves the way toward multifunctional and high-integrity optical systems, showing great potential in mobile devices, optical imaging, robotics, chiral materials, and optical interconnections.

Synchronized rotation of unit cells in a periodic structure provides a novel design perspective for manipulation of band topology. We then design a two-dimensional version of higher-order topological insulator (HOTI) by such rotation in a triangular photonic lattice with C3 symmetry. This HOTI supports the hallmark zero-dimensional corner states and, simultaneously, the one-dimensional edge states. We also find that our photonic corner states carry chiral orbital angular momenta locked by valleys, whose wave functions are featured by the phase vortex (singularity) positioned at the maximal Wyckoff points. Moreover, when excited by a fired source with various frequencies, the valley topological states of both one-dimensional edges and zero-dimensional corners emerge simultaneously. Extendable to higher or synthetic dimensions, our paper provides access to a chiral vortex platform for HOTI realizations in the terahertz photonic system.

The chiral coupling of an emitter to waveguide mode, i.e., the propagation direction of the excited waveguide mode is locked to the transverse spin (T-spin) of a circularly polarized emitter, has exhibited unprecedented applications in nanophotonics and quantum information processing. This chiral coupling can be largely enhanced in terms of unidirectivity, efficiency, and spontaneous emission rate by introducing resonant modes as coupling interfaces. However, this indirect chiral coupling still undergoes limitations in flexibility and miniaturization, and the underlying physical mechanisms are to be clarified. Here, we present an intuitive and rigorous approach for analyzing the direct/indirect chiral coupling, and thereout, derive some general relations between the chiral-coupling directionality and the T-spin of the field or emitter. Based on the theories, we propose an indirect chiral-coupling system on the platform of surface plasmon polariton (SPP), with a nanocavity supporting Fabry–Perot (FP) resonance of dual SPP modes serving as a novel coupling interface. The FP resonance provides flexible design freedoms which can modulate the chirality of the T-spin (and the resultant chiral-coupling directionality) to flip or disappear. A unidirectivity up to 99.9% along with a high coupling efficiency and enhancement of spontaneous emission rate is achieved. Two first-principles-based SPP models for the reciprocal and original problems are built up to verify the decisive role of the FP resonance in achieving the chiral coupling. The proposed theories and novel chiral-coupling interface will be beneficial to the design of more compact and flexible chiral-coupling systems for diverse applications.

Optical metamaterials present opportunities and challenges for manipulation of light. However, metamaterials with visible and near infrared responses are still particularly challenging to fabricate due to the complex preparation process and high loss. Here, a visible light poly(amidoamine) (PAMAM)-Ag metamaterial is prepared with the assistance of fifth-generation PAMAM (5G PAMAM), based on the dendritic structure. The large area of metamaterials, where Ag nanoparticles are spherical with diameters of ∼9 nm and distributed in a multilevel netlike sphere, results in broadband resonance. The negative Goos–Hänchen shift and anomalous spin Hall effect of light generated by 5G PAMAM-Ag in visible broadband are observed, and a strong slab focusing effect at 750–1050 nm is demonstrated. In addition, the simulation shows possible application of the dendritic structure in topological photonics. The results offer advances in the preparation of large-scale visible light metamaterials, showing the potential for subwavelength super-resolution imaging and quantum optical information fields.

We introduce non-Hermitian plasmonic waveguide-cavity systems with topological edge states (TESs) at singular points. The compound unit cells of the structures consist of metal-dielectric-metal (MDM) stub resonators side-coupled to an MDM waveguide. We show that we can realize both a TES and an exceptional point at the same frequency when a proper amount of loss is introduced into a finite three-unit-cell structure. We also show that the finite structure can exhibit both a TES and a spectral singularity when a proper amount of gain is introduced into the structure. In addition, we show that we can simultaneously realize a unidirectional spectral singularity and a TES when proper amounts of loss and gain are introduced into the structure. We finally show that this singularity leads to extremely high sensitivity of the reflected light intensity to variations of the refractive index of the active materials in the structure. TESs at singular points could potentially contribute to the development of singularity-based plasmonic devices with enhanced performance.

We demonstrate four-wave mixing (FWM) in the graphdiyne (GDY) microfiber based on the synchronized dual-wavelength pump pulses that are transformed from a mode-locked fiber laser. Benefiting from the large nonlinear refractive index of GDY and the synchronized pump pulses, a maximum conversion efficiency of -39.05 dB can be achieved in GDY with only an average pump power of 6.9 mW, greatly alleviating the possible damage compared to previous investigations employing the continuous-wave pump. In addition, our proposal can be applied to measure the effective nonlinear coefficient γ of the GDY-microfiber, which could be extended as a practical measurement tool for γ of nanomaterials-based devices.

Active metasurfaces whose optical properties can be tuned by an external stimulus have attracted great research interest recently. Introduction of VO2 phase change material in all-dielectric metasurfaces has been demonstrated to modulate the resonance wavelength and amplitude in the visible to near-infrared wavelength range. In this study, we report a mid-infrared active metasurface based on Si/VO2 hybrid meta-atoms. By incorporating VO2 thin films in different locations of Si/VO2 all-dielectric nanodisks, we demonstrate different modulation amplitude of the electric or magnetic resonance scattering cross sections, leading to drastically different transmission spectrum upon VO2 insulator to metal phase transition. The physical mechanism is originated from the field profiles of the resonance modes, which interact with VO2 differently depending on its locations. Based on this mechanism, we experimentally demonstrated a large modulation of the transmittance from 82% to 28% at the 4.6 μm wavelength. Our work demonstrates a promising potential of VO2-based active all-dielectric metasurface for mid-infrared photonic applications such as infrared camouflage, chemical/biomedical sensing, optical neuromorphic computing, and multispectral imaging.

Resonance between light and object is highly desired in optical manipulation because the optical forces reach maximum values in this case. However, in traditional waveguide structures, the resonant interaction also greatly perturbs the incident field and weakens or completely destroys the manipulation on the subsequent particles. In order to avoid this dilemma, we propose to perform optical manipulation in a topological photonic structure. Owing to the topological protection, the light mode can almost keep its original form when an object is being manipulated. Therefore, resonant optical sorting can be achieved in a multiple and high throughput manner. The mechanism and results presented here pave the way for efficient on-chip optical sorting for biophysical and biochemical analysis.

Dynamical control of the constitutive properties of a light beam is important for many applications in photonics and is achieved with spatial light modulators (SLMs). Performances of the current demonstrations, such as liquid-crystal or micro-electrical mechanical SLMs, are typically limited by low (∼kHz) switching speeds. Here, we report a high-speed SLM based on the electro-optic (EO) polymer and silicon hybrid metasurface. The specially configured metasurface can not only support a high-Q resonance and large “optical–electrical” overlap factor, but also overcome the challenge of polarization dependence in traditional EO modulators. Combined with the high EO coefficient of the polymer, a 400 MHz modulation with an RF driving source of 15 dBm has been observed in the proof-of-concept device near the wavelength of 1310 nm. The device with the desired merits of high speed, high efficiency, and micrometer size may provide new opportunities for high-speed smart-pixel imaging, free-space communication, and more.

High-Q metasurfaces have important applications in high-sensitivity sensing, low-threshold lasers, and nonlinear optics due to the strong local electromagnetic field enhancements. Although ultra-high-Q resonances of bound states in the continuum (BIC) metasurfaces have been rapidly developed in the optical regime, it is still a challenging task in the terahertz band for long years because of absorption loss of dielectric materials, design, and fabrication of nanostructures, and the need for high-signal-to-noise ratio and high-resolution spectral measurements. Here, a polarization-insensitive quasi-BIC resonance with a high-Q factor of 1049 in a terahertz all-silicon metasurface is experimentally achieved, exceeding the current highest record by 3 times of magnitude. And by using this ultra-high-Q metasurface, a terahertz intensity modulation with very low optical pump power is demonstrated. The proposed all-silicon metasurface can pave the way for the research and development of high-Q terahertz metasurfaces.

Optical bound states in the continuum (BICs) are spatially localized states with vanishing radiation, despite their energy embedded in the continuum spectrum of the environment. They are expected to greatly enhance light–matter interaction due to their long lifetime and high quality factor. However, the BICs in all-dielectric structures generally exhibit large mode volumes and their properties are difficult to manipulate. In this paper, we propose a metal–dielectric hybrid nanostructure where a silver film is inserted into the silicon (Si) substrate under the Si nanopillar array. We show that symmetry-protected BIC in this system can couple with surface plasmon polaritons (SPPs) to form a hybridized mode. Compared with previous symmetry-protected BICs in all-dielectric structures, the SPP-coupled BIC has a significantly decreased mode volume, and its corresponding electric field is strongly localized below the Si nanopillars. We also show that the SPP mode makes the original polarization-independent symmetry-protected BIC become polarization-dependent. In addition, we demonstrate that the silver film in the considered structure can induce a metal mirror effect. The destructive interference between the magnetic dipole inside the Si nanopillars and the mirror magnetic dipole in the silver film can lead to the formation of accidental BICs. Our hybrid structure provides a versatile platform for the manipulation of light–matter interaction in the nanoscale.

The photonic topological insulator has become an important research topic with a wide range of applications. Especially the higher-order topological insulator, which possesses gapped edge states and corner or hinge states in the gap, provides a new scheme for the control of light in a hierarchy of dimensions. In this paper, we propose a heterostructure composed of ordinary-topological-ordinary (OTO) photonic crystal slabs. Two coupled edge states (CESs) are generated due to the coupling between the topological edge states of the ordinary-topological interfaces, which opens up an effective way for high-capacity photonic transport. In addition, we obtain a new band gap between the CESs, and the two kinds of coupled corner states (CCSs) appear in the OTO bend structure. In addition, the topological corner state is also found, which arises from the filling anomaly of a lattice. Compared with the previous topological photonic crystal based on C-4 lattice, CESs, CCSs, and the topological corner state are all directly observed in experiment by using the near-field scanning technique, which makes the manipulation of the electromagnetic wave more flexible. We also verify that the three corner states are all robust to defects. Our work opens up a new way for guiding and trapping the light flow and provides a useful case for the coupling of topological photonic states.

We experimentally demonstrate a novel quasi-bound state in the continuum (BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot mode within a low-refractive-index medium sandwiched between high-index layers. The structures studied here comprise coupled amorphous germanium guided-mode resonance (GMR) structures with a top one-dimensional grating layer and bottom uniform layer separated by a low-index silicon nitride layer. The slot-mode profile within the silicon nitride layer with mode field confinement >30% is achieved as a solution to the electromagnetic wave propagation through the coupled GMR structure with the dominant field component being perpendicular to the layers. The quasi-BIC resonance in symmetric 1D grating structures can be observed even at normal incidence when considering a realistic excitation beam with finite angular spread. The measured transmission peak is found to redshift (remain almost unchanged) under classical (full-conical) mounting conditions. The highest quality factor of ∼400 is experimentally extracted at normal incidence under a classical mounting condition with a resonance peak at 3.41 μm wavelength. Such slot-mode GMR structures with appropriately chosen low-index intermediate layers can find applications in resonantly enhanced sensing and active photonic devices.

Topological systems containing near-field or far-field couplings between unit cells have been widely investigated in quantum and classic systems. Their band structures are well explained with theories based on tight-binding or multiple scattering formalism. However, characteristics of the topology of the bulk bands based on the joint modulation of near-field and far-field couplings are rarely studied. Such hybrid systems are hardly realized in real systems and cannot be described by neither tight-binding nor multiple scattering theories. Here, we propose a hybrid-coupling photonic topological insulator based on a quasi-1D dimerized chain with the coexistence of near-field coupling within the unit cell and far-field coupling among all sites. Both theoretical and experimental results show that topological transition is realized by introducing near-field coupling for given far-field coupling conditions. In addition to closing and reopening the bandgap, the change in near-field coupling modulates the effective mass of photonics in the upper band from positive to negative, leading to an indirect bandgap, which cannot be achieved in conventional dimerized chains with either far-field or near-field coupling only.

Opto-thermophoretic manipulation is emerging as an effective way for versatile trapping, guiding, and assembly of biological nanoparticles and cells. Here we report a new opto-thermophoretic tweezer based on an all-dielectric one-dimensional photonic crystal (1DPC) for reversible assembly of biological cells with a controllable center. To reveal its ability of long-range optofluidic manipulation, we demonstrate the reversible assembly of many yeast cells as well as E. coli cells that are dispersed in water solution. The 1DPC-based tweezer can also exert short-range optical gradient forces associated with focused Bloch surface waves excited on the 1DPC, which can optically trap single particles. By combining both the optical and thermophoretic manipulation, the optically trapped single polystyrene particle can work as a controllable origin of the reversible cellular assembly. Numerical simulations are performed to calculate the temperature distribution and convective flow velocity on the 1DPC, which are consistent with the experimental observations and theoretically confirm the long-range manipulations on the all-dielectric 1DPC platform. The opto-thermophoretic tweezers based on all-dielectric 1DPC endow the micromanipulation toolbox for potential applications in biomedical sciences.

Laplace operation, the isotropic second-order differentiation, on spatial functions is an essential mathematical calculation in most physical equations and signal processing. Realizing the Laplace operation in a manner of optical analog computing has recently attracted attention, but a compact device with a high spatial resolution is still elusive. Here, we introduce a Laplace metasurface that can perform the Laplace operation for incident light-field patterns. By exciting the quasi-bound state in the continuum, an optical transfer function for nearly perfect isotropic second-order differentiation has been obtained with a spatial resolution of wavelength scale. Such a Laplace metasurface has been numerically validated with both 1D and 2D spatial functions, and the results agree well with that of the ideal Laplace operation. In addition, the edge detection of a concerned object in an image has been demonstrated with the Laplace metasurface. Our results pave the way to the applications of metasurfaces in optical analog computing and image processing.

Metasurface provides miniaturized devices for integrated optics. Here, we design and realize a meta-converter to transform a plane-wave beam into multiple Laguerre-Gaussian (LG) modes of different orders at various diffraction angles. The metasurface is fabricated with Au nano-antennas, which vary in length and orientation angle for modulation of both the phase and the amplitude of a scattered wave, on a silica substrate. Our error analysis suggests that the metasurface design is robust over a 400 nm wavelength range. This work presents the manipulation of LG beams through controlling both radial and azimuthal orders, which paves the way in expanding the communication channels by one more dimension (i.e., radial order) and demultiplexing different modes.

Negative refraction might occur at the interface between a two-dimensional photonic crystal (PhC) slab and a homogeneous medium, where the guiding of the electromagnetic wave along the third dimension is governed by total internal reflection. Herein, we report on the observation of negative refraction in the PhC slab where the vertical guiding is enabled by a bound state in the continuum and essentially beyond the light cone. Such abnormal refraction and guiding mechanism are based on the synchronous crafting of spatial dispersion and the radiative lifetime of Bloch modes within the radiative continuum. Microwave experiments are provided to further validate the numerical proposal in an all-dielectric PhC platform. It is envisioned that the negative refraction observed beyond the light cone might facilitate the development of optical devices in integrated optics, such as couplers, multiplexers, and demultiplexers.

In silicon photonics, the cavity mode is a fundamental mechanism to design integrated passive devices for on-chip optical information processing. Recently, the corner state in a second-order topological photonic crystal (PC) rendered a global method to achieve an intrinsic cavity mode. It is crucial to explore such a topological corner state in silicon photonic integrated circuits (PICs) under in-plane excitation. Here, we study both theoretically and experimentally the topological nanophotonic corner state in a silicon-on-insulator PC cavity at a telecommunications wavelength. In theory, the expectation values of a mirror-flip operation for the Bloch modes of a PC slab are used to characterize the topological phase. Derived from topologically distinct bulk polarizations of two types of dielectric-vein PCs, the corner state is induced in a 90-deg-bend interface, localizing at the corner point of real space and the Brillouin zone boundary of reciprocal space. To implement in-plane excitation in an experiment, we fabricate a cross-coupled PC cavity based on the bend interface and directly image the corner state near 1383 nm using a far-field microscope. Finally, by means of the temporal coupled-mode theory, the intrinsic Q factor of a cross-coupled cavity (about 8000) is retrieved from the measured transmission spectra. This work gives deterministic guidance and potential applications for cavity-mode-based passive devices in silicon PICs, such as optical filters, routers, and multiplexers.

When light propagates through the edge or middle part of a microparticle’s incoming interface, there is a basic rule that light converges and diverges rapidly or slowly at the output port. These two parts are referred to as the region of rapid change (RRC) and region of slow change (RSC), respectively. Finding the boundary point between RRC and RSC is the key to reveal and expound upon this rule scientifically. Based on the correlation between light convergence–divergence and the slope of emergent light, combined with the relationship between a natural logarithm and growth in physical reality and the second derivative of a function in practical significance, we determine the boundary point between RRC and RSC, namely, the inflection point. From such a perspective, a photonic nanojet (PNJ) and near-field focusing by light irradiation on RSC and RRC, as well as the position of the inflection point under different refractive index contrasts and the field distribution of light focusing, are studied with finite-element-method-based numerical simulation and ray-optics-based theoretical analysis. By illuminating light of different field intensity ratios to the regions divided by the inflection point, we demonstrate the generation of a photonic hook (PH) and the modulation of PNJ/PH in a new manner.

Micro- and nanomechanical resonators have emerged as promising platforms for sensing a broad range of physical properties, such as mass, force, torque, magnetic field, and acceleration. The sensing performance relies critically on the motional mass, mechanical frequency, and linewidth of the mechanical resonator. Herein, we demonstrate a hetero optomechanical crystal (OMC) cavity based on a silicon nanobeam structure. The cavity supports phonon lasing in a fundamental mechanical mode with a frequency of 5.91 GHz, an effective mass of 116 fg, and a mechanical linewidth narrowing in the range from 3.3 MHz to 5.2 kHz, while the optomechanical coupling rate is as high as 1.9 MHz. With this phonon laser, on-chip sensing can be predicted with a resolution of δλ/λ=1.0×10-8. The use of a silicon-based hetero OMC cavity that harnesses phonon lasing could pave the way toward high-precision sensors that allow silicon monolithic integration and offer unprecedented sensitivity for a broad range of physical sensing applications.

An optomechanical crystal cavity with nonsuspended structure using As2S3 material is proposed. The principle of mode confinement in the nonsuspended cavity is analyzed, and two different types of optical and acoustic defect modes are calculated through appropriate design of the cavity structure. An optomechanical coupling rate of 82.3 kHz is obtained in the proposed cavity, and the designed acoustic frequency is 3.44 GHz. The acoustic mode coupling between two nonsuspended optomechanical crystal cavities is also demonstrated, showing that the proposed cavity structure has great potential for realizing further optomechanical applications in multicavity systems.

Photonic crystals (PhCs) have been demonstrated as a versatile platform for the study of topological phenomena. The recent discovery of higher-order topological insulators introduces new aspects of topological PhCs that are yet to be explored. Here, we propose an all-dielectric PhC with an unconventional higher-order band topology. Besides the conventional spectral features of gapped edge states and in-gap corner states, topological band theory predicts that the corner boundary of the higher-order topological insulator hosts a 2/3 fractional charge. We demonstrate that in the PhC such a fractional charge can be verified from the local density-of-states of photons, through the concept of local spectral charge as an analog of the local electric charge due to the band filling anomaly in electronic systems. Furthermore, we show that by introducing a disclination in the proposed PhC, localized states and a 2/3 fractional spectral charge emerge around the disclination core. The emergence of the fractional spectral charges and topological boundary modes here, however, is distinct from the known cases; particularly by the 2/3 fractional spectral charges and the unique topological indices. The predicted effects can be readily observed in the state-of-the-art experiments and may lead to potential applications in integrated and quantum photonics.

Photonic crystals have revolutionized the field of optics with their unique dispersion and energy band gap engineering capabilities, such as the demonstration of extreme group and phase velocities, topologically protected photonic edge states, and control of spontaneous emission of photons. Time-variant media have also shown distinct functionalities, including nonreciprocal propagation, frequency conversion, and amplification of light. However, spatiotemporal modulation has mostly been studied as a simple harmonic wave function. Here, we analyze time-variant and spatially discrete photonic crystal structures, referred to as spatiotemporal crystals. The design of spatiotemporal crystals allows engineering of the momentum band gap within which parametric amplification can occur. As a potential platform for the construction of a parametric oscillator, a finite-sized spatiotemporal crystal is proposed and analyzed. Parametric oscillation is initiated by the energy and momentum conversion of an incident wave and the subsequent amplification by parametric gain within the momentum band gap. The oscillation process dominates over frequency mixing interactions above a transition threshold determined by the balance between gain and loss. Furthermore, the asymmetric formation of momentum band gaps can be realized by spatial phase control of the temporal modulation, which leads to directional radiation of oscillations at distinct frequencies. The proposed structure would enable simultaneous engineering of energy and momentum band gaps and provide a guideline for implementation of advanced dispersion-engineered parametric oscillators.

Hybrid photonic-plasmonic cavities have emerged as a new platform to increase light–matter interaction capable to enhance the Purcell factor in a singular way not attainable with either photonic or plasmonic cavities separately. In the hybrid cavities proposed so far, the plasmonic element is usually a metallic bow-tie antenna, so the plasmonic gap—defined by lithography—is limited to minimum values of several nanometers. Nanoparticle-on-a-mirror (NPoM) cavities are far superior to achieve the smallest possible mode volumes, as plasmonic gaps smaller than 1 nm can be created. Here, we design a hybrid cavity that combines an NPoM plasmonic cavity and a dielectric-nanobeam photonic crystal cavity operating at transverse-magnetic polarization. The metallic nanoparticle can be placed very close (1 nm) to the upper surface of the dielectric cavity, which acts as a low-reflectivity mirror. We demonstrate through numerical calculations of the local density of states that this hybrid plasmonic-photonic cavity exhibits quality factors Q above 103 and normalized mode volumes V down to 10-3, thus resulting in high Purcell factors (FP≈105), while being experimentally feasible with current technology. Our results suggest that hybrid cavities with sub-nanometer gaps should open new avenues for boosting light–matter interaction in nanophotonic systems.

Metalenses are ultrathin optical elements that can focus light using densely arranged subwavelength structures. Due to their minimal form factor, they have been considered promising for imaging applications that require extreme system size, weight, and power, such as in consumer electronics and remote sensing. However, as a major impediment prohibiting the wide adoption of the metalens technology, the aperture size, and consequently the imaging resolution, of a metalens are often limited by lithography processes that are not scalable. Here, we propose to adopt a synthetic aperture approach to alleviate the issue, and experimentally demonstrate that, assisted by computational reconstruction, a synthetic aperture metalens composed of multiple metalenses with relatively small aperture size can achieve an imaging resolution comparable to a conventional lens with an equivalent large aperture. We validate the concept via an outdoor imaging experiment performed with a synthetic aperture metalens-integrated near-infrared camera using natural sunlight for target illumination.

Reconfigurable nanophotonic components are essential elements in realizing complex and highly integrated photonic circuits. Here we report a novel concept for devices with functionality to dynamically control guided light in the near-visible spectral range, which is illustrated by a reconfigurable and non-volatile (1×2) switch using an ultracompact active metasurface. The switch is made of two sets of nanorod arrays of TiO2 and antimony trisulfide (Sb2S3), a low-loss phase-change material (PCM), patterned on a silicon nitride waveguide. The metasurface creates an effective multimode interferometer that forms an image of the input mode at the end of the stem waveguide and routes this image toward one of the output ports depending on the phase of PCM nanorods. Remarkably, our metasurface-based 1×2 switch enjoys an ultracompact coupling length of 5.5 μm and a record high bandwidth (22.6 THz) compared to other PCM-based switches. Furthermore, our device exhibits low losses in the near-visible region (∼1 dB) and low cross talk (-11.24 dB) over a wide bandwidth (22.6 THz). Our proposed device paves the way toward realizing compact and efficient waveguide routers and switches for applications in quantum computing, neuromorphic photonic networking, and biomedical sensing and optogenetics.

Polarization manipulation of electromagnetic wave or polarization multiplexed beam shaping based on metasurfaces has been reported in various frequency bands. However, it is difficult to shape the beam with multi-channel polarization conversion in a single metasurface. Here, we propose a new method for terahertz wavefront shaping with multi-channel polarization conversion via all-silicon metasurface, which is based on the linear shape birefringence effect in spatially interleaved anisotropic meta-atoms. By superimposing the eigen- and non-eigen-polarization responses of the two kinds of meta-atoms, we demonstrate the possibility for high-efficiency evolution of several typical polarization states with two independent channels for linearly polarized waves. The measured polarization conversion efficiency is higher than 70% in the range of 0.9–1.3 THz, with a peak value of 89.2% at 1.1 THz. In addition, when more other polarization states are incident, combined with the integration of sub-arrays, we can get more channels for both polarization conversion and beam shaping. Simulated and experimental results verify the feasibility of this method. The proposed method provides a new idea for the design of terahertz multi-functional metadevices.

Nano-kirigami enables direct and versatile shape transformations from two-dimensional predesigns to three-dimensional (3D) architectures in microscale/nanoscale. Here a new and extensible strategy for cascaded multilayer nano-kirigami is demonstrated in a gold/silicon nitride (Au/SiN) bilayer nanofilm for 3D nanofabrication and visible light manipulation. By employing a focused ion-beam-based Boolean irradiation, rich 3D shape transformation and nested multilayer nanostructures are precisely manufactured, which are well reproduced by developing a modified mechanical model. Based on the multilayer and deformable features of the nano-kirigami structures, potentials in manipulating the phase and intensity of visible light are explored. The developed new nano-kirigami strategies, as well as the novel exotic 3D nanostructures, could be helpful to build a novel platform for 3D nanofabrication and find potential applications in microelectromechanical/nanoelectromechanical systems, holographic display, plasmonics, nanophotonics, biophotonics, etc.

Despite very efficient superconducting nanowire single-photon detectors (SNSPDs) reported recently, combining their other performance advantages such as high speed and ultralow timing jitter in a single device still remains challenging. In this work, we present a perfect absorber model and the corresponding detector design based on a micrometer-long NbN nanowire integrated with a 2D photonic crystal cavity of ultrasmall mode volume, which promises simultaneous achievement of near-unity absorption, gigahertz counting rates, and broadband optical response with a 3 dB bandwidth of 71 nm. Compared to previous stand-alone meandered and waveguide-integrated SNSPDs, this perfect absorber design addresses the trade space in size, efficiency, speed, and bandwidth for realizing large on-chip single-photon detector arrays.

High-order temporal soliton compression in dispersion-engineered silicon photonic crystal waveguides will play an important role in future integrated photonic circuits compatible with complementary metal–oxide–semiconductors. Here, we report the physical mechanisms of high-order temporal soliton compression affected by third-order dispersion (TOD) combined with free carrier dispersion (FCD) in a dispersion engineered silicon photonic crystal waveguide with wideband low anomalous dispersion. Through numerical temporal soliton evolution analysis, we report what we believe is the first demonstration of the dual opposite effects of TOD on temporal soliton compression, which are strengthening or weakening through two different physical mechanisms, not only depending on the sign of TOD but also the relative magnitude of TOD-induced equivalent group velocity dispersion (GVD) β2,equ to the original GVD β2. We further find that FCD counteracts the effects of negative TOD on the soliton compression, while it reinforces the effects of positive TOD on the soliton compression. These results will help to design suitable dispersion-engineered silicon waveguides for superior on-chip temporal pulse compression in optical communications and processing application fields.

Plasmonic devices using periodic metallic nanostructures have recently gained tremendous interest for color filters, sensing, surface enhanced spectroscopy, and enhanced photoluminescence, etc. However, the performance of such plasmonic devices is severely hampered by the solid substrates supporting the metallic nanostructures. Here, a strategy for freestanding metallic nanomembranes is introduced by taking advantages of hollow substrate structures. Large-area and highly uniform gold nanomembranes with nanohole array are fabricated via a flexible and simple replication-releasing method. The hollow structures include a hollow core fiber with 30 μm core diameter and two ferrules with their hole diameter as 125 and 500 μm, respectively. As a proof-of-concept demonstration, 2 times higher sensitivity of the bulk refractive index is obtained with this platform compared to that of a counterpart on a solid silica substrate. Such a portable and compact configuration provides unique opportunities to explore the intrinsic properties of the metal nanomembranes and paves a new way to fabricate high-performance plasmonic devices for biomolecule sensing and color filter.

To be useful for most scientific and medical applications, compact particle accelerators will require much higher average current than enabled by current architectures. For this purpose, we propose a photonic crystal architecture for a dielectric laser accelerator, referred to as a multi-input multi-output silicon accelerator (MIMOSA), that enables simultaneous acceleration of multiple electron beams, increasing the total electron throughput by at least 1 order of magnitude. To achieve this, we show that the photonic crystal must support a mode at the Γ point in reciprocal space, with a normalized frequency equal to the normalized speed of the phase-matched electron. We show that the figure of merit of the MIMOSA can be inferred from the eigenmodes of the corresponding infinitely periodic structure, which provides a powerful approach to design such devices. Additionally, we extend the MIMOSA architecture to electron deflectors and other electron manipulation functionalities. These additional functionalities, combined with the increased electron throughput of these devices, permit all-optical on-chip manipulation of electron beams in a fully integrated architecture compatible with current fabrication technologies, which opens the way to unconventional electron beam shaping, imaging, and radiation generation.

The mid-infrared spectrum can be recorded from almost any material, making mid-infrared spectroscopy an extremely important and widely used sample characterization and analysis technique. However, sensitive photoconductive detectors operate primarily in the near-infrared (NIR), but not in the mid-infrared, making the NIR more favorable for accurate spectral analysis. Although the absorption cross section of vibrational modes in the NIR is orders of magnitude smaller compared to the fundamental vibrations in the mid-infrared, different concepts have been proposed to increase the detectability of weak molecular transitions overtones. Yet, the contribution of magnetophotonic structures in the NIR absorption effect has never been explored so far. Here we propose high-Q magnetophotonic structures for a supersensitive detection of weak absorption resonances in the NIR. We analyze the contributions of both magnetic and nonmagnetic photonic crystal configurations to the detection of weak molecular transitions overtones. Our results constitute an important step towards the development of highly sensitive spectroscopic tools based on high-Q magnetophotonic sensors.

Metasurfaces have been used to realize optical functions such as focusing and beam steering. They use subwavelength nanostructures to control the local amplitude and phase of light. Here we show that such control could also enable a new function of artificial neural inference. We demonstrate that metasurfaces can directly recognize objects by focusing light from an object to different spatial locations that correspond to the class of the object.

The spin Hall effect of light (SHEL) is a photonic version of the spin Hall effect in electronic systems and has been studied for more than 10 years. However, the lack of effective methods for dynamic modulation of spin-dependent splitting may hinder its applications. By introducing additional spin-orbit coupling of photons or nonreciprocal phase shift (NRPS), the magneto-optical Kerr effect may be one of the methods to alleviate the situation. Here, we experimentally reveal an enhanced and tunable SHEL in magneto-optical oxide thin films under the transverse magneto-optical Kerr effect configuration for the first time, to the best of our knowledge, which can be regarded as the magneto-optical SHEL (MOSHEL). We study the magneto-optical response of the multilayer structure and select the optimal structural parameters by the magneto-optical transfer matrix method. With a transverse magnetic field along opposite directions, an obvious SHEL shift difference of H-polarized light caused by NRPS is observed via a weak measurement method. With optimal parameters, the maximum measured shift difference of the SHEL achieves about 70 μm. The demonstrated MOSHEL phenomenon may accelerate the application of the SHEL in the field of spin photonics devices and precision metrology.

A compact single-shot complementary metal-oxide semiconductor (CMOS) spectral sensor for the visible range (wavelength 400–700 nm) is presented. The sensor consists of two-dimensional silicon nitride-based photonic crystal (PC) slabs atop CMOS photodetectors. The PC slabs are fabricated using one-step lithography and amenable to monolithic integration into CMOS image sensors. Featuring a small footprint of 300 μm×350 μm, the sensor can successfully measure the spectra over the 400–700 wavelength range with a best resolution of 1 nm. The footprint of the sensor may be further reduced to enable hyperspectral imaging with high spatial resolution.

We show optical waves passing through a nanophotonic medium can perform artificial neural computing. Complex information is encoded in the wavefront of an input light. The medium transforms the wavefront to realize sophisticated computing tasks such as image recognition. At the output, the optical energy is concentrated in well-defined locations, which, for example, can be interpreted as the identity of the object in the image. These computing media can be as small as tens of wavelengths and offer ultra-high computing density. They exploit subwavelength scatterers to realize complex input/output mapping beyond the capabilities of traditional nanophotonic devices.

Achieving electromagnetic wave scattering manipulation in the multispectral and broad operation band has been a long pursuit in stealth applications. Here, we present an approach by using single-layer metasurfaces composed of space-variant amorphous silicon ridges tiled on a metallic mirror, to generate high-efficiency dual-band and ultra-wideband photonic spin-orbit interaction and geometric phase. Two scattering engineered metasurfaces have been designed to reduce specular reflection; the first one can suppress both specular reflectances at 1.05–1.08 μm and 5–12 μm below 10%. The second one is designed for an ultra-broadband of 4.6–14 μm, which is actually implemented by cleverly connecting two bands of 4.6–6.1 μm and 6.1–14 μm. Furthermore, the presented structures exhibit low thermal emission at the same time due to the low absorption loss of silicon in the infrared spectrum, which can be regarded as an achievement of laser–infrared compatible camouflage. We believe the proposed strategy may open a new route to implement multispectral electromagnetic modulation and multiphysical engineering applications.

Here we study theoretically the optical responses of hybrid structures composed of dielectric nanostructures and quantum emitters with magnetic dipole transitions. Coherent couplings between magnetic dipole transitions and magnetic modes can occur, leading to giant modifications of the extinction spectra of the constituents in the hybrid structures. For a given hybrid structure, the extinction-cross-section spectra show linear or nonlinear behaviors depending on the strength of the excitation field. For a weak excitation, the extinction of the quantum emitters is greatly enhanced. The hybrid structure shows a dip on its extinction spectrum. For a strong excitation, the resonant extinction of the quantum emitters is weakly enhanced while the extinction spectrum is broadened obviously. The hybrid structure shows a Fano-like line shape on its extinction spectrum, which is different from that with a weak excitation. This difference is highly related to the behaviors of the magnetic polarizabilities of the quantum emitters in the hybrid structure. The optical responses of hybrid structures can be largely tuned by varying the geometric and material parameters.

Optical trapping techniques hold great interest for their advantages that enable direct handling of nanoparticles. In this work, we study the optical trapping effects of a diffraction-limited focal field possessing an arbitrary photonic spin and propose a convenient method to manipulate the movement behavior of the trapped nanoparticles. In order to achieve controllable spin axis orientation and ellipticity of the tightly focused beam in three dimensions, an efficient method to analytically calculate and experimentally generate complex optical fields at the pupil plane of a high numerical aperture lens is developed. By numerically calculating the optical forces and torques of Rayleigh particles with spherical/ellipsoidal shape, we demonstrate that the interactions between the tunable photonic spin and nanoparticles lead to not only 3D trapping but also precise control of the nanoparticles’ movements in terms of stable orientation, rotational orientation, and rotation frequency. This versatile trapping method may open up new avenues for optical trapping and their applications in various scientific fields.