The imaginary Poynting momentum (IPM), as an intrinsic yet mysterious property of light, is nearly as ubiquitous as its real counterpart. However, prior research has only traced its origin to intensity asymmetry and showcased its capability in particle manipulation using evanescent waves and structured light. Fundamentally, the interplay between the IPM and polarization topology remains unexplored, significantly restricting its potential applications. Here, we observe the high-order polarization topological charges (PTCs) in the IPM and expand their capabilities in versatile particle manipulation. PTCs originate from spatial distributions of linear polarizations in vector beams. When the PTC equals 1, the IPM can be utilized to rotate particles. Distinctively, for higher-order PTCs, rotational potential-well arrays with controllable rotation directions emerge to trap and rotate various numbers of particles. This work uncovers a holistic and complete understanding of the IPM by investigating its link to the polarization distribution. It also suggests a credible way to harness polarization-topology optical forces, offering significant potential for biophysical and quantum applications.

Topological photonics has revolutionized light transport by enabling robust unidirectional propagation immune to structural disorder. However, conventional topological designs confine light to narrow interfacial channels, necessitating optically unoccupied topological-insulator regions and imposing a fundamental trade-off between topological protection and spatial efficiency. Here, we introduce an insulator-free topological waveguide architecture that eliminates this trade-off, enabling multi-lane unidirectional light guiding with both 100% spatial utilization efficiency and topological protection. By strategically combining time-reversal and inversion symmetry breaking in gyromagnetic honeycomb photonic crystals, we achieve four inequivalent photonic valley half-semimetals at distinct critical transition boundaries between trivial and Chern insulator phases. We arrange these four structures parallelly and cyclically, such that each domain simultaneously functions as a valley-selective waveguide and a topological barrier for the other valley in neighboring channels, circumventing the need for additional topological insulating layers. Our experimental and theoretical results demonstrate that this multi-lane configuration transforms conventional edge states into densely packed large-area one-way modes. These modes exhibit alternating unidirectionality across the four domains while maintaining robustness even under arbitrary sharp bends and drastic shape variations. This work establishes a paradigm for ultracompact topological photonic circuits, with direct implications for high-density integrated optics.

Photonic double-zero-index media, distinguished by concurrently zero-valued permittivity and permeability, exhibit extraordinary properties not found in nature. Remarkably, the notion of zero-index can be substantially expanded by generalizing the constitutive parameters from null scalars to nonreciprocal tensors with nonzero matrix elements but zero determinants. Here, we experimentally realize such a new class of gyromagnetic double-zero-index metamaterials possessing both double-zero-index features and nonreciprocal hallmarks. As an intrinsic property, this metamaterial always emerges at a spin-1/2 Dirac point of a topological phase transition. We discover and rigorously prove that a spatiotemporal reflection vortex singularity is always anchored to the metamaterial's Dirac point, with the vortex charge being determined by the topological invariant leap across the phase transition. This establishes a unique bulk-spatiotemporal vortex correspondence that extends the protected boundary effects into the time domain and exclusively characterizes topological phase transition points, setting it apart from any pre-existing bulk-boundary correspondence. Based on this correspondence, we propose and experimentally demonstrate a mechanism to deterministically generate optical spatiotemporal vortex pulses with firmly fixed central frequency and momentum, hence showing unparalleled robustness. Our findings uncover deep connections between zero-refractive-index photonics, topological photonics, and singular optics, opening the avenue for the manipulation of space-time topological light fields via the inherent topology of extreme-parameter metamaterials.

Photonic crystals are artificial materials characterized by a photonic band structure that governs the propagation of light waves. The photonic gap was originally introduced to inhibit spontaneous emission and facilitate photon localization. In this essay, I will highlight how, despite the established understanding of photonic crystals, they remain highly relevant today. Their design flexibility, the duality symmetry inherent in Maxwell's equations, and their functionality across a wide frequency range all allow the exploration of new areas in physics, each revealing unique phenomena. Examples include band topology and topological effects in structured light fields, as well as geometric concepts such as quantum geometry and the properties of non-Euclidean spaces. Furthermore, photonic crystals provide a valuable platform for studying non-Hermitian physics, including exceptional points and the non-Hermitian skin effect. Beyond their fundamental significance, these properties hold promise for advancing photonic technologies.

Topology multimode provides a promising avenue for developing multi-channel robust photonic devices and optical power manipulation. Particularly, the backscattering-immune property of topological multimode holds potential for ideally lossless optical beam combination. In this work, the combination of topological multimode in topologically non-trivial photonic crystals, is experimentally observed achieving a beam combination efficiency of up to 93%. The near-perfect combination can be explained by the power orthogonal excitation principle of topological multimode. Furthermore, controllable beam steering is successfully achieved by incorporating topological one-way large-area waveguide states into the output spatial channel with tunable excitation. The work paves the way for the application of topological photonics in the directional manipulation of high-power electromagnetic radiation.

Integrated photonic chips hold substantial potential in optical communications, computing, light detection and ranging, sensing, and imaging, offering exceptional data throughput and low power consumption. A key objective is to build a monolithic on-chip photonic system that integrates light sources, processors and photodetectors on a single chip. However, this remains challenging due to limitations in materials engineering, chip integration techniques and design methods. Perovskites offer simple fabrication, tolerance to lattice mismatch, flexible bandgap tunability and low cost, making them promising for hetero-integration with silicon photonics. Here we propose and experimentally realize a near-infrared monolithic on-chip photonic system based on a perovskite/silicon nitride photonic platform, developing nano-hetero-integration technology to integrate efficient light-emitting diodes, high-performance processors and sensitive photodetectors. Photonic neural networks are implemented to perform photonic simulations and computer vision tasks. Our network efficiently predicts the topological invariant in a two-dimensional disordered Su–Schrieffer–Heeger model and simulates nonlinear topological models with an average fidelity of 87

We propose a novel topological defect called Janus bound states in the continuum (BICs), featuring asymmetric topological charges in upward and downward radiation channels. Our approach involves a photonic crystal slab (PCS) that initially exhibits both out-of-plane and in-plane mirror symmetry, and this PCS possesses one BIC at the Γ point and two BICs off the Γ point. By introducing certain perturbations that break the out-of-plane mirror symmetry, the two off-Γ BICs decompose into four circularly polarized states (C points) with identical topological charges (each with half the topological charge of the original BIC) while the at-Γ BIC is preserved. Then, we selectively manipulate the four C points associated with the downward radiation channel to converge at the at-Γ BIC, forming a Janus BIC with distinct topological charges for upward and downward radiation. By further introducing in-plane mirror symmetry perturbation, we can bring two of the C points with the same handedness and identical topological charges for upward radiation to merge into the Janus BIC. This process results in a Janus chiral BIC which exhibits large intrinsic chirality and an infinite Q factor. Janus BICs can induce distinct Pancharatnam-Berry phase singularities in momentum space for different incident channels, providing a new approach to control orbital angular momentum. Janus chiral BICs hold promise in enhancing direction-dependent and spin-dependent asymmetric light-matter interaction, opening new pathways for improving chirality-dependent operation for on-chip devices.

The stable on-chip deterministic arbitrary-phase-controlling of signal light in micro/nanometer spatial scale is an extremely important basis for large-scale and high-density integrated photonic information processing chips. Conventional phase-controlling methods face with serious limitation of unavoidable crosstalk, length distortion, and fabrication error. To date, it is still a great challenge to achieve deterministic and wide-range on-chip arbitrary-phase-controlling. Here, we report an effective strategy of three-waveguide coupled configuration to realize on-chip deterministic arbitrary-phase-controlling (ranging from 0 to 2π) by combing the dynamic phase and the geometric phase. Based on this strategy, quantum gate operations in an optical permutation-group circuit are successfully realized in femtosecond-laser direct writing sample. To extend the feasibility of this method, on-chip silicon-based deterministic arbitrary-phase-controlling in the optical communication range is also experimentally verified. Our work not only paves the way for fundamental research in chip-scale novel optical devices but also promotes the study of topological quantum computing.

Topological photonic structures exhibit resilience to defects, allowing unidirectional light flow and promoting the development of robust devices with large information processing capacities. However, the diversity of topological boundary modes is typically governed by bulk-edge correspondence, which limits multidimensional multiplexing and the integration density of next-generation photonic systems. Here, we present a polycrystal approach based on domain wall engineering to configure multi-band dispersion in a synthetic hybrid dimension by utilizing orientation freedom. As a prototype, we demonstrate that an all-dielectric platform for hybrid topological polycrystalline photonic integrated circuits can support up to eight edge channels and four corner modes via pseudospin-valley Hall effect, empowering controllable directionality of multi-frequency and spinful channels with highly localized performance. Our findings reveal a photonic architecture that significantly advances the on-chip integration of topological photonics, offering valuable potential for future information processing technologies across optical and microwave frequencies.

Exceptional degeneracies, unique to open systems, are important in non-Hermitian topology. While bulk-Fermi-arcs connecting second-order exceptional points (EP2s) have been observed, the existence of bulk-Fermi-arcs linking higher-order exceptional points remains unexplored. Here, we introduce an unconventional bulk-Fermi-arc in systems with parity-time and pseudo-Hermitian symmetries, which links paired third-order exceptional points (EP3s), where three eigenvalues share identical real parts but distinct imaginary parts. We realize these systems using topological circuits and experimentally demonstrate this unconventional bulk-Fermi-arc. A winding number defined from resultant vector shows that the bulk-Fermi-arc is stabilized by the exchange of Riemannian sheets. Furthermore, analysis via eigenframe deformation and rotation reveals that the EP3 pair is topologically nontrivial and equivalent to a single defective triple point. The EP3s can split from the triple point by varying system parameters, with this splitting protected by topological equivalence. This finding offers insights into non-Hermitian topology with potential applications in wave engineering.

We study the complex band hybridization induced by nonreciprocal local resonances in photonic crystals. Composed of trimer unit cells, a two-dimensional magnetophotonic crystal with an analytically obtainable solution is considered. We find that a nonreciprocal spectral gap may appear without nonreciprocal transmission and that the imaginary parts of the complex wave vectors Im(k) may blow up at resonance to give extreme nonreciprocal transmission. We further show that, for a subwavelength lattice, the isolation ratio for the nonreciprocal transmission is determined solely by Im(k) instead of the extensively studied real part Re(k). Our finding contradicts the common belief that "spectral nonreciprocity [w(k) not equal w(-k)] always implies nonreciprocal transmission."

Electromagnetic wave propagation in three-dimensional (3D) space typically suffers omnidirectional scattering when encountering obstacles. In this study, we used Chern vectors to construct a topological heterostructure, where large-volume nonreciprocal topological transport in 3D is achieved. The shape of the cross section in the heterostructure can be arbitrary designed, and we experimentally observed the distinctive cross-shaped field pattern transport, nonreciprocal energy harvesting, and the remarkable ability of electromagnetic wave to traverse obstacles and abrupt structure changes without encountering reflections in 3D space.

Holography plays a crucial role in optics, yet traditional methods require complex setups and bulky devices, being unfavourable for optical integration. Although metasurface-based holograms can be ultra-compact, holographic images generated by previously realized metadevices were mostly scalar ones, with a few vectorial holograms realized so far suffering from restrictions on efficiency, incident polarization, and resolution. We propose and experimentally demonstrate an efficient meta-platform to generate vectorial holographic images with high resolutions under arbitrary incident polarizations. Combining Gerchberg-Saxton algorithm and the wave-decomposition technique, we establish a generic strategy to retrieve the optical properties (e.g., reflection phases and polarization-conversion capabilities) of meta-atoms required to construct a metasurface for generating a predesigned vectorial holographic image under a predesigned incident polarization. We next design a series of high-efficiency and deep-subwavelength single-structure meta-atoms exhibiting tailored reflection phases and polarization-conversion capabilities governed by both structural resonances and the Pancharatnam-Berry effect, and experimentally characterize their optical scattering properties. We finally construct a series of ultra-thin metadevices with these meta-atoms and experimentally demonstrate that they can generate pre-designed vectorial holographic images under illuminations of circularly polarized light at 1064 nm. We provide a highly efficient and ultra-thin platform to generate predesigned vectorial holographic images under illuminations of light with arbitrary given polarization, which can inspire numerous future applications in on-chip photonics.

Over the last few decades, the predominant strategies for controlling spontaneous emission have involved tailoring the spatial surroundings of quantum emitters or atoms to create resonant or spatially periodic photonic structures. However, the rise of time-varying photonics has prompted a reevaluation of spontaneous emission in dynamically changing environments, especially within photonic time crystals, where optical properties undergo time-periodic modulation. Here, we apply classical light-matter interaction theory together with Floquet analysis to reveal a substantial enhancement of the spontaneous emission decay rate at the momentum gap frequency in photonic time crystals. Moreover, our findings suggest that photonic time crystals enable a nonequilibrium light-matter interaction process: the spontaneous excitation of an atom from its ground state to an excited state, accompanied by the concurrent emission of a photon, referred to as spontaneous emission excitation.

We report an on-chip photonic system based on a heterogeneously integrated metal-halide-perovskite/Si3N4 photonic platform, which incorporates efficient LEDs, high-performance processors, and sensitive photodetectors on a single chip. © 2025 The Author(s)

It is known that the eigenspectra and eigenfields of wave systems are sensitive to boundary conditions. We show a counterintuitive result that two hard-boundary sound cavities of different boundary shapes and of different sizes have a common set of eigenfrequencies. Moreover, their eigenfields at shared frequencies are the same in some of their common subregions. The reason is that both cavities can be created by cutting along locally mirror-symmetric lines in a honeycomb lattice, with their boundaries compatible with the hexagonal cavity's eigenfields at those common eigenfrequencies. Based on this observation, we have discovered a phenomenon where cavity eigenfields remain unaffected by obstacles of certain shapes within the cavity. The key lies in aligning the obstacles' boundaries with the antinodal lines of the cavity eigenfields, which are easy to find when the cavity contains some local mirror symmetries. All the results can be generalized to electromagnetic waves and three-dimensional cavities.

Materials possessing an effective zero refractive index are often associated with Dirac-like cone dispersion at the center of the Brillouin zone (BZ). It has been reported the presence of hidden symmetry-enforced triply degenerate points [nexus points (NP)] away from the Brillouin zone center with the stacked dielectric photonic crystals. The spin-1 Dirac-like dispersion in the xy plane near the nexus point suggests a method for achieving zero refractive index materials. The stacked photonic crystals at the nexus points can be deemed as an effective moving double-zero-index medium (MDZIM) traveling with a speed relative to the laboratory reference. The ability of this moving double-zero-index medium to enable perfect wave tunneling across barriers without reflection has been demonstrated, dependent on the incident waves' specific angular orientations.

The hybrid skin-topological effect (HSTE) has recently been proposed as a mechanism where topological edge states collapse into corner states under the influence of the non-Hermitian skin effect (NHSE). However, directly observing this effect is challenging due to the complex frequencies of eigenmodes. In this study, we experimentally observe HSTE corner states using synthetic complex frequency excitations in a transmission line network. We demonstrate that HSTE induces asymmetric transmission along a specific direction within the topological band gap. Besides HSTE, we identify corner states originating from non-chiral edge states, which are caused by the unbalanced effective onsite energy shifts at the boundaries of the network. Furthermore, our results suggest that whether the bulk interior is Hermitian or non-Hermitian is not a key factor for HSTE. Instead, the HSTE states can be realized and relocated simply by adjusting the non-Hermitian distribution at the boundaries. Our research has deepened the understanding of a range of issues regarding HSTE, paving the way for advancements in the design of non-Hermitian topological devices.

Boundary modes localized on the boundaries of a finite-size lattice experience a finite size effect (FSE) that could result in unwanted couplings, crosstalks and formation of gaps even in topological boundary modes. It is commonly believed that the FSE decays exponentially with the size of the system and thus requires many lattice sites before eventually becoming negligibly small. Here we consider a two-dimensional strip geometry that is periodic along one direction and truncated along the other direction, in which we identify a special type of FSE of some boundary modes that apparently vanishes at some particular wave vectors along the periodic direction. Meanwhile, the number of wave vectors where the FSE vanishes equals the number of lattice sites across the strip. We analytically prove this type of FSE in a simple model and prove this peculiar feature. We also provide a physical system consisting of a plasmonic sphere array where this FSE is present. Our work points to the possibility of almost arbitrarily tunning of the FSE, which facilitates unprecedented manipulation of the coupling strength between modes or channels such as the integration of multiple waveguides and photonic non-abelian braiding.