Fig. 1. Concept and classification of BIC. (a) Correspondence between BIC in one-dimensional quantum well and planar dielectric waveguide^[27]; (b) diagram for classification of BIC in photonics; (c) symmetry-protected BIC and accidental BIC in PCS^[21]

Fig. 2. Quasi-BIC and chiral BIC. (a) Transition from BIC to quasi-BIC^[41], periodic Mie resonance structures supporting BIC (upper), N×N periodic structure with symmetric defects that supports quasi-BIC (center), relationship between Q-value of quasi-BIC eigenmodes and degrees of in-plane symmetry breaking (lower); (b) schematic diagram of chiral quasi-BIC^[17], spectral diagram of device with full CD at resonance (upper), schematic diagram of structure with adjustable chiral perturbation units (center), bound state leaks out of right-handed circularly polarized light, when magnetic field component is disturbed (lower)

Fig. 3. Transition from BIC to chiral BIC. (a)-(c) Schematic diagram of PCS unit symmetry structure, in-plane symmetry breaking structure, in-plane and out of plane symmetry breaking structure;(d)-(f) schematic diagram of evolution of C point over momentum space corresponding to structures (a)-(c)

Fig. 4. Chiral BIC induced by breaking one-dimensional symmetry. (a) Chiral BIC induced by breaking in-plane symmetry^[48], schematic diagram of structure (upper), corresponding structural band diagram (lower); (b) momentum space polarization main axis diagram corresponding to TE₂ mode in (a)^[48] (upper), and experimental observation diagram of Poincaré sphere presented by visible transmittance along TE₂^[48] (center and lower); (c) chiral BIC induced by breaking out-of-plane symmetry^[52], schematic diagram of PCS supporting unidirectional guided resonance (upper), upward and downward radiation intensities from resonances at 1551 nm (lower); (d) trajectories of two half-integer charges in momentum space as θ of structure in (c) is varied^[52] (left), experimental measurement trajectories of half-integer charges from five samples^[52] (right)

Fig. 5. Chiral BIC induced by complete symmetry breaking structure. (a) Design and fabrication metasurfaces with intrinsic chirality^[20], scanning electron microscope (SEM) images of metasurface, scale bar is 300 nm (upper), evolution of C points over momentum space for metasurfaces of different structural parameters (lower); (b) experimental characterization of metasurfaces with intrinsic chirality, angle-resolved transmission spectra of metasurface under LCP (left) and RCP (right) incidence^[20]; (c) chiral emission from resonant metasurfaces^[19], schematic diagram of nanostructures (left), SEM images of metasurface, scale bar is 200 nm (right); (d) spectra of chiral emissions in normal directions (left), chiral laser radiation spectra in normal direction, pumping densities are 58.9 mJ/cm² (upper right) and 16.5 mJ/cm² (lower right)

Fig. 6. Experimental methods to break out-of-plane symmetry. (a) Grayscale electron beam lithography and sample fabrication^[67], schematic diagram of manufacturing metasurfaces with different height bars defined by grayscale electron beam lithography (upper), relationship between electron beam exposure dose and depth of electron beam resist (lower); (b) SEM image of developed electron beam photoresist after development (upper), cross-sectional SEM image of etched glass substrate (lower)^[67]; (c) schematic diagrams of fabrication workflow of multi-height metasurfaces^[77]; (d) SEM images of fabricated multi-height metasurface^[77] (upper), and variation of transmittance spectra with asymmetric height difference^[77] (lower)

Fig. 7. Nonlinear CD effect. (a) Chiral BIC induced nonlinear third harmonic generation^[75], circular polarization transmission spectra and linear CD near quasi-BIC (left), third harmonic CD signal induced by quasi-BIC (right); (b) third harmonic CD response of asymmetric Si metasurface^[85], structural schematic diagram and SEM image of unit structure (left), quasi-BIC and Mie resonance generate strongest third harmonic under critical coupling conditions (center), third harmonic CD and its resonant peak (right)

Fig. 8. Generation of vortex beams by PCS. (a) Schematic diagrams of generating vortex beams using the polarization characteristics of BIC in momentum space^[90], electric field distribution of BIC in momentum space (upper right), schematic diagram of vortex beam generation (lower right); (b) ultra-fast control of quasi-BIC micro-laser^[92], principle diagram of dual beam pumping experiment (upper), dynamic switching process between linearly polarized laser and vortex laser (lower)

Fig. 9. Superchiral fields and spin Hall effect of light enhanced by BIC. (a) Experimental observation of vector BIC^[104], SEM images of experimental sample and experimental testing optical path (upper), band diagrams of experimental sample, indicating overlap between TE- and TM-like modes (lower); (b) BIC enhanced directional coupling^[113], sample transmittance and experimental setup diagram, resonance at 541.4 nm as BIC (upper), normal incident light reorientation under BIC (lower)