Fig. 1. Schematic of BICs mode coupling and high quality factor application in different optical systems

Fig. 2. Physical mechanism of optical BICs. (a) Periodic photon structure^[35]; (b) temporal coupled mode theory; (c) multipole deconposition method^[37]

Fig. 3. Gratings and BICs in photonic slabs. (a) Metal-photonic crystal structure (top) and photonic mode energy levels (bottom)^[67]; (b) far fields decomposition of resonances in photonics slabs^[68]; (c) BICs of polarization singularity^[68]; (d) experimental observation of polarization singularities and quality factor measurements^{[Download full size}

Fig. 4. Metasurfaces and BICs in single subwavelength resonators. (a) Terahertz superstructure surface of intra-planar non C2-symmetric structural element^[107]; (b) symmetry breaking in vertical direction on all dielectric superstructure surfaces^[109]; (c) schematic of terahertz superstructure surface and pseudo-color diagram of transmission spectrum of scanned structural parameters^[

Fig. 5. Application of optical BICs. (a) Field distributions of quasi-BICs in dielectric grating structure^[129]; (b) enhanced harmonic generation with asymmetric nonlinear metasurfaces^[106]; (c) enhancement of nonlinear effect in single nanostructures^[123]; (d) laser genection based on BICs^[131]; (e) hi