2022/10/02 by Cole Reynolds, Reynolds, Cole, Kocharovsky, Vladimir +2
Engineering · Materials Science · Physics and Astronomy · #FOS: Physical sciences #Optical Coatings and Gratings #Optics (physics.optics) #Photonic Crystals and Applications #Photonic and Optical Devices
paper · pdf · doi:10.48550/arxiv.2210.00628
openalex publication_date 2022/10/02 · openalex created_date 2022/10/06 · openalex updated_date 2026/07/28
We explain the origin of bound states in the continuum (BICs) in a planar grating waveguide, in particular, a mechanism for formation of degenerate BICs, via the analytical theory of the infinite-grating eigenmodes. Conventional symmetry-protected BICs are formed at normal incidence mainly by a single infinite-grating eigenmode that has an odd spatial parity on both sides of the BIC resonance. The odd parity is the reason for a cutoff from the radiation-loss channel and appearance of such BICs. The mechanism of emergence of a degenerate BIC in a vicinity of a degenerate frequency of two infinite-grating eigenmodes is different. The degenerate BIC is formed by an anti-phased coherent superposition of two crossing infinite-grating eigenmodes both of which possess a mixed parity and experience parity symmetry breaking as the frequency scans through the degeneracy point. In this case a cutoff from the radiation-loss channel and extremely high-Q narrow resonance is achieved due to the destructive interference of the two crossing eigenmodes. Implementation of such a mechanism can be instructive for designing BICs in other photonic crystals and structures.