Jumping and Merging of Bound States in the Continuum on Degenerate Bands

This study delves into the complex behavior of bound states in the continuum (BICs) on degenerate bands, focusing on the interactions and transitions among high-order symmetry-protected BICs, Friedrich–Wintgen BICs (FW BICs), and accidental BICs. We provide a visualization for understanding the mechanisms by which careful manipulation of the photonic crystal slab (PhCS) period can facilitate the topological behavior of BICs on a band as well as the reversal of their topological charges. Our results reveal that the accidental BICs move close to the Dirac points and jump to another band, reversing their own topological charges after the jump. As the FW BICs approach the symmetry-protected BICs with the topological charge of −2, the six Dirac points converge toward the Γ point simultaneously, causing a jumping of the high-order Γ-point BICs to a different band. The breaking of the C6z rotation symmetry prompts accidental BICs to jump to the focused band and merge with two FW BICs and a split BIC. Our research underscores the universal characteristic of BIC jumping at degenerate points, providing a deeper insight into the topological properties of photonic systems and their practical implications for the design of optical devices.