Ultrasensitive optical sensing via structural symmetry breaking-induced high-Q merging bound states in the continuum

The bound states in the continuum (BICs) in all-dielectric metasurfaces bring new degrees of freedom for controlling light-matter interactions at the nanoscale due to their low losses and ultrahigh Q-factor. The merging BIC spawning from symmetry-protected and accidental BICs has garnered significant attention by reason that it can exhibit a remarkable enhancement in Q-factor compared to that in isolated BIC configuration, thereby providing better robustness against fabrication imperfection and advancing high-performance compact optical sensing. In this study, we show that a silicon metasurface consisting of double-rod structures can evince the presence of ultrasensitive optical sensing empowered by structural symmetry breaking-induced high-Q merging BICs. In particular, the two accidental BICs can be achieved and evolved into a high-Q merging BIC at the Brillouin zone Γ point by breaking the in-plane symmetry. Moreover, the merging quasi-BIC resonance mode can be exploited for realizing ultrasensitive single-mode refractive index (RI) sensing. Our results provide an alternative approach for merging BIC-based optical sensing, which can also be extended to low-threshold lasers and nonlinear optical responses.