Degenerate merging BICs in resonant metasurfaces.

Resonant metasurfaces driven by bound states in the continuum (BIC) offer an intriguing approach to engineering high-Q resonances. Merging multiple BICs in the momentum space could further enhance the Q-factor as well as its robustness to fabrication imperfections. Here, we report the doubly degenerate guided mode resonances (GMR) in a resonant metasurface, whose radiation losses could be totally suppressed due to merging BICs. We show that the GMRs and their associated accidental BICs can evolve into degenerate merging BICs by parametric tuning of the metasurface. Significantly, these two GMRs share the same critical parameter (i.e., lattice constants or thickness) that the merging BICs occur. Interestingly, thanks to the degenerate property of two GMRs, a larger (smaller) period will split one of the merging BICs into eight accidental BICs at an off-Γ point but annihilate the other. Such an exotic phenomenon can be explained by the interaction of GMRs and background Fabry-Perot resonances. Our result provides new, to the best of our knowledge, strategies for engineering high-Q resonances in resonant metasurfaces for light-matter interaction.