Super-bound states in the continuum through merging in grating

We consider bound states in the continuum (BICs) in grating composed of infinitely long silicon rods of rectangular cross-section. We reveal merging off-Γ Friedrich-Wintgen BIC with symmetry protected BIC. We present CMT and multipole decomposition theory, complementing each other, to analyze the merging phenomenon. The theories show a crossover of the behavior of Q factor from standard inverse square law kx,z-2 towards extremely fast boosting law kx,z-6 in momentum space. In turn that crossover gives rise to another crossover from Q∼N2 to Q∼N3 for symmetry protected quasi-BIC in finite grating of N rods owing to suppression of radiation leakage of quasi-BIC mode from surface of grating. As a result, the Q factor of quasi-BIC is determined by residual leakage from ends of grating. We show numerically that this leakage can also be suppressed considerably if grating is stretched from the ends. © 2023 American Physical Society.