| Summary: | In this paper, we elucidate the formation of bound states in the continuum (BICs) in compound grating waveguide structures from the perspective of momentum mismatch. As the complex lattice reduces to a simple lattice, the excitable guided resonance turns into an unexcitable BIC due to momentum mismatch. Herein, we refer to this state as momentum mismatch driven BIC. Interestingly, as the incidence changes from normal to oblique, the single momentum mismatch driven BIC splits into dual momentum mismatch driven BICs due to the nonzero tangential momentum of the incident light. Distinct from conventional symmetry-protected and accidental BICs, momentum mismatch driven BICs lie in sections of photonic bands. Consequently, Q factors of momentum mismatch driven quasi-BICs exhibit angular robustness far beyond conventional symmetry-protected and accidental quasi-BICs. Empowered by the momentum mismatch driven BIC, a pair of ellipsometric phase singularities emerge. Enabled by the drastic ellipsometric phase change, ultrasensitive refractive index sensing can be achieved. These results not only provide unique insights into the relation between momentum, BICs, and ellipsometric phase, but also offer a recipe for developing high-performance phase-based optical devices, such as ultrasensitive sensors, wave plates, and spatial light modulators.
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