Suppressed angular dispersion of quasi-bound states in the continuum in conical mounting

Abstract

Bound states in the continuum (BICs) enable high- Q resonances in photonic lattices but are highly sensitive to incident angle, where small deviations broaden resonances and degrade Q -values. We analyze guided-mode resonance (GMR) of one-dimensional (1D) photonic lattices under classical mounting (CM) and fully conical mounting (FM). Using numerical simulations, we show that FM reorganizes leaky-mode bands to form a robust dark band from the same symmetry-protected BIC as in CM, but with much lower angular dispersion. In FM, resonant Bloch modes propagate obliquely to the grating vector, creating polarization-altered skew leaky modes that sustain the dark band. As a result, a quasi-BIC in CM can be accessed in FM at nearly the same wavelength with significantly improved angular tolerance and Q -factor without changing device geometry. Numerical examples in the long-wave infrared demonstrate orders-of-magnitude improvement in angular robustness under FM. These results clarify the physical origin of quasi-BIC formation in conical mounting and provide a practical strategy for high- Q resonant filters and devices that are tolerant to angular misalignment.