Multiple Circular Polarizations Coexisting with Bound States in the Continuum Without Breaking Symmetry

Bound states in the continuum (BICs) have been engineered in periodic photonic systems to achieve diverse polarization behavior in momentum space. By breaking C₂^z symmetry of photonic crystal slabs (PhCS), purely circular polarization, which hold significant potential for applications in topological physics and chiral optics, can be achieved near the BICs. In this study, the intriguing phenomenon of BICs on the degenerate band of PhCS with a triangular lattice featuring cylindrical holes are investigated. Unlike previous studies that mainly focused on BICs on non-degenerate photonic bands, this research reveals a sophisticated interplay between BICs and Dirac points on the more intricate degenerate photonic band. This interaction gives birth to two pairs of purely circular polarizations with opposite chirality, even without breaking any symmetry of the PhCS. Additionally, It is find that by further breaking the σ_z mirror symmetry of the PhCS, these purely circular polarization states can be significantly amplified. This findings not only enrich the polarization responses of high-Q photonic devices but also enables the modulation of chiral light, laying the groundwork for the creation of high-quality optical devices with precisely engineered polarization properties.