Obtuse-angled separation of chiral resonances with planar asymmetry-induced tunability of quality factors.

Abstract

Photonic crystal (PhC) supports Bloch resonances that confine electromagnetic energy within the subwavelength thickness and enable polarization modulation through their intrinsic mode states. If a PhC generates chiral resonances, then it can selectively enhance or suppress specific circular polarizations, making it ideal for chiral optics. Here, we devise a design strategy to realize chiral resonant modes with large angular divergence and tunable quality factors (<i>Q</i> factors) by introducing planar structural perturbations. The <i>Q</i> factor exhibits an inverse-square dependence on perturbation strength, consistent with the behavior of quasi-bound states in the continuum. Theoretical and experimental results demonstrate chiral modes with high circular dichroism, large separation angles, and high-<i>Q</i> factors. We further couple 2D excitons to these resonant modes, achieving spatially separated chiral emission. Using Brillouin zone folding, we translate bound modes at high-symmetry points into the radiative region, suggesting a strategy to control polarization, group velocity, and topology in photonic systems.