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

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 (Q factors) by introducing planar structural perturbations. The Q 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-Q 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.