Tailoring intrinsic chirality in a two-dimensional planar waveguide grating via quasibound states in the continuum

The strong chiral light-matter interaction is crucial for various important fields such as chiral optics, quantum optics, and biomedical optics, driving a quest for the extreme intrinsic chirality assisted by ultrahigh quality (Q-) factor resonances.In this quest, we propose a straightforward method to achieve extreme intrinsic chirality in lossless planar structures by manipulating the quasi-BIC through in-plane perturbation.The temporal coupled-mode theory is employed to derive the conditions necessary for achieving maximal intrinsic chirality.The quasi-BIC should be excited within the transparent spectral range of the structure and couple with xand y-polarized waves with the same intensity but a phase difference of π/2.For an illustration, a planar chiral dielectric dimeric waveguide grating is designed that strong interacts with left circularly polarized (LCP) light while decouples from right circularly polarized (RCP) light through in-plane symmetry engineering.Furthermore, by adjusting the magnitude of the in-plane asymmetry, we can independently manipulate the Q-factors of the chiral quasi-BIC while maintaining nearly unity circular dichroism.Our results provide a simple yet powerful paradigm for achieving extreme intrinsic chirality on an easily manufacturable platform, which may have potential applications in chiral emission, chiral sensing, and enantiomer separation.