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 ultra-high-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 $x$- and $y$-polarized waves with the same intensity but a phase difference of $\pi /2$. For an illustration, a planar chiral dielectric dimeric waveguide grating is designed that strongly interacts with left circularly polarized light while decoupling from right circularly polarized 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 near 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.