Photonic crystal slabs with maximal chiroptical response empowered by bound states in the continuum

To enhance the strength of chiral light–matter interaction for practical applications, the chirality and quality factors (Q$Q$-factors) of current methods need to be strengthened simultaneously. Here, we propose a design of photonic crystal slabs (PhCs) supporting chiral bound states in the continuum (BICs) of transverse electric (TE) and transverse magnetic (TM) modes, exhibiting maximal chiroptical responses with high Q$Q$-factors and near-unity circular dichroism (CD=0.98$\mathrm{CD}=0.98$). Different from the past, the PhCs we employed only have reduced in-plane symmetry and can support simultaneously chiral quasi-BICs (q$q$-BICs) of TE and TM mode with two-dimensional ultra-strong external and internal chirality. Based on the temporal coupled-mode theory, two analytical expressions of CD of chiral q$q$-BICs response are revealed, which are consistent with the simulation results. Furthermore, we elucidate these results within the charge-current multipole expansion framework and demonstrate that the co-excitation of higher-order multipole electric/magnetic modes is responsible for near-perfect CD. Our results may provide more flexible opportunities for various applications requiring high Q$Q$-factors and chirality control, such as chiral lasing, chiral sensing, and enantiomer separation.