Magnetically tunable ultra-high Q-factor intrinsic chirality based on merging bound states in the continuum

Achieving ultra-high quality factors (Q-factors) and near-unity circular dichroism (CD) in intrinsically chiral metasurfaces remains a critical challenge due to scattering losses and structural limitations. Here, we propose the magnetically tunable intrinsic chirality with an ultra-high Q-factor by utilizing merging bound states in the continuum (BICs). By introducing a single in-plane symmetry-breaking perturbation and modulating the off-diagonal elements of the dielectric tensor of the magneto-optical photonic crystal slab via an external magnetic field, we achieve a chiral quasi-BIC (QBIC) with a Q-factor of 14262 and a near-unity CD (-0.99). The magnetic field can enable a continuous tuning of the CD from -1 to 1 by shifting circularly polarized singularities to the Γ point. Furthermore, when both structural asymmetric parameters and the off-diagonal elements of the dielectric tensor are simultaneously reduced, the Q-factor of the chiral QBIC can be enhanced (up to 10⁷) while maintaining a near-unity CD. This work provides a method for obtaining the chiral response with an ultra-high Q-factor, opening up new possibilities for applications in chiral optical devices such as chiral lasers and chiral imaging.