Observation of tunable polarization states evolution in bound states in the continuum based on terahertz metasurfaces

Bound states in the continuum (BICs) are unique optical modes characterized by infinite quality (Q) factors and a topological vortex configuration. The introduction of chiral perturbations transforms BICs into chiral quasi-BICs with high-Q-factors, thereby enhancing chiral optical responses. However, this method lacks dynamic modulation flexibility. Here, we demonstrate a magneto-optical (MO) metasurface capable of dynamically tuning the polarization states of BICs at off-Γ points by adjusting the strength of an external magnetic field while preserving structural symmetry. The resulting circularly polarized states enhance the chiral optical response. Furthermore, the applied external magnetic field breaks time-reversal symmetry, causing the double-degenerate BICs to split into two chiral BICs. Finally, breaking the in-plane inversion symmetry transforms the polarization singularity at-Γ into a linearly polarized state, and applying an external magnetic field enables the MO metasurface to support high-Q intrinsic chirality. Our findings provide enhanced options for polarization control, chiral sensing, and chiral emission applications.