Steerable intrinsic chirality mediated by toroidal dipole bound states in the continuum in an active metasurface

Chiral metasurfaces supporting bound states in the continuum (BICs) have been extensively researched and gradually applied in biology, sensing, and optical communication due to their strong chiroptical response and ultrahigh-quality (Q) factor. Here, we suggest a feasible pathway to realize steerable chiral metasurfaces with near-unity circular dichroism (CD), high figure of merit (FOM), and sensing sensitivity supported by a toroidal dipole (TD) response, that empower BICs. Through the study of symmetry of metasurfaces, we find that when the up-down and rotational symmetry of the system are broken, one circular (C) polarization state, carrying a fractional topological charge and separated from the eliminated BIC, can be moved to the Γ point, thereby forming a chiral quasi-BIC mode. The intrinsic chirality induced by the chiral quasi-BIC mode can be tuned over a wide frequency range and maintain a large CD value (>0.99) by virtue of the active tunable properties of bulk Dirac semimetal (BDS). We find that the chiral quasi-BIC mode is also dominated by TD through multipolar decomposition, which makes the mode possess high-Q factor. The chiral quasi-BIC mode can hold high FOM and sensing sensitivity (S) as the analyte changes. The proposed active tunable chiral metasurfaces can achieve excellent sensing characteristics in a variety of environments, which may have important applications in dynamic chiral detectors and other chiral optical instruments.