Giant and Rapidly Switching Intrinsic Chirality Enabled by Toroidal Quasi‐Bound States in the Continuum

Abstract Circular dichroism (CD), arising from spin‐selective light‐matter interactions controlled by chirality, is critical for advanced applications such as chiral imaging and ultrasensitive biosensing. However, CD of chiral natural materials is inherently constrained owing to molecular symmetry and thermodynamic stability. Recently, artificially engineered metasurfaces incorporating chiral quasi‐bound states in the continuum (Q‐BICs) have emerged as a promising solution, which enables near‐unity CD responses. However, their current designs heavily rely on complex 3D geometries, posing significant challenges for integration with planar on‐chip platforms. To address the stringent challenges, a truly planar metasurface is demonstrated that achieves giant intrinsic chiral responses by utilizing a chiral Q‐BIC dominated by out‐of‐plane component ( T z ) of toroidal dipole (TD). With deep‐subwavelength (<λ/20) thickness, the metasurface exhibits outstanding intrinsic CD values in both simulations (>0.90) and experiments (≈0.80). Moreover, in contrast to previous electric or magnetic chiral Q‐BICs, the toroidal Q‐BIC produces a rapidly switching CD response—transitioning sharply between positive and negative giant CD values within ≈0.2 GHz, and the switching is highly sensitive to small oblique incidence of opposite angles. Therefore, the scheme provides a planar platform for studying chiral light‐matter interactions involving TD, important for future development of polarization‐ and angle‐sensitive photonic and optoelectronic devices.