Maximum intrinsic chirality in terahertz metasurfaces with bound states in the continuum

Chiral metasurfaces with strong chirality and high Q-factors provide a uniquely powerful platform for enhancing light–matter interactions. Here, we propose a terahertz (THz) chiral metasurface based on bound states in the continuum (BICs). Through temporal coupled-mode theory, we derive the conditions for maximizing intrinsic chirality in quasi-BICs metasurfaces. By introducing the in-plane radius difference perturbation and out-of-plane slant angle perturbation, and balancing these two perturbations, quasi-BICs metasurface achieves strong interaction with left circularly polarized light while remaining decoupled from right circularly polarized light. Maximum intrinsic chirality is reached with near-unity circular dichroism (CD = 0.99) and ultrahigh Q-factor (Q = 2728). The THz chiral metasurface holds significant for applications in chiral lasers, optical filters, and enantiomer separation.