Parametric Control of Extrinsic Chiral Response in THz Nonlocal Metasurfaces via Quasi-Bound States in the Continuum

Abstract Symmetry-protected bound states in the continuum (BICs) exhibit polarization-selective radiation characteristics governed by the symmetry-imposed decoupling from free-space modes. Leveraging this intrinsic property, we demonstrate that controlled symmetry breaking in nonlocal metasurfaces formed by tilted bars converts a symmetry-protected BIC into a quasi-BIC with controlled radiative leakage. The resulting symmetry-engineered coupling pathways simultaneously enable cross-polarization conversion and helicity-dependent scattering. Both experimental and numerical results reveal that strong cross-polarized transmission and circular dichroism (CD) co-emerge and reach their maxima at an optimal tilt angle, reflecting a Q-factor-mediated balance between radiative leakage and field confinement. The cross-polarized channels exhibit a characteristic π phase jump and strong dispersion, indicating significant group delay associated with quasi-BIC-dominated radiation. Optical chiral density analysis further confirms that polarization conversion and chiral response originate from the same symmetry-breaking-induced leakage channel. These findings establish a mechanism linking symmetry breaking, Q-factor modulation, and polarization-selective radiation, providing a versatile strategy for chiral terahertz polarization control.