3D intrinsic chiral BIC metasurface enabling strong linear and nonlinear CD with efficient THG

A three-dimensional (3D) intrinsic chiral bound state in the continuum (BIC) metasurface is presented in this paper, which achieves strong linear circular dichroism (CD) with a high Q-factor and pronounced polarization-selective third harmonic generation (THG) arising from nonlinear circular dichroism (NLCD). The metasurface is composed of an all-dielectric silicon plate patterned with a periodic array of right-trapezoidal holes. By breaking all mirror symmetries, the metasurface supports a quasi-BIC (QBIC) and exhibits a CD value of 0.95 and a Q-factor of 11965 at 3.1318 THz. Meanwhile, under right-handed circularly polarized wave (RCP) excitation, the metasurface demonstrates an NLCD-driven THG efficiency of 1.06 × 10⁻³, which is approximately seven orders of magnitude higher than the 1.82 × 10⁻¹⁰ obtained under left-handed circularly polarized wave (LCP) excitation. Multipole decomposition reveals that the chiral BIC resonance is predominantly governed by a magnetic dipole mode, providing strong local field enhancement under RCP illumination. Far-field polarization and optical chirality density (OCD) analyses further confirm that complete mirror symmetry breaking is essential for inducing the 3D intrinsic chiral BIC. The proposed metasurface offers a promising route toward chiral-selective radiation, high-efficiency nonlinear optics, and terahertz photonic devices.