High-Q terahertz circular dichroism based on an all-silicon metasurface

Existing terahertz (THz) chiral structures are constrained by their linewidth, which severely limits their utility for narrowband signal processing and high-resolution spectroscopy. To achieve a stable and high quality factor (Q-factor) chiral response, an all-silicon THz metasurface supporting two distinct types of high-Q chiral resonances is proposed. The metasurface hosts both the symmetry-broken quasi-BIC mode (Mode 1/Mode 2) and the geometry-governed Mie-resonant mode (Mode 3/Mode 4). The Mie-resonant mode is the primary focus of this work, owing to its insensitivity to asymmetry parameter perturbations. The numerical results indicate that this mode supports the high-Q chiral response with the value of |CD|>0.9 and >10 4 , confirming its potential for robust chiral photonic applications. Furthermore, leveraging the enhanced local fields and chiral selectivity, the metasurface achieves third-harmonic generation (THG) with a conversion efficiency several orders of magnitude higher than that of unstructured silicon films. The study can promote applications in multiband chiral sensing, quantum communication, and compact THz photonic devices.