Ultra-Narrowband Circular Dichroism Induced by Chiral Quasi-Bound States in All-Silicon Dielectrics in the Terahertz Range

Circular dichroism has extensive applications in fields such as ultra-sensitive biosensing and wavefront manipulation within chiral structures. However, traditional terahertz chiral structures face challenges in effectively addressing narrowband issues due to linewidth limitations. Recently, the introduction of the concept of quasi-bound states in the continuum (quasi-BIC) in optical resonant systems has demonstrated extremely high quality factors and very low radiation losses, providing a theoretical foundation for achieving ultra-narrow linewidth wave control. In this study, we innovatively construct bound states in the continuum (BIC) in a planar all-silicon terahertz metasurface with in-plane C2 symmetry and broken mirror symmetry. This system not only exhibits symmetry-protected BICs but also reveals parameter-tuned BICs. Through methods such as band structure calculations and far-field polarization analysis, we achieve an ultra-narrow linewidth with an extremely high quality factor at the quasi-BIC frequency, successfully realizing ultra-narrowband terahertz chirality. Our findings provide a novel solution for terahertz narrowband chirality and spin-selective asymmetric transport, potentially driving technological advancements in related fields.