Quasi-BICs triggered by Brillouin zone folding toward tunable terahertz circular dichroism.

Chiral quasi-bound states in the continuum (QBICs) have demonstrated significant potential in light-matter interaction research. However, chiral responses are primarily constrained by the rapid decay of the quality factor (Q) under symmetry breaking, making the stability of high Q-values particularly crucial. In this paper, we propose a novel high-Q QBIC dual-ring structure, which employs Brillouin zone folding (BZF) to fold the bound states at the edges of the first Brillouin zone into the radiative continuum. While maintaining circular dichroism (CD) unchanged, it achieves independent control over the Q-factor. This structure can achieve a high Q-factor of 106, along with nearly perfect CD response and 100% depth modulation. This stable high-Q chiral metasurface technology shows promising applications in fields such as terahertz switches and chiral polarization detection.