Brillouin Zone Folding Metasurface for Near Perfect Circular Dichroism

Planar chiral metasurfaces have demonstrated their proficiency in manipulating chiral light‐matter interactions for diverse applications. Nevertheless, it proves arduous to simultaneously attain elevated quality factors and potent chirality response. In this work, a planar chiral metasurface is proposed, that supports Brillouin zone folding‐induced bound states in the continuum (BZF‐BICs) featuring a high‐Q factor resonance and near‐perfect circular dichroism response. By introducing periodic perturbation and breaking symmetry, the BZF‐BICs exhibit a folded band structure and decreased radiative channels, leading to a superior quality factor relative to unfolded quasi‐BICs. Exploiting the topological nature of BZF‐BICs, the polarization singularity is eradicated, resulting in the appearance of two elliptical eigenstate polarizations. Consequently, an almost perfect circular dichroism response is demonstrated due to the selective coupling between the eigenstate and the incident light. Furthermore, it is revealed that the symmetry of the distribution of eigenstates in the BZF‐BICs lattice is determined by the perturbed cell, and the more intricate orthogonal polarization states can be distinguished through the Brillouin zone design, enabling the splitting of complex polarization state beams. This study presents an innovative method for creating easily fabricated chiral metasurfaces and holds promise for applications in biochemical sensing, emission, lasing, quantum optics, and nonlinear optics. A novel Planar chiral metasurface that utilizes Brillouin zone folding is introduced to create bound states in the continuum for achieving ultrahigh‐Q resonances with nearly perfect circular dichroism, left‐ and right‐circularly polarized waves exhibit segregation in transmission and reflection. Besides, by incorporating structural changes in the Brillouin zone design, it is possible to discern more sophisticated orthogonal polarization states.