Double-layer cross-shaped cylinder terahertz all-dielectric metasurface with a high quality factor and giant chiral response governed by bound states in the continuum

The combination of high-quality-factor (Q-factor) bound states in the continuum (BIC) and chiral metasurfaces has attracted much attention in the field of photonics. Here, we design and analyze a “sandwich” all-dielectric metasurface with two silicon cross-shaped cylinders distributed on the upper and lower surfaces of the silica. The transition from symmetry-protected BIC to chiral quasi-BIC (QBIC) is achieved by innovatively breaking both the mirror symmetry and the in-plane inversion symmetry of the structure, resulting in a transmittance-ratio circular dichroism (RCD) close to unity and a high Q-factor. In particular, three chiral QBICs (QBIC1, QBIC2, and QBIC3) are realized in the terahertz band without increasing the complexity of the structure at each layer. Multipole decomposition and near-field analysis demonstrate that QBIC1 and QBIC2 are dominated by the toroidal dipole and magnetic quadrupole, while QBIC3 is mainly affected by the electric quadrupole and magnetic quadrupole. In addition, the presence of positive and negative states due to the RCD values of the structure suggests a spin selectivity for different frequency bands. Theoretical assessment shows a maximum Q-factor of 3.94 × 10⁴, a maximum sensitivity of 245 GHz per refractive index unit (RIU), and a figure of merit of 7927 RIU⁻¹. The results reveal a novel approach for selectively modulating circularly polarized light, demonstrating significant potential in biomolecular detection, high-spectral-resolution chiral biosensors, and chemical analysis.