High-spectral-resolution metasurface based on quasi-bound states in the continuum

Bound states in the continuum (BIC) have attracted significant attention due to their theoretically infinite Q factors and excellent electromagnetic field confinement. However, achieving both high spectral resolution and strong circular dichroism (CD) in a single metasurface structure remains challenging. Here, we propose a silicon-based metasurface with fourfold rotational symmetry (C 4 v ) and demonstrate the controlled evolution from an ideal symmetry-protected BIC to a chiral quasi-BIC by sequentially introducing in-plane and out-of-plane symmetry breaking. Numerical simulations show that the structure simultaneously supports a high-spectral-resolution quasi-BIC mode with a pronounced CD response, reaching a CD magnitude of approximately 0.90 with a corresponding Q factor of 713.82. This work provides what we believe to be a new design strategy for narrowband chiral photonic devices and may find applications in chiral sensing and spin-selective wavefront control.