High-Q intrinsic and nonlinear chirality in planar silicon metasurfaces enabled by Brillouin zone folding

Realizing a planar metasurface with a high quality (Q) factor that simultaneously possesses perfect intrinsic chirality and nonlinear chirality remains a key challenge. Here, through theoretical analysis and full-wave numerical simulations, we propose a high-Q planar silicon metasurface with intrinsic chirality enabled by Brillouin zone folding. Through band engineering, high-Q quasi-guided modes are excited at the Γ point, while breaking the in-plane mirror symmetry enables selective coupling to circularly polarized light. The optimized structure achieves a near-perfect intrinsic chiral response with a Q factor above 2200, a linear circular dichroism (CD) larger than 0.99, and a nonlinear CD exceeding 0.999. Moreover, the Q factor of the chiral quasi-guided mode can be effectively tuned while maintaining near-unity CD. Strong chiral selectivity and a stable high-Q response are preserved over an incidence-angle range of ±2.5°. This work establishes a theoretical framework based on Brillouin zone folding for engineering chiral metasurfaces, providing useful design guidelines for future high-performance chiral photonic and optoelectronic devices.