Switchable chirality in non-volatile planar metasurfaces based on quasi-bound states in the continuum.

Bound states in the continuum (BICs) provide an exceptional platform for light-matter interactions with ultrahigh quality factors. However, multifunctional metasurfaces with simple structures for polarization manipulation and dynamic modulation, especially chiroptical response, remain challenging. Here, we propose a planar dielectric metasurface capable of generating and dynamically switching optical chirality. Maximum intrinsic chirality with reversible near-unity circular dichroism (|CD| > 0.992) is realized via a simple one-step in-plane geometric perturbation. Furthermore, by integrating non-volatile Sb₂S₃, we demonstrate dynamic switching of the chiral response between "on" and "off" states with high chirality contrast (ΔCD~1), functioning as an ultra-narrowband transmissive optical switch for circular polarization. This work provides a promising avenue for compact, reconfigurable chiral photonic devices.