Chiral quasi-bound states in the continuum with near-unity and switchable circular dichroism in a photosensitive silicon metasurface

High-Q chiral quasi-bound states in the continuum (q-BICs) have great prospects for applications in detectors, modulators, switches, etc. Here, we demonstrate the chiral q-BIC with near-unity and switchable circular dichroism (CD) in a photosensitive silicon metasurface. By breaking the out-of-plane symmetry of the windmill-shaped structure, the high-Q q-BIC and near-unity CD are simultaneously realized. Interestingly, CD keeps the near-unity values as the asymmetry parameter varies, which is an important merit for chiroptical devices. The CD could be dynamically switched from 0.98 to 0.04 by optically pumping photosensitive silicon. Besides, an extrinsic chiral q-BIC is demonstrated and switched by optically pumping photosensitive silicon. The intrinsic and extrinsic chirality are both obtained by exciting electric quadrupole (EQ) resonance, which are verified by quantitative multipole decompositions and electric field distribution analysis.