Near-unity third-harmonic circular dichroism driven by a quasibound state in the continuum in asymmetric silicon metasurfaces

We use numerical simulations to demonstrate third-harmonic generation with near-unity nonlinear circular dichroism (CD) and high conversion efficiency $({10}^{\ensuremath{-}2}\phantom{\rule{4pt}{0ex}}{\text{W}}^{\ensuremath{-}2})$ in asymmetric Si-on-$\mathrm{Si}{\mathrm{O}}_{2}$ metasurfaces. The working principle relies on the spin-selective excitation of a quasibound state continuum, characterized by very $(>{10}^{5})$ quality factor. By tuning multimode interference variation metasurface geometrical parameters, we show possibility control both linear CD CD. Our results pave way for development all-dielectric metasurfaces chiro-optical devices efficiency.