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\nnear-unity nonlinear circular dichroism (CD) and high conversion efficiency ($\n10^{-2}\\ \\text{W}^{-2}$) in asymmetric Si-on-SiO$_2$ metasurfaces. The working\nprinciple relies on the selective excitation of a quasi-bound state in the\ncontinuum, characterized by a very high ($>10^5$) quality-factor. By tuning\nmulti-mode interference with the variation of the metasurface geometrical\nparameters, we show the possibility of independent control of linear CD and\nnonlinear CD. Our results pave the way for the development of all-dielectric\nmetasurfaces for nonlinear chiro-optical devices with high conversion\nefficiency.\n