  We use numerical simulations to demonstrate third-harmonic generation with
near-unity nonlinear circular dichroism (CD) and high conversion efficiency ($
10^{-2}\ \text{W}^{-2}$) in asymmetric Si-on-SiO$_2$ metasurfaces. The working
principle relies on the selective excitation of a quasi-bound state in the
continuum, characterized by a very high ($>10^5$) quality-factor. By tuning
multi-mode interference with the variation of the metasurface geometrical
parameters, we show the possibility of independent control of linear CD and
nonlinear CD. Our results pave the way for the development of all-dielectric
metasurfaces for nonlinear chiro-optical devices with high conversion
efficiency.
