Enhanced second-harmonic generation in high- Q all-dielectric metasurfaces with backward frequency conversion

Here we employ the quasibound state in the continuum (quasi-BIC) resonance in all-dielectric metasurfaces for efficient nonlinear processes in consideration of the backward frequency conversion. We theoretically study second-harmonic generation (SHG) from symmetry-broken AlGaAs metasurfaces and reveal the efficiency enhancement empowered by high-$Q$ quasi-BIC resonances. By introducing the correction term of nonlinear polarization at the fundamental wave field to the conventional undepleted approximation, we uncover the effect of backward frequency conversion on the nonlinear conversation efficiency. The SHG efficiency of $2.45\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ with the developed depleted model shows a $14.3%$ decrease compared with $2.86\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ in the conventional undepleted approximation, under an incident intensity of $10\phantom{\rule{4pt}{0ex}}\mathrm{MW}/{\mathrm{cm}}^{2}$. Our results are of importance for designing efficient nonlinear metasurfaces supporting high-$Q$ resonances.