  Here we employ the quasi-bound 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 the
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 as $2.45\times10^{-2}$ with the developed depleted model, shows
a $14.3\%$ decrease compared with $2.86\times10^{-2}$ in conventional
undepleted approximation, under the incident intensity of 10 MW/cm$^{2}$. Our
results are of significant importance for designing efficient nonlinear
metasurfaces supporting high-$Q$ resonances.
