Enhanced second harmonic generation in high-$Q$ all-dielectric metasurfaces with backward coupling

Abstract

Here we employ the quasi-bound state in the continuum (quasi-BIC) resonance in all-dielectric metasurfaces for efficient nonlinear processes in consideration of backward coupling. 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 coupling on the nonlinear conversation efficiency. The SHG efficiency as $3.94\times10^{-3}$ with the developed depleted approximation, shows a 9$\%$ decrease compared with $4.33\times10^{-3}$ in conventional undepleted approximation, under the incident intensity of 10 GW/cm$^{2}$. Our results are of significant importance for designing efficient nonlinear metasurfaces supporting high-$Q$ resonances.