Second Harmonic and Sum-Frequency Generations from a Silicon Metasurface Integrated with a Two-Dimensional Material

Silicon-based nonlinear metasurfaces were implemented only with third-order nonlinearity due to the crystal centrosymmetry and the efficiencies are considerably low, which hinders their practical applications with low-power lasers. Here, we propose to integrate a two-dimensional GaSe flake onto a silicon metasurface to assist high-efficiency second-order nonlinear processes, including second-harmonic generation (SHG) and sum-frequency generation (SFG). By resonantly pumping the integrated GaSe metasurface, which supports a Fano resonance, second-order nonlinear processes in the GaSe layer are strongly enhanced. The obtained SHG from the GaSe metasurface is about two orders of magnitude stronger than the third-harmonic generation from the bare silicon metasurface. In addition, thanks to the resonant field enhancement and GaSe's strong second-order nonlinearity, SHG of the integrated structure could be excited successfully with a low-power continuous-wave laser, which makes it possible to further implement SFG. The high-efficiency second-order nonlinear processes assisted by the two-dimensional materials present potentials to expand functionalities of silicon metasurfaces in nonlinear regime.