  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, the obtained SHG 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
two-dimensional materials present potentials to expand silicon metasurface's
functionalities in nonlinear regime.

Optics