Second harmonic generation in monolithic gallium phosphide metasurfaces

Gallium phosphide (GaP) offers unique opportunities for nonlinear and quantum nanophotonics due to its wide optical transparency range, high second-order nonlinear susceptibility, and the possibility to tailor the nonlinear response by a suitable choice of crystal orientation. However, the availability of single crystalline thin films of GaP on low index substrates, as typically required for nonlinear dielectric metasurfaces, is limited . Here we design resonant monolithic GaP metasurfaces optimized for efficient second harmonic (SH) generation. We experimentally realized the metasurfaces from bulk (110) GaP wafers using electron-beam lithography and an optimized inductively coupled plasma etching process. SH generation measurements show good agreement with numerical simulations and a high NIR-to-visible conversion efficiency reaching up to 10^(-5) for SH emission along the optical axis. Furthermore, we investigated the potential of the suggested monolithic GaP metasurface for SH wavefront shaping applications.