Impact of nonlocal response in plasmonic metasurfaces on four-wave mixing

We report the impact of nonlocal response in metallic nanostructures on four-wave mixing (FWM) process in nonlinear plasmonic metasurfaces which consist of Au nanostrips coupled with Au film spaced by an ultrathin dielectric film. When the dielectric film is linear and FWM only from the Au nanostructures, the FWM efficiency of around two orders of magnitude enhancement is obtained when the nonlocal effect of Au, not the traditionally local Drude model of Au, is considered. However, when the dielectric film is nonlinear and FWM from the Au nanostructures is negligible, the almost half FWM response from the nonlinear metasurface under the nonlocal, not the local model, of Au is confirmed. These results are both ascribed to the different local electric field distributions near the surface of Au nanostructures and in the gap at the local and nonlocal response of Au. The results have an important significance to design ultra-compacted integrated nonlinear optical devices or to explain the experimental measurements of nonlinear response involving plasmonic nanostructures of ultra-small gaps.