Large Nonlinear Efficiency Enhancement in the Visible and UV Range from Plasmonic Gold Nanogratings.

We report a combined experimental/theoretical investigation on second and third harmonic generation from a plasmonic gold nanograting, that exhibits a plasmonic resonance in the near IR. The intense field localization leads to significant enhancement in the efficiency of nonlinear optical processes from the UV to the near IR range: second harmonic generation (SHG) conversion efficiencies increase three orders of magnitude compared to a flat gold mirror, while third harmonic generation (THG) conversion efficiency increases by nearly four orders of magnitude. The spectral and angular dependence (qualitative and quantitative) of the harmonics were experimentally recorded and validated within the framework of our own theoretical model, that we employ to simulate SHG and THG efficiencies. Our approach consists of an expanded microscopic hydrodynamic model that captures the dynamics of both free and bound electrons, and faithfully describes the basic electrodynamic behaviour of linear and nonlinear optics of metals at the nanoscale.