Enhanced nonlinear optical conversion from a periodically nanostructured metal film

We demonstrate an $\sim {10}^4$ increase in conversion efficiency for optical second-harmonic generation (SHG) from a periodically nanostructured metal structure consisting of a single subwavelength aperture in a thin silver film surrounded by a set of concentric surface grooves. The forward-transmitted second-harmonic radiation from this structure is measured relative to that from an identical aperture with no surrounding surface periodicity. We explain the observed SHG enhancement quantitatively in terms of a measured $120\times$ increase in the strength of the fundamental radiation in the vicinity of the aperture resulting from the periodic nanostructuring.