Critical Role of Modal Spatial Overlap in Nanoscale Nonlinear Optics

We unambiguously demonstrate the critical role of modal spatial overlap in \textit{nonlinear} optics for nanoscale structures. Our experimental and theoretical investigations show that, within a sub-wavelength metallic hole, spatial overlap between the linear and nonlinear modes strongly correlates to the conversion efficiency. Our results provide an accurate explanation for the long-emphasized but elusive shape effect. Moreover, our investigation stimulates new angles for and deeper insights into general nonlinear optics at nanoscale.