Concerted effects of local and nonlocal plasmons on the broadband nonlinear optical response in tip-enhanced nanophotonics

We report critical impacts of local and nonlocal geometries of plasmonic tips on the broadband nonlinear optical responses in tip-substrate nanocavities. Using gold tips with varied geometries, we demonstrated for the first time that not only the nanometer-scale sharpness of tip apexes but also the micrometer-scale geometry of tip shafts directly affects the enhancement properties of second-harmonic generation over the visible-to-infrared wavelength range. Numerical simulations of the tip-substrate plasmonic field revealed concerted contributions from spatially nonlocal and local plasmonic modes. Micrometer-scale tip shafts enable the excitation of nonlocal plasmonic modes throughout the tip, enhancing near-to-mid-infrared incoming light. Subsequent radiation of visible-to-near-infrared second harmonics is boosted by localized plasmons at the nanogap. Based on the agreement between experiments and calculations, our results indicate the importance of nanometer- and micrometer-scale geometrical engineering of plasmonic tips and provide a firm basis to understand and finely manipulate nonlinear optical phenomena in tip-substrate nanocavities.