Nonlinear optical imaging of single plasmonic nanoparticles with 30 nm resolution.

We show that background free nonlinear optical imaging of Au nanostructures with a resolution down to 30 nm can be achieved. To attain such performance, an ultrafast laser source (110 fs pulse duration) has been integrated into a parabolic mirror assisted scanning near-field optical microscope. Combining nonlinear hyperspectral imaging and the simultaneously obtained topography, the setup allows one to directly correlate/assign locations with nonlinear signals originating from either second harmonic generation or two-photon excitation processes. The contrast mechanisms of the far-field background free nonlinear optical image are discussed based on the different tip-sample coupling schemes and the selective excitation of the plasmonic modes.