A dual-color plasmonic focus for surface-selective four-wave mixing.

We describe a dual-color plasmonic lens suitable for focusing two femtosecond surface plasmon polariton wavefronts to a common focal spot. We show that the overlapping evanescent fields, which are confined to (sub-) micrometer dimensions, form a surface-selective excitation volume for four-wave mixing (FWM) experiments. We demonstrate that stable and virtually background-free FWM signals from single nano-objects placed in the plasmonic focus can be generated. The plasmonic focal spot constitutes a precisely controlled excitation source for surface-selective nonlinear spectroscopic measurements. [ABSTRACT FROM AUTHOR]