Nonlinear excitation of surface plasmon polaritons by four-wave mixing.

We demonstrate nonlinear excitation of surface plasmons on a gold film by optical four-wave mixing. Two excitation beams of frequencies omega(1) and omega(2) are used in a modified Kretschmann configuration to induce a nonlinear polarization at a frequency of omega(4wm)=2omega(1)-omega(2), which gives rise to surface plasmon excitation at a frequency of omega(4wm). We observe a characteristic plasmon dip at the Kretschmann angle and explain its origin in terms of destructive interference. Despite a nonvanishing bulk response, surface plasmon excitation by four-wave mixing is dominated by a nonlinear surface polarization. To interpret and validate our results, we provide a comparison with second-harmonic generation.