Surface-Enhanced Nonlinear Four-Wave Mixing

We report on a particularly strong third-order nonlinear response from nanostructured gold surfaces. Two incident laser beams with frequencies omega(1) and omega(2) give rise to four-wave mixing (4WM) fields with frequencies 2 omega(1) - omega(2) and 2 omega(2) - omega(1). We demonstrate that the nonlinear response can be purely evanescent and that nanostructured surfaces convert the evanescent energy into propagating radiation, thereby increasing the efficiency of frequency conversion. The emitted 4WM radiation is found to be directional, polarized, coherent, and both frequency and angle tunable. The ability to perform efficient frequency conversion in reduced dimensions provides new opportunities for nanophotonics and active plasmonics.