Direct observation of localized second-harmonic enhancement in random metal nanostructures

Second harmonic (SH) scanning optical microscopy in reflection is used to image the gold film surface covered with randomly placed scatterers. SH images obtained with a tightly focused tunable (750-830 nm) laser beam show small (similar to 0.7 mum) and very bright (similar to 10(3) times the background) spots, whose locations depend on the wavelength and polarization of light. Comparing SH and fundamental harmonic (FH) images, we conclude that the localized SH enhancement occurs due to the overlap of FH and SH eigenmodes. The probability density function of the SH signal is found to follow the power-law dependence.