Enhancement of two-photon photoluminescence and SERS for low-coverage gold films.

Electromagnetic field enhancement (FE) effects occurring in thin gold films 3-12-nm are investigated with two-photon photoluminescence (TPL) and Raman scanning optical microscopies. The samples are characterized using scanning electron microscopy images and linear optical spectroscopy. TPL images exhibit a strong increase in the level of TPL signals for films thicknesses 3-8-nm, near the percolation threshold. For some thicknesses, TPL measurements reveal super-cubic dependences on the incident power. We ascribe this feature to the occurrence of very strongly localized and enhanced electromagnetic fields due to multiple light scattering in random nanostructures that might eventually lead to white-light generation. Raman images exhibit increasing Raman signals when decreasing the film thickness from 12 to 6-nm and decreasing signal for the 3-nm-film. This feature correlates with the TPL observations indicating that highest FE is to be expected near the percolation threshold.