Ion-beam-assisted deposition of Au nanocluster/Nb2O5 thin films with nonlinear optical properties

Gold nanocluster thin films (similar to 200 nm thickness) consisting of metal clusters similar to 5 nm in size embedded in a matrix of Nb2O5 were deposited by ion beam-assisted deposition (IBAD) by coevaporation of Au and Nb with O-2(+) ion bombardment. The microstructure and optical characteristics of these films were examined as-deposited and after annealing at 600 degrees C. Annealing crystallized the amorphous oxide matrix and ripened the nanoclusters. A strong linear absorption at the wavelength of the surface plasmon resonance for Au developed as a result of annealing. The linear optical behavior was modeled using Mie scattering theory. Good agreement was found between the nanocluster sizes predicted by the theory and the particle sizes observed experimentally using transmission electron microscopy (TEM). The nonlinear optical (NLO) properties of the nanocluster films were probed experimentally using degenerate four wave mixing and nonlinear transmission. The wavelength was near the peak of the surface plasmon resonance as measured by VIS/UV spectroscopy. Values of \chi(xxxx)((3))\ were 7.3 x 10(-8) and 3.0 x 10(-10) esu for annealed and unannealed samples, respectively. The dominant mechanism for the nonlinear response was change in dielectric constant due to the generation of a distribution of hot, photoexcited electrons.