Large third-order nonlinear optical susceptibility of Au-Al2O3 composite films near the resonant frequency

Au:Al2O3 composite films with high Au concentrations (30\%-60\% in volume fraction) were prepared by reactive co-sputtering and post rapidly thermal annealing. The structure of the films and the size distributions of the Au nanoclusters were examined by TEM, and the third-order nonlinear optical susceptibility chi((3)) was measured by degenerated four-wave mixing using a 70-ps pulse laser at 532 nm. The maximum value of the chi((3)) was about 1.2 x 10(-6) esu in the annealed films and occurred at around 45\% Au concentration. The figure of merit, chi((3))/alpha (alpha is the absorption coefficient), has a value of 7 x 10(-12) esu cm over a wide range of Au concentrations