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

O3 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 χ(3) was measured by degenerated four-wave mixing using a 70-ps pulse laser at 532 nm. The maximum value of the χ(3) was about 1.2×10-6 esu in the annealed films and occurred at around 45% Au concentration. The figure of merit, χ(3)/α (α is the absorption coefficient), has a value of 7×10-12 esu cm over a wide range of Au concentrations.