LARGE 3RD-ORDER OPTICAL NONLINEARITY IN AU-SIO2 COMPOSITE FILMS NEAR THE PERCOLATION-THRESHOLD

Very large third-order optical nonlinearity, chi((3))similar to 2.5X10 (-6) esu, measured by a degenerate four wave mixing method using a sho rt pulse (70 picosecond) laser, has been found in the rapid-thermal an nealed Au:SiO2 composite films at concentrations below the Au percolat ion threshold. The dependence of the chi((3)) on Au concentration, p, follows a cubic power law. The maximum figure of merit, chi((3))/alpha (with alpha being the absorption coefficient) is about 10(-11) esu cm . We explain this result as due to local field enhancement arising fro m the Mie resonance of the Au nanoclusters, with strong interaction be tween the nanoclusters further promoting the effect. (C) 1997 American Institute of Physics.