Nonlinear optical properties of a gold-silica composite with high gold fill fraction and the sign change of its nonlinear absorption coefficient

We study the intensity-dependent absorption coefficient of gold-silica composites as a function of fill fraction at frequencies near the plasmon resonance. The samples we have studied act as saturable absorbers at all wavelengths and all fill fractions, whereas pure gold is an optical limiter. We provide an explanation based on the intensity-dependent damping of the surface plasmon for this reversal of the sign of the nonlinear absorption. (C) 2008 Optical Society of America.