Z-scan determination of the third-order optical nonlinearity of gold:silica nanocomposites

Third-order nonlinear optical properties of Au:SiO2 thin films were studied at the surface plasmon resonance wavelength by the z-scan technique using a nanosecond laser. Films were prepared by a multilayer deposition sputtering technique. They were composed of 2 nm mean diam gold particles, with a metal volume fraction of 20%. Z-scan measurements performed both with and without aperture showed a very large nonlinear absorption masking the nonlinear refraction. The nonlinear absorption coefficient beta was found to be negative and equal to -1.1x10(-2) cm/W. The different mechanisms contributing to this absorption are discussed and the large value of beta is correlated to the duration of the laser pulses. Moreover, it is shown that a mean field theory is not appropriate to evaluate the effective susceptibility at high metal concentrations. (C) 2000 American Institute of Physics. [S0021- 8979(00)10519-5].