Saturation of nonlinear optical absorption of metal-nanoparticle composites

Metal-nanoparticle composites consisting of copper and gold nanoparticles embedded in insulators (Cu:SiO2, Au:SiO2, and Cu:Al2O3) have been fabricated by metal-ion implantation. Nonlinear optical (NLO) absorption of the nanocomposites has been examined by the Z-scan method with a tunable femtosecond laser. We have measured (a) NLO absorption spectra near the surface-plasmon resonance and (b) laser-intensity dependence of NLO absorption, alpha(I). The results are compared to the data reported on NLO response for various laser pulse durations. By analyzing the laser-intensity dependence of Z-scan signals, the saturation intensity of light, I-S, the coefficient of NLO absorption, beta, and the radius of the beam waist, w(0), are derived. This approach takes into account the saturation of optical absorption, whereas the conventional approach using the linear approximation alpha(I)approximate to alpha(0)+beta I cannot do it. The analysis shows that transient optical absorption, parallel to alpha(I)-alpha(0)parallel to, decreases with shortening the laser pulse duration. The decrease in transient optical absorption is explained in terms of small contributions from slow components of the NLO response. (C) 2008 American Institute of Physics.