Saturation of nonlinear optical absorption of metal-nanoparticle composites

摘要

Metal-nanoparticle composites consisting of copper and gold nanoparticles embedded in insulators (Cu:SiO_2, Au:SiO_2, and Cu:Al_2O_3) 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, α(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, β, 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 α(I) ≈ α_0+βI cannot do it. The analysis shows that transient optical absorption, |α(I)-α_0|, 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.