Annealing Induced Aggregations and Sign Alterations of Nonlinear Absorption and Refraction of Dense Au Nanoparticles in TiO₂ Films

Au-TiO₂ dis-conductive composite films with Au atom concentrate as high as 82% are prepared by using reactive co-sputtering technique. The annealing effect on the nanostructures and optical nonlinearities of the composite films are investigated. Au nanoparticles aggregated to semi-continuum network structures during the annealing processes. As the annealing temperature increases from 25°C to 400°C, the surface plasmon absorption band is shifted and its strength is increased, consequently, the nonlinear absorption coefficient β decreases from 5.6×10⁴cm/GW to -1.7×10⁴cm/GW, while the nonlinear refractive index γ increases from -0.95cm²/GW to 1.3cm²/GW.