A Sign Alternation of Nonlinear Absorption in Gold Composite Films in Z-Scan

Au--TiO₂ composite films with Au atom content varying from about 15% to 82% are prepared by co-sputtering technique. Both open- and closed-aperture Z-scan of the samples are performed in the femtosecond time region. A conversion of the nonlinear absorption from negative to positive is observed as the Au atom content increases due to the saturation of reverse saturable absorption. The nonlinear refractive index γ and effective nonlinear absorption coefficient β_eff at the Au atom content of 54% are measured to be 1.6×10^-2cm²/GW and -2.6×10³cm/GW, respectively. The corresponding third-order optical nonlinearity x⁽³⁾ is about 6.3×10^-8esu.