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 βeff at the Au atom content of 54% are measured to be 1.6×10−2 cm2/GW and −2.6×103 cm/GW, respectively. The corresponding third-order optical nonlinearity χ(3) is about 6.3×10−8 esu.