Enhanced optical nonlinear absorption of graded Au--TiO₂ composite films

This paper reports that single-layer and graded Au--TiO_2 granular composite films with Au atom content 15%--66% were prepared by using reactive co-sputtering technique. The third-order optical nonlinearity of single-layer and graded composite films was investigated by using s- and p-polarized Z-scans in femtosecond time scale. The nonlinear absorption coefficient \beta_\rm eff of single-layer Au--TiO_2 films is measured to be --2.3\times 10^3-0.76\times 10^3 cm/GW with Au atom content 15%--66%. The \beta_\rm eff value of the 10-layer Au--TiO_2 graded film is enhanced to be --2.1\times 10^4cm/GW calculated from p-polarized Z-scans, which is about ten times the maximum \beta _\rm eff of single-layer films. Broadened response in the wavelength region 730--860 nm of the enhanced optical nonlinearity of graded Au--TiO_2 composite films was also investigated.