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

This paper reports that single-layer and graded Au--TiOAu--TiOAu--TiO$_{2}$ granular composites, graded composite films, $Z$-scan, optical nonlinear absorptionProject supported by the National Natural Science Foundation of China (Grant No 10474075).4270, 7820This 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 \textit{$\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 \textit{$\beta $}$_{\rm eff}$ value of the 10-layer Au--TiO$_{2}$ graded film is enhanced to be --2.1$\times $10$^{4}$cm/GW calculated from p-polarized $Z$-scans, which is about ten times the maximum \textit{$\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.