Large third-order optical nonlinearity in Au:TiO2 composite films measured on a femtosecond time scale

The wavelength dependence of the third-order nonlinear optical susceptibilities, chi X-(3), Of the Au:TiO2 composite films with Au concentration varying from 15% to 60% (volume fraction), tvas measured by a degenerate four-wave mixing (DFWM) technique using a probe laser with a pulse width of 200 fs. It was found that, with the wavelength of the probe laser close to the surface plasmon resonance (similar to 680 nm), both the chi((3)) and the figure of merit, chi((3))/alpha (alpha is optical absorption coefficient) were significantly enhanced. The maximum value of the chi((3)) was 6 x 10(-7) esu and occurred at an Au concentration of about 38%. Femtosecond time-rr:solved DFWM measurements revealed that the response time of the optical nonlinearity in the Au:TiO2 films is extremely fast. The time-resolved DFWM results suggest that the main physical mechanism involved in the optical nonlinearity in Au:TiO2 films on the femtoseconds time scale is the interband electric-dipole transition, and the hot electron excitation only partially contributes to the chi((3)) on the femtosecond time scale and it becomes dominant only in the picosecond region. (C) 1998 American Institute of Physics.