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, χ⁽³⁾, of the Au:TiO₂ composite films with Au concentration varying from 15% to 60% (volume fraction), was 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 (∼680nm), both the χ⁽³⁾ and the figure of merit, χ⁽³⁾/α (α is optical absorption coefficient) were significantly enhanced. The maximum value of the χ⁽³⁾ was 6×10^-7esu and occurred at an Au concentration of about 38%. Femtosecond time-resolved 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 χ⁽³⁾ on the femtosecond time scale and it becomes dominant only in the picosecond region.