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, ${\chi }^{\text{(3)}},$ of the ${\mathrm{Au:TiO}}_{\mathrm{2}}$ 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 $\text{(∼680\hspace{0.17em}}\mathrm{nm}\mathrm{),}$ both the ${\chi }^{\text{(3)}}$ and the figure of merit, ${\chi }^{\text{(3)}}\mathrm{/\alpha }$ (α is optical absorption coefficient) were significantly enhanced. The maximum value of the ${\chi }^{\text{(3)}}$ was ${\text{6×10}}^{\text{−7}}\hspace{0.17em}\mathrm{esu}$ 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 ${\mathrm{Au:TiO}}_{\mathrm{2}}$ films is extremely fast. The time-resolved DFWM results suggest that the main physical mechanism involved in the optical nonlinearity in ${\mathrm{Au:TiO}}_{\mathrm{2}}$ films on the femtoseconds time scale is the interband electric–dipole transition, and the hot electron excitation only partially contributes to the ${\chi }^{\text{(3)}}$ on the femtosecond time scale and it becomes dominant only in the picosecond region.