Origin of third-order optical nonlinearity in Au:SiO2 composite films on femtosecond and picosecond time scales

Three sorts of probe laser, which have pulse durations of 200 fs, 35 ps, and 70 ps, were employed in the measurement of the third-order nonlinear optical susceptibility chi((3)) in Au:SiO2 composite films in a degenerate four-wave mixing scheme. me found that the composite films at their absorption peak (similar to 550 nm) had a maximum chi((3)), which depends strongly on the pulse width of the probe laser. The value of chi((3)) measured with a 70-ps laser was similar to 30 times larger than that measured with a 200-fs laser. The time-resolved measurements revealed that the optical nonlinearity on the femtosecond time scale is attributable mainly to contributions from the interband electric-dipole transition (especially at low concentrations) and partly to those From hot electrons rather than being dominated by hot-electron excitation in the picosecond regime. (C) 1998 Optical Society of America.