Nonlinear absorption in Au films: Role of thermal effects

The effective nonlinear optical absorption coefficient beta(eff) is measured for 20-nm-thick Au films at 630 nm as a function of pulse width. The z-scan measurements show that beta(eff) increases from 6.8x10(-7) to 6.7x10(-5) cm W-1 as the pulse width is varied from 0.1 to 5.8 ps. To help interpret this similar to 100x increase, differential transmission and reflectivity measurements are performed using 775 nm pump and 630 nm probe pulses. All experiments are simulated with a two-temperature model for electrons and lattice. The pulse width dependence of beta(eff) is consistent with thermal smearing of d-band to conduction-band transitions, with beta(eff) arising from changes in the linear (Im chi((1))) absorption coefficient.