Nonlinear absorption in Au films: Role of thermal effects

The effective nonlinear optical absorption coefficient βeff is measured for 20-nm-thick Au films at 6300.3em0exnm as a function of pulse width. The z-scan measurements show that βeff increases from 6.8×10^-7 to 6.7×10^-50.3em0excm0.2em0exW^-1 as the pulse width is varied from 0.10.3em0exto0.3em0ex5.80.3em0exps. To help interpret this ~100× increase, differential transmission and reflectivity measurements are performed using 7750.3em0exnm pump and 6300.3em0exnm probe pulses. All experiments are simulated with a two-temperature model for electrons and lattice. The pulse width dependence of βeff is consistent with thermal smearing of d-band to conduction-band transitions, with βeff arising from changes in the linear (Im0.2em0exχ⁽¹⁾) absorption coefficient.