Time-resolved thermoreflectivity of thin gold films and its dependence on film thickness

Transient linear reflectivities of thin gold films in the thickness range from 10 nm to 500 nm have been measured by femtosecond time-resolved pump-probe experiments. A conspicuous change of the relaxation behavior was found around 100 nm for pump pulse fluences of 1 mJ/cm(2). Thicker films show a nearly exponential decay of the transient linear reflectivity, which turns into a linear decay during the first 5-7 ps for films with thicknesses of 100 nm or less. This observation is evidence of a mean free path of about 100 nm for hot electrons with temperatures around 1500 K.