Damage threshold dependence on electron–phonon coupling in Au and Ni films

Multi-shot damage thresholds of thin Au and Ni films in the thickness range from 20 nm to 1500 nm for Au and 10 nm to 100 nm for Ni induced by 200-fs pulses at 400 nm have been investigated by light scattering. A dramatic difference in the dependence of damage threshold on film thickness for Au and Ni is observed. Both metals show a linear thickness dependence of the absorbed fluence at the damage threshold for thin films and a saturation as the film thickness increases, but the onset of saturation differs for both metals by one order of magnitude 50 nm for Ni and 500 nm for Au.. This behavior can be explained by the different electron–phonon coupling in both metals. By applying the two-temperature model, we show that the energy transport after absorption of a fs-laser pulse is mainly determined by diffusion of hot electrons during the first picoseconds. This is in contrast to the case of ns-laser pulses, where the energy transport is governed by the heat diffusion in the lattice. The thermal diffusion lengths for Au and Ni are similar, whereas the electron diffusion length depends on the strength of the electron–phonon coupling, which differs by two order of magnitude for Au and Ni. This fact is responsible for the different thickness dependence of the ablation threshold for fs-pulses. q1998 Elsevier Science B.V.