Sub-picosecond electron and phonon dynamics in nickel

Electron thermalization and electron–phonon relaxation in metals can be studied by measuring the transient change in sample reflectivity after excitation with a femtosecond laser pulse. This paper describes experiments on ferromagnetic nickel thin films, in particular addressing the analysis of the reflectivity data and its dependence on temperature. Specifically engineered samples of optically thin films on a thermally insulating barrier layer are used to minimize heat diffusion and hot-electron transport out of the volume under consideration. It is found that lattice expansion and strain wave phenomena significantly contribute to the transient reflectivity, and therefore have to be accounted for when determining the electron–phonon relaxation time. The measurements reveal an electron thermalization time of 80 fs, and an electron–phonon energy relaxation time of 0.3–0.4 ps.