Energy relaxation in a strongly excited electron subsystem of a Nickel film on a substrate

Non-destructive and non-contact all-optical methods for characterization of thermal and mechanical properties of layered materials with nanoscale spatial and femtosecond/picosecond temporal resolutions are in ever-increasing demand. Here, we report on an experimental study of energy relaxation in a strongly excited electron subsystem of a Nickel film on a substrate upon irradiation with an ultrashort laser pulse using time-resolved pump-probe reflectivity measurements with phase-sensitive signal detection. The temporal dynamics of changes in reflectivity at initial electron temperatures reaching several thousand kelvins contains essential information about the ultrafast electron relaxation, electron-phonon coupling parameter, electron thermal conductivity and heat capacitance of the metal film, as well as elastic and mechanical properties revealed by detection of propagating picosecond coherent acoustic phonons in the gigahertz frequency range both in the metal and substrate. We believe these experimental data to be useful for validation of the theories existing at present time and the further development of predictive modelling.