Study of Energy Relaxation in a Nickel Nanofilm after Ultrafast Heating of the Electronic Subsystem by a Femtosecond Laser Pulse

In this paper, we investigate the dynamics of energy relaxation in a 73-nm-thick nickel film in a highly nonequilibrium two-temperature state induced by a femtosecond laser pulse. Experimental measurements of changes in reflectance ΔR/R₀ from the front side of the nanofilm were carried out in a pump-probe scheme with the phase-sensitive detection technique at a wavelength of 793 nm in the time range of up to 300 ps with a temporal resolution of 60 fs at the maximum possible non-destructive absorbed fluence F_abs = 10.87 mJ/cm² of the heating (pump) pulse at a wavelength of 396 nm and with a duration of 150 fs. Signal ΔR/R₀ contains information on both the dynamics of thermal processes and the propagation of picosecond acoustic pulses in the nanofilm and in the substrate. The longitudinal speed of sound in the nanofilm was 5.73 ± 0.16 nm/ps, and the Brillouin frequency shift in the substrate was about 21.15 GHz. Two-temperature hydrodynamic calculation yields values of the maximum electron and lattice temperatures T_e = 2.9 kK and lattices T_i = 1.1 kK, respectively. The peak pressures of the acoustic pulses in the nanofilm and in the substrate were 6.8 and 1.2 GPa, respectively. There are hardly any data in the literature on studies of picosecond dynamics of thermal and acoustic processes in metal nanofilms at high initial temperatures of the electronic subsystem, excited with a femtosecond laser pulse at the energy flux rate near the modification (destruction) threshold of the material.