Synchronous detection of nonlinear phenomena in opto-acoustic vibrations induced in a nanofilm by a femtosecond laser pulse

Studies of the laser-induced ultrafast processes in thin films are of significant importance for the development of microelectronics. These processes include the heating of an electron subsystem, relaxation and transport of the absorbed energy, and generation and propagation of picosecond acoustic waves. In view of this circumstance, to study the dynamics of variation of the differential reflection coefficient $\Delta R(t)/R_0$ of a $73$-nm-thick Ni film on a glass substrate, pump–probe measurements have been performed in this work with the synchronous detection of a $\Delta R(t)/R_0$ signal. High absorbed fluences up to $11$ mJ/cm$^2$ have been reached by increasing the pulse-repetition interval $t_{\text{cool}}$ of heating (pump) pulses. An increase in tcool makes it possible to better cool the film after heating. As a result, record temperatures $(T_e\approx 3$ and $T_i\approx 1$ kK and stresses up to $7$ GPa have been reached for the first time to the best of our knowledge. These high values have allowed the observation of nonlinear effects for the first time in experiments with synchronous detection.