Ultrafast dynamics of non-equilibrium electrons and strain generation under femtosecond laser irradiation of Nickel

We present a theoretical study of the ultrafast electron dynamics in transition metals of large electron-phonon coupling constant using ultrashort pulsed laser beams. The significant influence of the dynamics of produced nonthermal electrons to electron thermalisation and electron-phonon interaction is thoroughly investigated for various values of the pulse duration (i.e., from 10 fs to 2.3 ps). The model correlates the role of nonthermal electrons, relaxation processes and induced stress-strain fields. Simulations are presented by choosing Nickel (Ni) as a test material to compute electron-phonon relaxation time due to its large electron-phonon coupling constant. We demonstrate that the consideration of the aforementioned factors leads to significant changes compared to the results the traditional two-temperature model provides. The proposed model predicts a substantially (similar to 33%) smaller damage threshold and a large increase of the stress (similar to 20%, at early times) which first underlines the role of the nonthermal electron interactions and second enhances its importance with respect to the precise determination of laser specifications in material micromachining techniques.