Interaction of intense ultrashort laser pulses with solids creates highly excited non-equilibrium states of matter with relatively high electron temperatures (several eV), but still at solid state density. Relaxation of such an excited system may lead to significant changes in the lattice structure of the irradiated target. Understanding these mechanisms is necessary to manufacture long lasting reflective optics for the rapidly developing x-ray free-electron laser light sources.
In our work we study the interaction of femtosecond laser pulses with 50 nm thick Ru films in the wide range of incident photon energies (~1 - 104 eV). Since reflective thin metal films operate at grazing incidence conditions, absorption of light occurs in the top part of the Ru (~1 – 10 nm). Hybrid multi-scale modeling is performed which includes photoabsorption and non-equilibrium electron cascading occurring on a fs timescale. Thermal diffusion and electron-phonon energy exchange occur on a ps timescale and lattice dynamics up to a ns timescale.
Different photon energies result into qualitatively different absorbed energy profiles at the moment of thermalization of the electronic system. The effects of such a difference on the hydrodynamical evolution and eventual damage of Ru are discussed. Details of processes such as melting, cavitation, ablation and recrystallization are revealed for selected photon energies. The results show good qualitative agreement with the experimental observations.
|Period||27 May 2019|
|Event title||E-MRS Spring Meeting 2019: European Materials Research Society|
|Degree of Recognition||International|