Modeling of femtosecond laser-induced non-equilibrium deformation in metal films

Abstract

This paper extends the dual-hyperbolic two temperature (Numer. Heat Transfer Part A 40 (2001) 1) and hot-electron blast (J. Exp. Theor. Phys. 88 (1999) 84) models to investigate the deformation in metal films subjected to ultrashort laser heating. A new set of fully coupled, transient thermoelasticity equations is derived based on the assumption of uniaxial strain but three-dimensional stress. Two potential material removal mechanisms, thermal (melting) and non-thermal (high stress), are identified. Numerical results show that the non-thermal damage could be a dominating mechanism in ultrashort laser-material ablation. The major driving force for the non-thermal damage is the so-called hot-electron blast force, which is generated by non-equilibrium hot electrons. It is also found that for gold films thicker than 200 nm, a thin layer of material near the heated surface could be removed, as experimentally observed (Opt. Cummun. 129 (1996) 134; J. Appl. Phys. 85 (1999) 6803). On the other hand, damage could initiate from the middle region and then extend over the entire film for a gold film of 50 nm in thickness or thinner.