Ultra-short double pulse laser ablation of metals: what can we know from numerical hydrodynamics and from molecular dynamics simulations?

International audienceThe past decade has witnessed a considerable development in the field of ultra-short, or femtosecond laser systems and of their applications. At the same time, many theoretical and numerical investigations have been proposed underlying the main physical processes involved the interactions of these laser pulses with various materials. In particular, classical two-temperature one fluid hydrodynamics (Hydro-TTM) was used to account for the target material motion and for the phase transitions.1,2 This model allowed us to elucidate the role of fragmentation of the metastable liquid phase as a result of fast heating and shock wave propagation inside metal targets. Hydrodynamics, however, does not describe non-equilibrium state of matter. To better explain such effects, two-temperature molecular dynamics simulations (MD-TTM) were furthermore performed to provide even more detailed insights into the laser ablation mechanisms, such as phase explosion, fragmentation, evaporation, and mechanical spallation. Nevertheless, many effects, such as a curious effect of ablation suppression observed for metals in the case of double laser pulses, required more investigations.A careful application of our models to the case of double pulse laser ablation required a self-consistent simulation of laser energy propagation and absorption, in particular by plasma formed by the 1st pulse. As a result of the performed MD-TTM calculations, two main mechanisms could be identified to be responsible for the suppression of ablation in the double pulse ablation experiments.3 The first one is associated with the suppression of the rarefaction wave,2 which leads to homogeneous nucleation in the liquid layer of the target under tensile stress. For the delays longer than 50 ps, the second pulse generates a novel high-pressured plasma region ahead of the ablated liquid layers. This region pushes a large fraction of the ablated layers back to the target. In this case, the ablation depth can be even smaller than the one in the case of a single pulse regime. These findings bring more light into the recent double-pulse LIBS experiments.References: 1 M.E. Povarnitsyn, T.E. Itina, M. Sentis, K.V. Khishchenko, P.R. Levashov, Phys. Rev. B, 75(23), (2007) 235414.2 M.E. Povarnitsyn, T.E. Itina, K.V. Khishchenko, P.R. Levashov, Phys. Rev. Lett., 103(19), (2009) 195002.3 M.E. Povarnitsyn, V. Fokin, P.R. Levashov, T.E. Itina, Phys. Rev. B, 92(17), (2015) 174104