Dynamic Material Parameters in Molecular Dynamics and Hydrodynamic Simulations on Ultrashort-Pulse Laser Ablation of Aluminum

Abstract

Molecular dynamics reveals a detailed insight into the material processes. Among various available codes, IMD features an implementation of the two-temperature model for laser-matter interaction. Reliable simulations, however, are restricted to the femtosecond regime, since a constant absorptivity is assumed. For picosecond pulses, changes of the dielectric permittivity 𝜖 and the electron thermal conductivity κ _e due to temperature, density and mean charge have to be considered. Therefore, IMD algorithms were modified for the dynamic recalculation of 𝜖 and κ _e for every timestep following the corresponding implementation in the hydrodynamic code Polly-2T. The usage of dynamic permittivity yields an enhanced absorptivity during the pulse leading to greater material heating. In contrast, increasing conductivity induces material cooling which in turn decreases absorptivity and heating resulting in a higher ablation threshold. This underlines the importance of a dynamic model for 𝜖 and κ _e with longer pulses which is commonly often neglected. Summarizing all simulations with respect to absorbed laser fluence, ablation depths in Polly-2T are two times higher than in IMD. This can be ascribed to the higher spallation strength in IMD stemming from the material-specific potential deviating from the equations of state used in Polly-2T.