Description
The long-term goal of this subproject is the comprehensive molecular dynamics simulation of laser ablation on an atomistic basis. It started with simple metals and the evaluation of surface effects in the first funding period. The goal of the next periods is, first, to simulate successively more complex materials, second, to investigate the effects in the bulk of the probe, and, third, to increase the size of the samples, especially the lateral diameter up to experimentally realizable dimensions.
State of the art
Molecular dynamics simulations of laser ablation is being investigated since around 1990. For large simulations (up to 100 million atoms) usually model potentials, like the Lennard-Jones potential are used. Many simulations are even done without a proper treatment of the electronic heat conduction, e.g. the Two-Temperature Model (TTM). The sample sizes are often comparable small, sometimes in the order of 1000 atoms. The systems may even be treated two dimensional. Besides all this, for the first time in 2008 a multi-scale simulation was showed. In the work done by Ivanov et al. a sample containing ~ 80 million atoms was embedded in a finite element method (FEM) area. Most of the simulations available are dealing with pure metals. A first approach towards more complex systems was done in 2010, where simulations of a multi-layer system were presented. Our group already did simulations on a metallic alloy (Al13Co4), which shows a rather complex behavior, due to the inherent anisotropic structure, compared to metals.
For material processing, like drilling a hole, many thousand laser pulses are needed before a macroscopic hole evolves – this clearly lies beyond the possibilities of today's molecular dynamics simulations.
However, the efficiency of the actual ablation process depends on the laser pulse geometry (spatial and time dependence) and for multi-pulse excitations on the separation of two consecutive pulses. In 2009, first results for two following Gaussian pulses were presented. Most simulations are done in vacuum, but the ambient gas or liquid clearly has an influence on the ablation characteristics. From a technological point of view, a simulation scheme for covalent materials like Si or Ge would be interesting. For semiconductors a generalized Two-Temperature Model was already proposed. However, detailed studies are missing up to now. To our knowledge, there are no simulations available in ionic systems or complex polymers. Even for simple metals, the TTM should me slightly modified to better match experimental results.