PHASE CHANGE AND ABLATION STUDY OF METALS BY FEMTOSECOND LASER IRRADIATION USING HYBRID TTM/MD SIMULATIONS

The interactions of femtosecond lasers with gold targets were investigated with a numerical method combining molecular dynamics (MD) and the two-temperature model (TTM). Previous works using MD-TTM method did not consider all the thermodynamic parameters and the interatomic potential dependent of the electron temperature simultaneously. Therefore, we developed a LAMMPS function to achieve this. To accurately capture the physics behind the interactions, we also included the electron blast force from free electron pressure and the modified Fourier law with steep electron temperature gradient in our model. For bulk materials, a stress non-reflecting and heat conducting boundary is added between the atomistic and the continuum parts. The modified boundary force in our study greatly reduces the reflectivity of the atomistic-continuum boundary compared with its original form. Our model is the first to consider all these factors simultaneously and manage to predict four femtosecond laser ablation phenomena observed in the experiments.
In this dissertation, the thermodynamic parameters in the two-temperature model were extensively explored. We considered three different approaches to calculate these parameters: namely interpolation, ab initio calculation, and analytical expression. We found that simple interpolation between solid state and plasma state could lead to high level of inaccuracy, especially for electron thermal conductivity. Therefore, ab initio calculation and analytical expression were used for the calculation of the thermodynamic parameters in more advanced studies. The effects of electron thermal conductivity and electron-phonon coupling factor on electron and lattice temperatures were analyzed.
Our studies considered electron temperature dependent (ETD) and electron temperature independent (ETI) interatomic potentials. The ETI interatomic potential is easier to implement and therefore it is used in our phase change study to investigate the effects of target thickness on melting. Homogeneous melting occurred for thin films, while melting can be observed through the movement of the solid-liquid interface in thick or bulk materials. However, the ETI potential overestimated the bond strength at high temperatures. Therefore, ablation process was studied with the ETD potential. Three ablation mechanisms were found in our simulations at different laser fluences. Short nonthermal ablation was only observed at the ablation threshold. With increasing laser fluence, spallation was then seen. In high laser fluence regime, phase explosion occurred on the surface and coexisted with spallation.
Lastly, we researched on the effects of the delay time between two femtosecond laser pulses. Various delay times did not have much influence on melting depth. In low laser fluence regime, with increasing delay time, the target went through nonthermal ablation, to spallation and to no ablation. In high laser fluence regime, longer delay time encouraged phase explosion while suppressed spallation.