Molecular dynamics simulation of shock waves in laser-material interaction

Gacek, Sobieslaw
Major Professor
Xinwei Wang
Committee Member
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Mechanical Engineering
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Mechanical Engineering

In recent decades, laser technology has been widely used in manufacturing, non-destructive measurement processes, and has been extensively implemented in medical applications. The detailed knowledge of the laser-target interaction along with accompanied effects in background environment is absolutely essential due to the significance of the intricate existing occurrences. Therefore, in this discourse, a number of phenomena in laser-material interaction at nanoscale are studied thorough.

Firstly, the dynamics and internal structure of shock waves in picosecond laser-material interaction are explored at the atomistic level. The pressure of the shock wave, its propagation, and interaction zone thickness between the plume and ambience are evaluated to study the effect of the laser absorption depth, ambient pressure, and laser fluence. Sound agreement is observed between the molecular dynamics simulation and theoretical prediction on shock wave propagation and mass velocity. Due to the strong constraint from the compressed ambient gas, it is observed that the ablated plume could stop moving forward and mix with the ambient gas, or move backward to the target surface, leading to surface redeposition. Under smaller laser absorption depth, lower ambient pressure, or higher laser fluence, the shock wave will propagate faster and have a thicker interaction zone between the target and ambient gas.

Secondly, the effects of shock driven process of the laser-ablated argon plume in the background gas environment are explored via molecular dynamics simulations. The primary

shock wave propagation and its influence on the backward motion of the target material are delineated. It has been observed that the strong pressure gradient inside the main shock wave overcomes the forward momentum of the plume and some compressed gas, leading to backward movement and re-deposition on the target surface. Reflection of the backward moving gas on the target surface results in the secondary shock wave. Detailed investigation of the secondary shock wave phenomenon is provided, which gives, for the first time, an insight into formation and evolution of the internal gaseous shock at the atomistic level.

Thirdly, the physics of plume splitting in pico-second laser material interaction in background gas are studied with MD simulations. The velocity distribution shows a clear split into two distinctive components. For the first time, detailed atom trajectory track reveals the behavior of atoms within the peaks and uncovers the mechanisms of peak formation. The observed plume velocity splitting emerges from two distinguished parts of the plume. The front peak of the plume is from the faster moving atoms and smaller particles during laser-material ablation. This region experiences strong constraint from the ambient gas and has substantial velocity attenuation. The second (rear) peak of the plume velocity originates from the larger and slower clusters in laser-material ablation. These larger clusters/particles experience very little constraint from the background, but are affected by the relaxation dynamics of plume and appear almost as a standing wave during the evolution. Density splitting only appears at the beginning of laser-material ablation and quickly disappears due to spread-out of the slower moving clusters. It is found that higher ambient pressure and stronger laser fluence favor earlier plume splitting.

Finally, the conclusions are drawn and author's contributions from performed work are delineated.