Laser Surface Engineering @ ALPS

Picosecond laser studies

In the context of his PhD work M Yiming Zhang studies the interaction of picosecond pulsed lasers with metal surfaces. His workat the laser surface engineering group has both a theoretical and experimental aspect.


Picosecond laser systems have been widely used in industrial micro-processing applications since they are a cost-effective tool to achieve high throughput. A better understanding of the underlying phenomena is often required.
Simulating the interaction between ultrashort and short pulsed laser and metals and interpreting the mechanisms of corresponding phenomena during laser ablation process, e.g. Two Temperature Model (TTM).


To better understand the ablation process, firstly the dependence of the ablation depth and the threshold fluence on the laser spot size were determined experimentally by performing ablation with a 10ps pulsed laser system. Further, a 2D axisymmetric model was established to demonstrate the possible mechanism of the phenomena. Three sets of spot radii, namely 15.5μm, 31.5μm and 49.6μm, respectively with equal laser peak fluences ranging from 0.6J/cm2 to 4.5J/cm2 were applied on copper.


It was found that the laser ablation depth increases while the threshold fluence decreases with decreasing spot size at identical peak fluence. A 2D axisymmetric thermo-mechanical model was developed to qualitatively illustrate the mechanism behind these phenomena. The numerical results of the position where the tensile stress exceed to ultimate tensile strength (UTS) of copper show the same trend as the experimental ones. The longitudinal tensile stress was seen to play a more crucial role than the radial tensile / compressive stress on laser ablation process. The impact of the thermal stress on the ablation depth and threshold fluence is derived from the lattice temperature gradient along the surface of the material, leading to spallation and possible modifications of the mechanical properties already at lower laser peak fluences. This is elucidated numerically and analytically.


laser ablation simulation with Two Temperature Model

laser ablation simulation with Two Temperature Model

Award at LAMON 16


The deviation of the experimental results from the simulation might be attributed to the fact that this simulation model is static. Nevertheless, at low laser fluences, this static approach can provide good explanations of the cold ablation with ultrashort pulsed laser. The limitation of this model is also illustrated.


Yiming Zhang registered as a PhD student at the laser department of the institute of applied physics of the University of Bern. He is doing a PhD in physics and his official advisor at the University of Bern is Valerio Romano also member of ALPS.