Self-organized structures in ultrashort pulsed laser processing

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In laser material processing with ultrashort laser pulses (USP) it is neither possible to quantitatively predict processing results nor to control potentially occurring effects like the formation of self-organized structures. Their controlled generation and avoidance is of high interest for production technology. On the one hand they cause a reduction in resolution and an increase in surface roughness; on the other hand changes in the surface structure can lead to useful properties like distinctive hydrophobicity. Therefore, a multi-physical process model to simulate USP processing of metals will be developed and applied in this research project. As intermediate aim, single pulses will be used to analyse if it is possible to completely determine temperature, pressure and density using a transient description of the material heat flows and the equation of states. Furthermore, it will be studied if spallation and phase explosion can be fully explained by a fluid dynamic (CFD) approach. The aim of the project is to explore the formation of self-organized structures like cone-like protrusions (CLP) and laser-induced periodic surface structures (LIPSS) by using multiple-pulses. The influence of intensity variations caused by surface roughness and fluid dynamics on their formation will be studied.The planned project will be organised in three phases. In the first one, a multi-physical USP process model, developed at the Institute of Photonic Technologies (LPT), will be coupled with a model for absorption and heat input, developed at the Lasercenter at the Munich University of Applied Sciences (LHM). Additionally, the equation of state and electronic material parameters will be modelled. A model to calculate the local intensity by solving the Maxwell’s equations will be developed and included. The development of the model will be continuously accompanied by experiments for comparison and iterative model improvement. This leads to a very detailed and comprehensive study of the thermo- and process dynamics, which builds the basic of the project.The second phase focuses on single pulse processing. Using simulations, pump-probe ellipsometry and high-speed videography, the consistency of the model descriptions will be verified and an improvement of the empirical process understanding is expected. The third phase addresses multi-pulse processing. Using simulations, in-situ pump-probe microscopy and SEM and LSM analyses the empirical process understanding of multi-pulse processing will be improved. The focus is kept on the influence of fluid dynamic effects on CLP- and LIPSS-formation and the additional impact of inhomogeneous, periodic energy coupling into the workpiece, caused by interference effects of the surface roughness and by plasmonic effects.