Computer modelling of short pulse laser processing of metals

The ability of short pulse laser irradiation to produce complex multiscale surface morphologies, metastable phases and unusual microstructures has been demonstrated in experiments and is generally attributed to the conditions of strong electronic, thermal, phase, and mechanical non-equilibrium created in irradiated targets by the fast laser excitation. Detailed understanding of the relations between the fast non-equilibrium processes caused by the laser energy deposition and the resulting structure and properties of laser-treated regions of the targets, however, is still lacking, thus limiting the expansion of laser technologies into the new domain of nanoscale material processing and fabrication. The main objectives of the proposed research program is (1) to provide, through advanced multiscale modeling, detailed information on the mechanisms and kinetics of fast non-equilibrium structural and phase transformations triggered by short pulse laser irradiation of metal targets and (2) to establish the main factors that control the surface morphology and microstructure of laser-modified targets. The key component of the program is the design and thorough verification of a novel multiscale computational approach that combines, in a synergistic manner, large-scale atomistic simulations of the initial material response to the short pulse laser excitation (rapid melting, cavitation, explosive boiling and phase decomposition of superheated liquid) with a continuum-level modeling of the subsequent slower liquid flow and solidification in the melted surface region. The new computational model will enable, for the first time, a detailed exploration of the complex interplay of laser-induced processes occurring at different time- and length-scales and will reveal the connections between the irradiation conditions, material properties, crystallographic orientation of grains in polycrystalline targets, and the configurations of crystal defects generated by laser irradiation. The research program will be closely coordination with collaborators at the University of Vienna (host institution) and other Austrian and European scientists. The computational predictions will be related to observations by the experimental collaborators and the results of the comparison will be used to verify and improve the multiscale model. The initial focus of the joint computational- experimental effort will be aimed at revealing the physical origin of the incubation effect in multi- pulse interactions with metals (pulse-to-pulse variation of laser modification of the target), explaining the peculiarities of laser processing of multi-layered targets, and providing insights into the mechanisms of laser processing of metal surfaces in liquid environment. It is expected that the proposed project will result in the establishment of long-term international collaborations that would last far beyond the time of the project supported by the Lise Meitner Program.

The most significant scientific advances of the project include: (1) elucidation of the mechanisms responsible for short pulse laser induced modification of surface microstructure of metal targets irradiated in the regime of melting and resolidification in vacuum and in the presence of solid or liquid overlayers; (2) establishment of processes responsible for the generation of subsurface voids, incubation effect, and formation of nanoparticles in short pulse laser interactions with bulk metal targets in liquids; and (3) detailed analysis of the conditions leading to the formation of distinct shapes of amorphous regions and generation of crystal defects in short pulse laser processing of silicon. These advances have been achieved through the application of atomistic and continuum-level computer simulation models of laser - materials interactions. Overall, the results of the simulations have provided important insights into the mechanisms of laser-induced modification of metals and silicon, suggested promising irradiation regimes and conditions for experimental exploration, and added to the set of intriguing research questions that will be addressed in future studies. The research results obtained within this project have been reported in five published and two submitted papers, and presented in 13 seminar lectures and conference talks. The project enabled close collaboration between the research groups of Prof. Kautek (University of Vienna) and Prof. Zhigilei (University of Virginia), which already resulted in two joint research papers and several ongoing collaborative projects. Additional collaborations with researchers from Germany, Romania, France, and Lithuania, have also been initiated. The collaborations involve graduate students and have a substantial educational impact.