Control of the microstructure of thin multilayer systems by ultrashort pulsed laser irradiation - process understanding by complementary in situ and ex situ characterizations and multiscale simulations

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Laser-assisted techniques belong to state of the art in the materials processing, and are widely used for laser ablation and structuring of materials in many fields of applications. The processes accompanying the laser ablation are typically simulated using hydrodynamic approaches in combination with molecular dynamics. On the nanometer scale, however, the application of high-intensity laser radiation leads additionally to the modifications of the materials microstructure, which are related to local melting, solidification, interdiffusion of involved species, formation of microstructure defects and recrystallization. These processes could be used for a targeted manipulation of the materials microstructure, but because the microstructure changes that are induced by the laser irradiation also affect the interaction between the laser beam and the material, the adjustment of a desired microstructure on the nanoscale is a very complex task. As a holistic model describing the effect of the microstructure on the interaction between the laser beam and the material is still missing, the adjustment of the parameters of the laser process is currently carried out using a trial-and-error approach in many cases.The aim of this project is to contribute to the understanding of laser-induced changes in the microstructure of thin metallic films and to the description of the effect of microstructure on the materials characteristics, which are relevant for the laser processes, e.g., absorption of the laser beam, electron-phonon coupling, heat transfer, etc. This aim should be achieved by combining in situ (ultrafast ellipsometry and reflectometry during the laser irradiation) and ex situ experiments (scanning and transmission electron microscopy, X-ray and electron spectroscopy) with simulations using mesoscopic (hydrodynamics) and microscopic (molecular dynamics) approaches. The evaluation of the electron micrographs will be supported by a multimodal analysis based on deep learning. The information obtained from the molecular dynamic simulations will complement the ex situ microstructure studies by providing, e.g., the atomic positions for the in situ microstructure analyses during the laser irradiation.The materials proposed for this study are single layers (Cr, Mo, Ti, Fe) and bilayers (Au/Cr, Mo/Ti, Au/Fe) consisting of unary metallic phases with different melting points, different sequences of high-temperature and high-pressure phases, and with different mutual solubilities and diffusivities in the respective binary system. Experimentally observed phase transitions and concentration profiles will be used as “sensors” for the temperatures and pressures induced by the laser irradiation. The effect of the microstructure on the laser process will be studied in samples having different grain size and preferred orientation of crystallites in the original state.