Delineating the interplay effects of microstructure topology and residual stresses in ultrafast laser irradiated thin films
Advanced nanodevices require high-precision machining of thin films using ultrafast lasers. However, thin-film fabrications cause variations in microstructure, crystallographic orientation, and residual stresses owing to coating conditions and substrate choice. This work investigates the complex interplay between these factors in ultrafast laser-irradiated gold (Au) thin films using a hybrid Two Temperature Model-Molecular Dynamics simulations. We realized microstructure-informed atomistic models with varying grain topologies (randomized vs. equiaxed), grain sizes, and residual tensile/compressive stress configurations. Our results reveal a clear hierarchy of influence on laser-metal interaction: 1.) Microstructure configuration 2.) Topology 3.) Grain Size 4.) Crystallographic orientations. In fine-grained thin films, grain boundaries act as primary melting precursors, while local crystallographic orientation determines the melting extent in coarser grains. Residual tensile stresses contribute to higher melting and greater laser-induced expansion than unstrained films. Conversely, residual compressive stresses resist deformation, as deposited thermal energy is utilized to overcome lattice compression, leading to reduced expansion. We found that microstructure grain topology and size exert a stronger fingerprint on film expansion than the initial defect density.