Abstract
Short pulse laser irradiation has the ability to bring material into a state of strong electronic, thermal, phase, and mechanical non-equilibrium and trigger a sequence of structural transformations leading to the generation of complex multi-scale surface morphologies, unusual metastable phases and microstructure that cannot be produced by any other means. High resolution and accuracy of laser-induced modification of material properties makes this technique indispensable for practical applications; however, better theoretical understanding of fundamentals of laser interaction with matter, non-equilibrium processes caused by ultrafast energy deposition, generation of crystal defects, and modification of surface morphology under highly non-equilibrium conditions created by the laser irradiation is vital for further advancement of laser-based processing methods.
In this work, a combination of large-scale atomistic simulations and theoretical analysis is used to reveal the mechanisms responsible for the laser-induced generation of crystal defects and the sensitivity of the structural modifications to the target crystallographic orientation, which is reported in experimental studies. The peculiarities of melting and solidification occurring under conditions of strong superheating/undercooling as well as the relaxation of intense laser-induced thermal stresses are found to result in emission of dislocations from the melting front, formation of dislocations during crystal growth, generation of growth twins and high concentration of vacancies. The effect of the choice of interatomic potential on the results of atomistic simulations of short pulse laser processing is illustrated by considering high-temperature properties of Ti predicted by nine potentials suggested in literature for this metal. Spatial confinement by a solid transparent overlayer in short pulse laser processing is revealed to suppress of the generation of unloading tensile waves in the irradiated target, leading to suppression of photomechanical spallation and cavitation, decrease of the maximum melting depth, and reduction or elimination of the emission of dislocations from the melting front. The stabilizing effect of the overlayer prevents complete melting near the interface and facilitates formation of a thin layer of slightly misoriented grains. Apart from the microstructure modification, large-scale atomistic simulations have provided insights into the mechanisms of laser-induced sub-surface cavitation and photomechanical spallation, and suggested a novel mechanism of the generation of Laser-Induced Periodic Surface Structures (LIPSS) in the phase explosion regime.
The relaxation of laser-induced stresses on the scale of entire laser spot is accompanied not only by generation of crystal defects and ablation but also by formation of intense bulk and surface acoustic waves (SAWs) that are shown in recent experimental studies to affect surface processes at atomic/molecular level. This observation is surprising since there is a large mismatch between the typical SAWs frequency and characteristic vibrational modes of atoms and molecules on a surface. A plausible explanation of this phenomenon is suggested by the results of our study of acoustically activated surface diffusion and desorption of small clusters. We have found that nonlinear wave evolution accompanied by generation of high frequency harmonics capable of direct coupling with vibrational modes of surface species is largely responsible for the acoustic activation of surface diffusion and desorption of atomic clusters. Moreover, atomistic simulations performed for graphene on a catalytic Cu substrate demonstrate and explain an acoustically-activated motion of graphene sheets on the substrate.
