Computational Investigation of Short Pulse Laser Interaction with Metals

Recent developments of short (femtosecond or picosecond) pulse laser techniques for probing and modifying materials provide an intriguing opportunity to study the material behavior and properties under extreme conditions that can hardly be achieved by any other means. The highly non-equilibrium nature of the transient processes occurring in the laser-irradiated targets, however, hinders the interpretation of the experimental results. In this work, the microscopic mechanisms of the ultrafast structural and phase transformations induced in the irradiated metal targets are investigated in a computational model combining classical molecular dynamic method with a continuum description of the electronic excitation and electron-phonon equilibration. The atomic-level structural rearrangements predicted in the simulations are analyzed through the calculation of the diffraction profiles and density correlation functions, and related to the diffraction spectra observed in time-resolved electron diffraction experiments. Computational investigation of the laser-induced generation of crystal defects in bcc and fcc metal targets suggests that a large number of vacancies can be created in the surface regions of the irradiated targets. Moreover, the emission of partial dislocations from the melting front is observed for Ni, where the interaction among the dislocations propagating along different glide planes leads to the formation of complex dislocation configurations. For Cr, the stacking faults, generated during the initial stage of the relaxation of the laser-induced stresses, disappear shortly after the laser-induced tensile stress wave leaves the surface region of the target. The connections between the electronic structure and the electron temperature dependences of the thermophysical properties, namely the electron-phonon coupling and the electron heat capacity, under strong laser-induced electron-phonon non-equilibrium are established through ab initio electronic structure calculations for several representative metals. As a direct consequence of the thermal excitation of lower d band electrons, the thermophysical properties of noble and transition metals investigated in this study exhibit significant deviations from the commonly accepted descriptions for the range of electron temperatures typically realized in femtosecond laser material processing applications. The comparison of computational predictions with experimental observations demonstrates that the transient modifications of the thermophysical properties have important practical implications for quantitative computational description of laser-materials interactions.

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