Two-stage model and simulation of ultra-short pulsed laser interaction with phase-change-films followed by ultra-fast cooling

In this work, we develop a computational model for ultra-short laser–matter interaction performing detailed simulations of fs pulsed 800 nm laser heating of amorphous Ge2Sb2Te5 (αGST) films, which are at the heart of re-writable optical disks and the latest generation of non-volatile electronic memory. The developed model shows good agreement with related experimental data revealing various effects of ultra-fast dynamics of the electronic subsystem photoexcitation followed by relaxation, ultra-fast cooling, re-amorphization, and partial crystallization. First, our simulations show that additional generation of free electrons by impact ionization leads to a significant decrease of the effective absorption coefficient combined with a significant temperature decrease at the film surface and a simultaneous temperature increase inside the film. Second, our simulations demonstrate the nonlinear dynamics of the dielectric function and related optical parameters. Third, our computations of the post-relaxation ultra-fast cooling dynamics combined with the equations of Arrhenius-type kinetics for crystallization elucidate the thermally controlled mechanism for the experimentally observed generation of the amorphous-crystalline-amorphous nanostructure inside the bulk of the αGST film irradiated by a fs pulsed laser. Additionally, the generation of this nanostructure is associated with an order of magnitude difference between the attempt rates involved in phase transitions (i) in the near-surface melted part and (ii) in the bulk non-melted part of the film. Finally, our study shows that a single fs pulsed laser-induced melting does not lead to the crystallization on the surface of irradiated amorphous and crystalline GST films due to onset of post-relaxation high-rate cooling 1011–1012 K/s.