Femtosecond laser-induced modifications in dielectric materials include localized densification, self-organization of sub-micrometer structures and phase transitions,  both on the surface and, thanks to the nonlinear absorption of the photons, in the bulk.
Recently, laser-induced crystallization and intermixing phenomena in the femtosecond regime occurring in layered systems have been reported in both dielectric thin films and multi-layer stacks. This method offers an interesting pathway for inducing localized micron to sub-micron crystallization in confined layers, but also for producing novel material compounds locally through laser-induced inter-mixing processes.
Here, we aim at understanding laser-induced crystallisation dynamics and structures in confined layers, in both thermal and a-thermal regimes. Specifically, we investigate crystallisation processes resulting from the exposure of dielectric multi-layers to femtosecond lasers, both in the a-thermal and thermal regime (in other words, at low and high (>1 MHz) repetition rates) that we compare to continuous-wave (CW) mid-IR laser-induced crystallization. As expected, the different  crystallization dynamics, as evidenced by electron microscopy observations, led to different by-products. Notably, despite higher ionic mobility induced by the elevated temperatures, during CW mid-IR laser exposure elemental intermixing is reduced compared to femtosecond laser exposure conditions, while crystallites are larger and  tend to be directional.