Simulation of crystallization dynamics in phase change materials induced by femtosecond laser radiation

In this study, we develop a phenomenological phase-field model of crystallization in phase change materials using Ge2Sb2Te5 as an example. We investigate the phase transition between amorphous and crystalline states, accounting for heating by a femtosecond laser with different radiation intensities and a Gaussian intensity profile. The relaxation constant of the order parameter is considered to be inversely proportional to the viscosity, having strong temperature dependence responsible for the formation of a non-equilibrium glassy state. The obtained crystalline structures exhibit a heterogeneous distribution throughout the material bulk, forming continuous and discontinuous parts, which are observed in the center and at the edge of the heated region, respectively. The direct electrodynamic calculations show that the region of continuous crystallized layer reflects electromagnetic waves better than a discontinuous one, which can be observed as bright and pale spots under visible light illumination, respectively. The increase in femtosecond laser beam intensity causes an increase in crystallized region size and crystallization depth of the sample, as well as the size of spots observed in the reflected light, which agrees with experimental data obtained in other studies.