Femtosecond laser-induced ultrafast transient snapshots and crystallization dynamics in phase change material

We report here femtosecond laser-driven transient snapshots of ultrafast crystallization of Ge₂Sb₂Te₅ films from its as-deposited amorphous phase, and the local structural change is validated by micro-Raman spectroscopy and x-ray diffraction. The decay time constant of ∼5  ps in transient spectra with a precise temporal resolution using 400 nm (pump) reveals about 68 volumetric percentage crystallization at a remarkably low fluence of 4.78  mJ·cm^-2. This is attributed to reiterated excitation after a complete carrier relaxation and formation of a long-lasting transient phase at sub-threshold fluences. Furthermore, Raman spectra of irradiated spots confirm defective-octahedral modes at 110 and 160  cm^-1 validating crystallization.