Ultrafast crystalline-to-amorphous phase transition in Ge ₂Sb₂Te₅ chalcogenide alloy thin film using single-shot imaging spectroscopy

We have observed an irreversible ultrafast crystalline-to-amorphous phase transition in Ge₂Sb₂Te₅ chalcogenide alloy thin film using broadband single-shot imaging spectroscopy. The absorbance change that accompanied the ultrafast amorphization was measured via single-shot detection even for laser fluences above the critical value, where a permanent amorphized mark was formed. The observed rise time to reach the amorphization was found to be ∼130-200 fs, which was in good agreement with the half period of the A₁ phonon frequency in the octahedral GeTe₆ structure. This result strongly suggests that the ultrafast amorphization can be attributed to the rearrangement of Ge atoms from an octahedral structure to a tetrahedral structure. Finally, based on the dependence of the absorbance change on the laser fluence, the stability of the photoinduced amorphous phase is discussed.