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Phase change alloys have seen widespread use from rewritable optical discs tothe present day interest in their use in emerging neuromorphic computingarchitectures. In spite of this enormous commercial interest, the physics ofcarriers in these materials is still not fully understood. Here, we describethe time and space dependence of the coupling between photoexcited carriers andthe lattice in both the amorphous and crystalline states of one phase changematerial, GeTe. We study this using a time-resolved optical technique calledpicosecond acoustic method to investigate the \textit{in situ} thermallyassisted amorphous to crystalline phase transformation in GeTe. Our workreveals a clear evolution of the electron-phonon coupling during the phasetransformation as the spectra of photoexcited acoustic phonons in the amorphous($a$-GeTe) and crystalline ($\alpha$-GeTe) phases are different. In particularand surprisingly, our analysis of the photoinduced acoustic pulse duration incrystalline GeTe suggests that a part of the energy deposited during thephotoexcitation process takes place over a distance that clearly exceeds thatdefined by the pump light skin depth. In the opposite, the latticephotoexcitation process remains localized within that skin depth in theamorphous state. We then demonstrate that this is due to supersonic diffusionof photoexcited electron-hole plasma in the crystalline state. Consequentlythese findings prove the existence of a non-thermal transport of energy whichis much faster than lattice heat diffusion.

Non-thermal transport of energy driven by photoexcited carriers in switchable solid states of GeTe