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

TL;DR

This study reveals that in GeTe, photoexcited carriers can transfer energy non-thermally over distances exceeding the optical skin depth, especially in the crystalline phase, due to supersonic electron-hole plasma diffusion, affecting phase change dynamics.

Contribution

It demonstrates the existence of non-thermal, rapid energy transport driven by photoexcited carriers in GeTe, highlighting differences between amorphous and crystalline states.

Findings

Photoexcited carrier energy transfer exceeds optical skin depth in crystalline GeTe.

Supersonic diffusion of electron-hole plasma occurs in crystalline GeTe.

Non-thermal energy transport is faster than lattice heat diffusion.

Abstract

Phase change alloys have seen widespread use from rewritable optical discs to the present day interest in their use in emerging neuromorphic computing architectures. In spite of this enormous commercial interest, the physics of carriers in these materials is still not fully understood. Here, we describe the time and space dependence of the coupling between photoexcited carriers and the lattice in both the amorphous and crystalline states of one phase change material, GeTe. We study this using a time-resolved optical technique called picosecond acoustic method to investigate the \textit{in situ} thermally assisted amorphous to crystalline phase transformation in GeTe. Our work reveals a clear evolution of the electron-phonon coupling during the phase transformation as the spectra of photoexcited acoustic phonons in the amorphous ($a$-GeTe) and crystalline ($\alpha$-GeTe) phases are different. In particular and surprisingly, our analysis of the photoinduced acoustic pulse duration in crystalline GeTe suggests that a part of the energy deposited during the photoexcitation process takes place over a distance that clearly exceeds that defined by the pump light skin depth. In the opposite, the lattice photoexcitation process remains localized within that skin depth in the amorphous state. We then demonstrate that this is due to supersonic diffusion of photoexcited electron-hole plasma in the crystalline state. Consequently these findings prove the existence of a non-thermal transport of energy which is much faster than lattice heat diffusion.