Ultrafast Coherent Phonon Dynamics in GeTe and GeSe: Contrasting Metavalent and Covalent Bonding

Phase‑change materials exhibit pronounced changes in optical and electronic properties between their amorphous and crystalline phases, which are closely related to differences in chemical bonding. Here, we investigate the ultrafast lattice dynamics of GeTe and GeSe thin films using femtosecond pump-probe transient reflectivity combined with Raman spectroscopy and structural characterization. By comparing amorphous and crystalline phases of both materials, we explore how different bonding mechanisms influence coherent phonon generation following optical excitation. Crystalline GeTe exhibits strong coherent phonon oscillations associated with the A₁ optical mode, pronounced fluence‑dependent phonon softening, and very short coherent phonon dephasing times, indicating a highly anharmonic and excitation-sensitive lattice potential. In contrast, amorphous GeTe shows weaker oscillations and only minor fluence dependence. For GeSe, coherent phonons are weak in the crystalline phase and absent in the amorphous phase. Structural relaxation of the amorphous films below the glass transition temperature does not significantly affect the ultrafast response. These observations can be understood in terms of their distinct bonding mechanisms. While GeSe remains covalently bonded in both phases, crystalline GeTe exhibits metavalent bonding that is highly susceptible to electronic excitation. Our findings demonstrate that coherent phonon spectroscopy provides a sensitive probe of bonding in chalcogenide materials.