Coherent Phonons in Epitaxial Thin Films of Phase‐Change Material GeTe

This study aims to characterize and control coherent phonons in thin GeTe films, a material which has significant potential in advancing optoelectronic technologies. Using femtosecond optical pump‐probe spectroscopy, the influence of film thickness, excitation fluence, and excitation polarization on Raman‐active phonon modes is investigated. Upon reducing the film thickness from bulk‐like to less than 5 nm, the fluence‐induced softening of the A 1g mode is enhanced by an order of magnitude. In addition, the time‐domain measurement of the E g phonon mode in GeTe is reported. By changing the excitation polarization and using double‐pulse excitation techniques, these coherent phonons can be enhanced or suppressed as desired. These findings help to develop ultrafast optoelectronic devices and enable novel applications of phase‐change materials.