Damped oscillations of exciton wave packets in the layered semiconductor GaSe observed by four-wave mixing spectroscopy

We have observed damped oscillations of the exciton wave packet through spectrally resolved four-wave mixing measurements in a layered semiconductor GaSe, and demonstrated that the four-wave mixing spectroscopy is a powerful tool to observe the wave-packet motion. The wave-packet motion is explained theoretically in terms of the configuration coordinate model, where the adiabatic potential curve is shifted due to the exciton-phonon interaction. We have found that the localization of excitons in a few layers enhances the coupling to acoustic phonons.