Effect of diffusion on acoustic deformation potential characterization through coherent acoustic phonon dynamics

Ultrafast spectroscopy of coherent acoustic phonon (CAP) dynamics has recently been proposed as a method to characterize acoustic deformation potential (ADP), a key standard to quantify carrier--acoustic phonon coupling in semiconductors. In this Letter, we illustrate the importance of addressing the diffusion effect in ADP characterization by this method, using Ge as the demonstration system. It is found that the ADP mechanism and the thermoelastic effect have comparable contributions to CAP generation in Ge. Due to the different dependences on pump photon energies, the roles of these two mechanisms were assessed by varying pump wavelengths, based on which the ADP coupling constant of Ge was obtained. The analysis reveals that the carrier diffusion has a considerable impact on the shape of the CAP wave packet and must be processed cautiously for the ADP characterization for Ge.