DENSITY-MATRIX THEORY OF COHERENT PHONON OSCILLATIONS IN GERMANIUM

The generation and detection processes of coherent phonon oscillations in germanium are described within an extended density-matrix model. In the relevant hierarchy of equations of motion for the generation of the phonon oscillation, the anisotropy of the hole distributions related to the anisotropy of the interband dipole matrix elements is identified as the driving force of the coherent vibration. The optical detection of coherent phonons in reflectivity is based on anisotropic band-gap modulations due to the coherent displacements, weighted with the deformation potential of the valence bands.