Power- and polarization-dependences of coherent phonons of tetrahedrally bonded semiconductors

Generation of coherent optical phonons in GaAs and Ge are systematically investigated by pump-probe reflectivity measureents. While the amplitude of the coherent optical phonon of Ge increases linearly with increasing optical excitation density, that of the coherent LO phonon of GaAs shows a saturation above a certain optical density. The coherent optical phonon of Ge follows the Raman selection rule, while that of GaAs is generated isotropically regardless of pump polarization. These differences are explained by different generation mechanisms of coherent phonons; impulsive stimulated Raman scattering and transient depletion field screening.