Subpicosecond time-resolved coherent-phonon oscillations in GaAs.

Coherent LO phonons are excited at the surface of bulk GaAs with femtosecond laser pulses. They are observed for the first time through electro-optic modulations of the transient reflectivity. The corresponding signal oscillations are superimposed upon an additional longitudinal polarization feature which decays exponentially and whose rise time is sufficiently short to act as a driving force for the coherent phonons. The components of the coherent phonon states dephase relative to each other in the order of picoseconds depending on the density of optically excited electron-hole pairs.