The lower branch of plasmon-phonon coupled modes

Coherent plasmon-phonon coupled modes in intrinsic bulk GaAs are investigat ed for excitation energies around the band gap. The frequencies of the cohe rent terahertz plasmons lie on the lower branch of the plasmon-phonon coupl ed modes. Their damping times show a strong increase for high excitation de nsities. Taking advantage of strong absorption saturation during the excita tion process, one can adjust the degree of coupling between coherent LO pho nons and plasmons by tuning the excitation energy by a few millielectronvol ts. Ensemble Monte Carlo simulations confirm the existence of an oscillator y built-in electric field after optical excitation of electron-hole pairs.