The lower branch of plasmon-phonon coupled modes

Coherent plasmon-phonon coupled modes in intrinsic bulk GaAs are investigated for excitation energies around the band gap. The frequencies of the coherent terahertz plasmons lie on the lower branch of the plasmon-phonon coupled modes. Their damping times show a strong increase for high excitation densities. Taking advantage of strong absorption saturation during the excitation process, one can adjust the degree of coupling between coherent LO phonons and plasmons by tuning the excitation energy by a few millielectronvolts. Ensemble Monte Carlo simulations confirm the existence of an oscillatory built-in electric field after optical excitation of electron-hole pairs.