Generation and relaxation of coherent majority plasmons

We report on the time-resolved study of the dynamics of coherent plasmon-phonon modes in n-doped GaAs at different doping levels. The measurements show that the initially thermalized background majority plasma participates in the coherent plasmon-phonon oscillation and that the observed frequency is determined by the total electron density including both the background doping and the optical excitation. The dephasing of the coupled plasmon-phonon oscillation is found to be slower (i) as the relative contribution of the optical excitation to the total carrier density decreases and (ii) as the frequency approaches that of the transverse-optical phonon resonance.