Coupling of coherent longitudinal optical phonons to excitonic quantum beats in GaAs/AlAs multiple quantum wells

We have investigated the dynamical properties of the coherent oscillations due to the GaAs-like longitudinal optical (LO) phonon and the quantum beat of excitons in two samples of GaAs/AlAs multiple quantum wells with different splitting energies of the heavy-hole and light-hole excitons. The coherent oscillations were measured with a reflection-type pump-probe technique. In the multiple quantum well with the splitting energy almost equal to the energy of the GaAs-like LO phonon, the intensity of the coherent LO phonon are markedly enhanced in comparison with that of the other sample, and the pump-energy dependence of the intensity of the coherent GaAs-like LO phonon shows similar profile to that of the excitonic quantum beat. Moreover, the intensity of the coherent GaAs-like LO phonon is linearly increased with an increase in pump power, which is contrary to a usual generation mechanism, i.e., the displacive excitation of coherent phonon leading to a square dependence. These facts indicate that the excitonic quantum beat acts as a driving force for the coherent GaAs-like LO phonon in the case that the splitting energy of the heavy-hole and light-hole excitons is almost equal to the LO phonon energy.