Coupling of Coherent Phonons to Excitonic Quantum Beats in GaAs/AlAs Multiple Quantum Wells

摘要

We report on the coupling between the coherent GaAs-like longitudinal optical (LO) phonons and the excitonic quantum beats in GaAs/AlAs multiple quantum wells with different splitting energies of the heavy-hole (HH) and light-hole (LH) excitons. The time-domain signal in each GaAs/AlAs multiple quantum well observed by using a reflection-type pump-probe technique shows a strong coherent oscillation with the fast dephasing time less than 0.5 ps and a weak coherent oscillation with the long dephasing time over 4 ps: The former and later oscillations correspond to the excitonic quantum beat and the coherent GaAs-like LO phonon, respectively. When the splitting energy between the HH and LH excitons becomes close to the energy of the GaAs-like LO phonon, the amplitude of the coherent GaAs-like LO phonon is resonantly enhanced, and the pump-energy dependence of the amplitude of the GaAs-like LO phonon shows the similar profile to that of the excitonic quantum beat. These results indicate that in the multiple quantum wells with the splitting energy almost equal to the LO phonon energy, the excitonic quantum beat acts as a driving force for the coherent GaAs-like LO phonon due to the coupling between the GaAs-like LO phonon and the excitonic quantum beat. The dependence of the frequency of the excitonic quantum beat on the splitting energy exhibits an anticrossing behavior in the vicinity of the resonance energy that the splitting energy becomes equal to the LO phonon energy, which demonstrates the existence of the coupled mode between the GaAs-like LO phonon and the excitonic quantum beat.