Coupled mode of the coherent optical phonon and excitonic quantum beat in GaAs∕AlAs multiple quantum wells

We report on the time-resolved study of the coherent coupled oscillation of excitonic quantum beats and GaAs-like longitudinal optical (LO) phonons in GaAs∕AlAs multiple quantum wells. The time-domain signals in GaAs∕AlAs multiple quantum wells observed by using a reflection-type pump-probe technique show the coherent coupled oscillation with fast dephasing time and the long-lived coherent GaAs-like LO phonon oscillation. Under the condition that the splitting energy of the heavy-hole and light-hole excitons is close to the energy of the GaAs-like LO phonon, the dependence of the coupled oscillation frequency on the splitting energy exhibits an anticrossing behavior. We discuss the dispersion relation of the coupled oscillation from the viewpoint of the linear coupling of the excitonic quantum beat and the GaAs-like LO phonon through the longitudinal polarization.

Figure 1

Transient reflectivity changes in the GaAs∕AlAs MQW’s observed at $10\mathrm{K}$. The insets show the oscillations magnified in the time region after $1.3\text{ps}$. $\Delta E_{\text{HH}\text{−}\text{LH}}$ indicates the splitting energy of the HH and LH excitons obtained by PLE spectroscopy.

Figure 2

Fourier transformed spectra of the time-domain signals divided into two time regions before and after $1.3\text{ps}$ in Fig. 1. The dashed line represents the frequency of the GaAs-like LO phonon. The arrows indicate the frequency of the quantum beat estimated from the HH-LH splitting energy in each MQW.

Figure 3

Peak frequencies of the coupled mode and GaAs-like LO phonon (open circles and open squares) and the ratio of the amplitude of GaAs-like LO phonon to that of the coupled mode (open triangles), $A_{\text{LO}}∕A_{\mathrm{C}}$, plotted as a function of the HH-LH splitting energy. The solid curves indicate the calculated dispersion relation of the LO phonon–QB coupled mode. The dotted curve indicates the fitted result of $A_{\text{LO}}∕A_{\mathrm{C}}$ on the basis of the forced oscillation model.