Well-width dependence of light-hole exciton dephasing in GaAs quantum wells

We investigate coherent emission from light-hole ground-state excitons (lh1 excitons) in 8 nm and 17.5 nm GaAs quantum wells (QWs) in transient degenerate four-wave mixing (DFNM) experiments. The lh1 excitons are resonantly excited using femtosecond laser pulses. The lh1-exciton energy in the 8 nm QW lies within the continuum of electron-heavy-hole pair states in the first subbands, but that in the 17.5 nm QW is discrete (nonresonant with the continuum). The DFWM signal decay and DFNM spectra show different behavior in the two cases. In particular, at vanishing excitation densities, the DFWM signal decay rate for lh1 excitons in the 8 nm QWs is found to be several times larger than the corresponding rate for the 17.5 nm QWs. The results suggest that the population lifetime of the resonant lh1 exciton may be very short (<0.4 ps).