Dynamics of doubly resonant Raman scattering and resonant luminescence in ultrathin InAs/GaAs quantum wells

We have studied the transient behavior of a sharp emission line, which emerges close to the heavy-hole (hh) exciton photoluminescence line in a 1.6-monolayer-thick InAs layer embedded in a GaAs matrix. The sharp emission line appears under resonant excitation of the light-hole (lh) exciton transition, whose energy separation from the hh exciton transition amounts to one GaAs LO phonon. We have measured the decay time of the sharp emission line in addition to the lifetime of the hh and the lh excitons, using time-correlated single-photon counting and two-wavelength picosecond time-resolved pump-prove transmission techniques. We find that the decay time of the sharp emission line is equal to the lh exciton lifetime for excitation on resonance but that it strongly decreases for excitation off resonance. We conclude that, whereas the sharp line originates from doubly resonant Raman scattering for off-resonant excitation, it is predominantly due to resonant luminescence for resonant excitation. We derive a lh exciton dephasing time of 40 ps, which suggests that the length scale of potential fluctuations from disorder is larger than the lh exciton Bohr radius.