Multiple quantum beats of a system of the coherently excited quantum confined exciton states in a high-quality heterostructure with a wide InGaAs/GaAs quantum well are experimentally detected by the spectrally resolved pump-probe method for the first
time. The beat signal is observed as at positive as at negative delays between the pump and probe pulses. A theoretical model is developed, which allows one to attribute the QBs at negative delay to the four-wave mixing (FWM) signal detected at the non-standard direction. The beat signal is strongly enhanced by the interference of the FWM wave with the polarization created by the probe pulse. At positive delay, the QBs are due to the mutual interference of the quantum confined exciton states. Several QB frequencies are observed in the experiments, which coincide with the interlevel spacings in the exciton system. The decay time for QBs is of order of several picoseconds at both the positive and negative delays. They are close to the relaxation time of exciton population that allows one to consider the exciton depopulation as the main mechanism of the coherence relaxation in the system under study.
Photoluminescence (PL) and reflectivity spectra of a high-quality InGaAs/GaAs quantum well structure reveal a series of ultra-narrow peaks attributed to the quantum confined exciton states. The intensity of these peaks decreases as a function of temp
erature, while the linewidths demonstrate a complex and peculiar behavior. At low pumping the widths of all peaks remain quite narrow ($< 0.1$ meV) in the whole temperature range studied, $4 - 30K$. At the stronger pumping, the linewidth first increases and than drops down with the temperature rise. Pump-probe experiments show two characteristic time scales in the exciton decay, $< 10$ps and $15 - 45ns$, respectively. We interpret all these data by an interplay between the exciton recombination within the light cone, the exciton relaxation from a non-radiative reservoir to the light cone, and the thermal dissociation of the non-radiative excitons. The broadening of the low energy exciton lines is governed by the radiative recombination and scattering with reservoir excitons while for the higher energy states the linewidths are also dependent on the acoustic phonon relaxation processes.
