Imaging Spectroscopy of Two-Dimensional Excitons in a Narrow GaAs/AlGaAs Quantum Well

We report the observation of a sharp absorption line in the photoluminescence excitation spectra of individual naturally occurring quantum dots in a narrow (2.8 nm) $\mathrm{GaAs}/\mathrm{Al}{}_{0.3}\mathrm{Ga}{}_{0.7}\mathrm{As}$ quantum well that is remarkably constant throughout the sample. We propose that it is identified as the delocalized two-dimensional exciton. This assignment is confirmed by photoluminescence and photoluminescence excitation diffusion experiments.