Effects of few-particle interaction on the atomiclike levels of a single strain-induced quantum dot

Abstract

We investigate the effects of few-particle population of a single strain-induced quantum dot by optical excitation. The low-power photoluminescence spectra consist of sharp lines with energy separation of a few meV, associated to the formation of excitonic molecules in the single dot. With increasing photoexcitation intensity, the population of higher states is observed; however, we also observe a clear intensity dependence of the transition energies, inconsistent with a simple filling of exciton levels. Based on a theoretical model for interacting electron-hole pairs in the dot, we obtain good agreement with experiment and show that excitonexciton interactions control the spectral changes as the number of pairs is increased. The realization of high-quality semiconductor quantum dots has recently lead to the demonstration of artificial atoms, in which addition energies 1 and Zeeman splitting 2 associated with three-dimensionally confined electron states mimic the characteristics of natural atoms. Similarly, the optical excitation of a single quantum dot ~QD! results in optical spectra reminiscent of atomic transitions with sharp and spectrally narrow lines. 3‐7 The understanding of the complex optical spectra from single dots and their dependence on photoexcitation intensity is, however, still quite poor. The atomiclike lines in the spectra were found to exhibit intriguing red- and blue-shifts, and new features are seen to appear and disappear depending on the number of injected electronhole pairs. 8‐10 Such a complicated dynamics originates from the Coulomb correlation among pairs of carriers ~excitons! confined in the dot, which is often neglected in the analysis of these experiments. In fact, we expect that whenever an electron-hole ~e-h! pair is added to the dot, the energy states must change because of the resulting additional Coulomb interactions. In this paper, we report a complete study of the single-dot spectra, both experimental and theoretical, which elucidates the atomiclike nature of the optical transitions and the important role of Coulomb correlations in an isolated quantum dot. As a prototype nanostructurewe have chosen a single strain-induced quantum dot physically isolated from the surrounding environment in a nanomesa defined by highresolution lithography. This system has never been investigated by local spectroscopy so far, but for an intriguing nearfield luminescence experiment in which no atomiclike