9.1 INTRODUCTION

In a quantum dot (QD), an electron is localized in all three spatial dimensions. In the case of optically excited QDs, electron-hole pairs (excitons) are excited, and it is the center-of-mass wavefunction of the exciton that is localized. Localization changes the energy spectrum of the exciton from a continuum into a set of discrete levels. As a result, there is a qualitative change in the dynamics of the exciton [1 3]. Localization of the exciton can occur in many ways, and there are a large number of different QD systems actively being studied [4]. However, much of the physics and experimental methodology is general. In this chapter, we focus on GaAs quantum dots defined by interface fluctuations in narrow GaAs quantum wells.

One severe difficulty in the study and application of quantum dots is the large inhomogeneous broadening present in ensembles of quantum dots due to size, shape, and other fluctuations. However, by looking at individual quantum dots, inhomogeneous broadening is eliminated, and it becomes possible to study a wide variety of quantum dot properties that previously were hidden. In the last few years, there has been considerable effort to perform optical spectroscopy on excitons localized in single QDs [5].

The first optical study of a single QD was published in 1992 by Brunner et al. [6], who used microscopic photoluminescence (PL) to study a QD created within a narrow GaAs quantum well. In 1994, there was an explosion of studies published...