Growth, spectroscopy, and quantum optics of self-assembled quantum dot molecules

Since their inception, semiconductor quantum dots (QDs) have attracted much attention due to properties which are analogous with individual atoms. In addition to classical applications, novel applications have been identified which exploit the strong confinement of both electrons and holes in self-assembled QDs. This artificial atom analogy can be extended to artificial molecules by coupling two neighboring quantum dots. This single quantum system has been proposed for entangled photon pair emission and two bit or quantum bit gate operations. This thesis focuses on self-assembled InAs/GaAs QD molecules, which have good optical quality and tunable electronic and optical properties.; One significant drawback of the spontaneous nature of self-assembly is random site nucleation. The lateral ordering of epitaxial semiconductor quantum dots is investigated using crystal growth techniques on pre-patterned substrates. Using localized surface chemical potential engineering, the ability to create ordered quantum dot lattices is demonstrated. However, this positioning technique is not precise enough to investigate coupling between neighboring QDs. Therefore, coupling between two vertically stacked QDs is explored. The strain field above the first QD induces a nucleation site for a second QD. The different nucleation conditions naturally yield different optical and electronic properties in the QD pair. By applying an electric field, carrier transfer between two QDs of different confining potentials is measured using micro-photoluminescence. The crystal growth kinetics in each QD were then carefully optimized to independently tune the two QDs ground state transitions to nearly identical energies. Optical spectroscopy of a QD molecule shows that excitons are strongly localized on each QD, therefore minimizing electronic tunneling. However, two types of electrostatic coupling are observed: Coulombic attraction and dipole-dipole interaction. Two-photon emission correlations from the rich spectra exhibit strong antibunching, unambiguously demonstrating the formation of an artificial molecule. Temperature dependent photoluminescence measurements show that directional energy transfer takes place from the high energy QD to the low energy QD. A simple rate-equation model is used to simulate the photon correlation experiment with qualitative agreement.