Book Description

This dissertation focuses on the optical properties of single InAs/GaAs quantum dot molecules. A quantum dot molecule consists of a pair of quantum dots coupled by a nanometer scale tunneling barrier. Compared to single quantum dots, quantum dot molecules provide greatly enhanced versatility - with the ease of an electric field control, one gains broad flexibility to tune electronic energy levels and manipulate particle tunneling within a QDM. As a consequence, individual QDMs are being intensely studied as controllable interfaces of charge, spin and photonic quantum states at the single particle level. On the other hand, phonons - the quantized vibrations of the underlying crystal lattice - have mostly been left outside the realm of coherent control. In the domain of solid-state quantum technologies, the ubiquitous phonons are mainly considered for the limitations they impose. Omnipresent electron-phonon interactions and the predominantly dissipative nature of phonons are typically a major source of decoherence of the atom-like quantum states hosted by low-dimensional solid-state structures, such as QDMs. Here, we report experimental and theoretical results on the interactions between charges, photons and phonons in electric field tunable quantum dot molecules. Using an effective mass perturbation model, we compute the low energy biexciton states of a quantum dot molecule and apply them to provide a theoretical description of the dipole-dipole interaction between two excitons occupying separate dots in a quantum dot molecule. The expected properties of these so called dipolar states are presented, and we highlight their potential application as a switch for manipulating the transition energy and tunneling properties of the ground state neutral exciton. We then present results from a comprehensive investigation of the quantum confined Stark effect in quantum dot molecules, in which we studied the electric-field dependent energy shifts of exciton states as a function of the tunneling barrier width. Our experimental and computational results reveal that molecular wavefunction formation in quantum dot molecules strongly affects the quantum confined Stark effect, even as the dots are tuned far from resonance for particle tunneling. This dissertation culminates with our report of a novel mechanism by which phonons are made non-dissipative and coherent via electric field control and the optically driven formation of a molecular polaron in a quantum dot molecule. The coherent interaction of a single optical phonon with individual electronic states is revealed via a Fano-type quantum interference that produces a phonon-induced transparency in the optical absorption of individual quantum dot molecules. Experimentally, we find that the transparency is widely tunable by electronic and optical means, and provides a mechanism for amplifying weak coupling channels. This work is significant in that it demonstrates a specific mechanism by which typically incoherent and dissipative phonons are made to behave in a coherent and non-dissipative manner. As such, we demonstrate that phonons may enter the realm of mutual control of quantum states on the single particle level, which so far has been dominated by photons, electrons and spins.