UC Merced

Quantum dots (QDs) are semiconductor nanoparticles that trap electrons and holes in all three dimensions, resulting in discrete energy levels with strong optical transitions. InAs/GaAs QDs are grown by molecular beam epitaxy of lattice-mismatched InAs on a GaAs substrate, resulting in strain-induced island formation on a two-dimensional wetting layer. In addition to optoelectronic applications such as lasing, infrared detection, and photovoltaics, QDs are capable of hosting optically-controlled spin qubits and emitting photonic qubits for quantum communication and quantum computation.

This dissertation focuses on InAs/GaAs coupled quantum dot pairs (CQDs) formed by strain-induced alignment of QDs in nearby layers, resulting in interdot charge tunneling that can be controlled with an applied electric field. We use a combination of theoretical modeling and optical spectroscopy to understand dynamical processes of bound photoexcited charges, aiming to enhance their usefulness for quantum information and sensing technologies and help overcome difficulties preventing their implementation.

We develop a model of electron and hole confinement in CQDs, including Coulomb and spin interactions, phonon coupling, and optical transitions. This model is used to simulate relaxation dynamics during neutral molecular biexciton cascades, identifying parameter regimes where two-photon polarization entanglement can be expected. While this process has been demonstrated in single QDs, we find that charge separation in interdot states of CQDs allows for tunable emission energies and a higher tolerance to anisotropic electron-hole exchange splitting.

Using low-temperature optical photoluminescence spectrosopy, we identify charge and spin states in single CQDs and investigate their interactions. Two-laser photoluminescence excitation spectroscopy demonstrates two-photon excitation into the molecular biexciton state via a stepwise process, while calculations identify conditions required for efficient simultaneous two-photon absorption. Further investigations find decoherence by electric field fluctuations from charged lattice defects, and identify a novel enhancement of acoustic phonon coupling at hole tunneling resonances from piezoelectric interactions.