Gabija Kiršanskė – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Research > PhD theses 2016 > 2016 > Gabija Kiršanskė

 

Gabija KiršanskėGabija Kiršanskė

A thesis submitted February 2016 for the degree of Doctor of Philosophy and defended April 20, 2016.

The PhD School of Science
Faculty of Science
Niels Bohr Institute, Quantum Photonics, University of Copenhagen

Supervisors:
Peter Lodahl
Søren Stobbe

 

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Summary

Electrical control of excitons in semiconductor nanostructures: from quantum dots in photonic-crystal devices to the exciton Mott transition in coupled quantum wells

The scope of this thesis covers investigation of the exciton Mott transition in coupled quantum wells, fabrication of photonic-crystal structures with embedded self-assembled quantum dots, and tuning of their properties by means of an external electric field. In the first part of the thesis the focus is on quantum dots in photonic nanostructures. The fabrication process of reproducible high-quality photonic-crystal structures on electrically gated GaAs samples is presented. This process is employed to investigate light localization in short photonic-crystal waveguides with a dispersion relation facilitating a slow-light effect. The effect of the variations in the local density of optical states on electrically tuned quantum dots embedded in photonic structures is investigated. An electric field is employed to induce strain in suspended GaAs structures, where a bidirectional spectral shift of the embedded quantum dots is observed. It is suggested that the spatial distribution of strain can be engineered by designing the sample geometry, which could have potential applications in quantum photonics.

The second part of the thesis concerns the exciton Mott transition, which is a phase transition occurring in a population of interacting electrons and holes, in which insulating excitons are ionized to a metallic phase of free carriers. It is still debated in the literature whether the Mott transition in quantum wells occurs gradually or abruptly as a function of the governing parameters — exciton density and temperature. In this work, the Mott transition is studied with indirect excitons with electrically extended radiative lifetime and is found to occur gradually as a function of exciton density and temperature. The exciton-density-temperature phase diagram of the transition exposes two regions with exciton-dominant and plasma-dominant populations that are separated by a linear boundary.

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