Out of the Dark and into the Light - Microscopic Analysis of Bright, Dark and Trapped Excitons

Licentiate thesis, 2018

Atomically thin transition metal dichalcogenides (TMDs) have been in the focus of current research due to their efficient light-matter interaction, as well as the remarkably strong Coulomb interaction that leads to tightly bound excitons. Due to their unique band structure, TMDs show a variety of optically accessible bright and inaccessible dark excitons. Moreover, due to their optimal surface-to-volume ratio, these materials are very sensitive to changes in their surroundings, which opens up the possibility of externally tailoring their optical properties.

The aim of this thesis is to present different strategies to control the optical fingerprint of TMD monolayers via molecules, strain and impurities. Based on a fully quantum-mechanical approach, we show that the coupling of excitons to high-dipole molecules can activate dark excitonic states, resulting in an additional and well-pronounced peak in the optical spectra. 

Moreover, we find that these dark excitonic states are very sensitive to strain, leading to crucial energy shifts and intensity changes of the dark exciton signature. Our findings reveal the potential for optical sensing of strain through activation of dark excitons. 

Finally, we investigate the possibility of local impurities to trap excitons resulting in localized states.
We study the formation, excitonic binding energies and wave functions of localized excitonic states, all of which depend on the trapping potential. With this, we are able to calculate the photoluminescence signal and investigate the possibility of single-photon emission.