Plasmon-Exciton Coupling Dynamics in Metal-ZnO Nanostructures

Abstract

Zinc oxide films or quantum wells and plasmonic elements, comprising rough metal films and nano-cylinder arrays of Ag, Al or Au, constitute an especially interesting model system for studying plasmon-exciton interactions. This dissertation focuses on the energetics, dynamics and control of the coupling between band-edge excitons and luminescent defect complexes in ZnO thin films and quantum wells, on the one hand, and localized or propagating plasmons in metallic films and nanostructures on the other. Multilayer structures of ZnO, MgO, and Ag or Au with varying thicknesses of MgO provide a workbench for analyzing interactions as a function of plasmon-emitter separation. In particular, the coupling of Ag and Au SPPs to excitons via Purcell-like interactions, and the dipole-dipole scattering of Ag and Au LSPs with ZnO DAPs were analyzed via photoluminescence and pump probe spectroscopy. Simultaneous transmission and reflection pump-probe spectroscopy performed on samples annealed under varying conditions provided an understanding of the ZnO defect dynamics, and demonstrated the dramatic Purcell enhancement of a long-lived Zn interstitial defect state. The selective decay rate enhancement of individual quantum emitters by tunable surface plasmon resonances should make available emitters currently too inefficient to be commercially practical. Aluminum nanodisc arrays deposited on ZnO/Zn0.85Mg0.15O single quantum wells provided a flexible template for the investigation of LSP-exciton coupling. By optimizing the LSP resonance and the QW emission, heterostructures were fabricated that demonstrated a hybridized Al LSP quadrupole – ZnO QW exciton state in the confocal extinction spectra, a strong coupling phenomenon that provides the foundation for the fabrication of nano-designed heterostructures with tunable dielectric functions throughout the near to mid ultraviolet spectrum.