Plasmon-Exciton Coupling Dynamics in Metal-ZnO Nanostructures

At surfaces, interactions between metal nanostructures and light can generate collective excitations in the form of localized surface plasmons (LSPs) or propagating surface plasmon polaritons (SPPs) that interact with quantum systems such as excitons and donor-acceptor pair (DAP) complexes in an adjacent semiconductor. The interactions of plasmons with excitons and other light-emitting complexes can enhance or reduce quantum efficiency, facilitate electromagnetically induced transparency, and elicit Fanolike resonance response, transient optical magnetism and disorder-induced light localization. Indeed, plasmonic elements are now being used to dramatically enhance photon absorption in fourth generation thin film solar cells, and semiconductor-metal heterostructures are being used to generate surface plasmon amplification by stimulated emission of radiation (SPASERs), Understanding this class of photon interactions with interfaces and thin films is thus significant for both the science and technology of the interactions of light with condensed phases.;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. ZnO has a wide bandgap, a large exciton binding energy, and optical transitions that extend from the near infrared to the UV and thus overlap surface plasmon resonance energies in Ag, Al and Au. Hence, heterostructures incorporating ZnO films or quantum wells and plasmonic elements constitute a unique workbench for probing and manipulating the interactions of surface plasmons and surface-plasmon polaritons with quantum states.;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. These general considerations and the experimental methods used to prepare and analyze the model heterostructures are described in Chapters 1 and 2.;Chapter 3 discusses photoluminescence spectroscopy on multilayer structures of ZnO, MgO, and Ag or Au with varying thicknesses of MgO as a template 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 is discussed. Further, the effect of SPP coupling into Si substrates is presented through comparison of heterostructures on Si and Al2O3.;Chapter 4 provides additional context for these interactions through examination of the Purcell enhancement calculated from bi-exponential fits to the transient reflection spectra of the Ag-MgO-ZnO heterostructures. Simultaneous transmission and reflection pump-probe spectroscopies on samples annealed under varying conditions demonstrate a long-lived Zn interstitial state that demonstrates a narrow photoluminescence emission in the presence of Ag SPPs.;Finally, Chapter 5 presents the results of extinction and photoluminescence experiments based on aluminum nanodisc arrays fabricated on ZnO/Zn0.85 Mg0.15O single quantum wells. The emergence of a Fano resonance in the Al nanodisc extinction spectra indicates the coherent LSP-exciton coupling characteristic of the strong-coupling regime.;While there are a significant number of papers that discuss plasmon-exciton coupling in semiconductor-metal heterostructures, and a growing number of papers that examine plasmon-exciton coupling in ZnO-metal heterostructures, the variable MgO spacer layers introduced in Chapter 3 allowed for a full analysis of the coupling energetics and dynamics as a function of exciton-plasmon separation. Because of this approach, we were able to clearly characterize the coupling energetics and dynamics of the various interactions seen in metal-ZnO heterostructures that had previously been unclearly classified or unreported in the literature: enhancement related to charge transfer processes, Purcell enhancement, and dipole-dipole scattering. Aside from the 'green PL,' ZnO defect-states have been largely ignored in the literature, and studies of the defect-state dynamics are also sorely lacking. The distinction between the differential transmission and reflection pump-probe spectra discussed in Chapter 4 provided a convenient technique for analyzing near-surface defect-states while also recording the dramatic control of the defect-state dynamics by Ag surface plasmon polaritons. Finally, the transition into the strong coupling regime allowed by the nanodesigned heterostructures discussed in Chapter 5 presents the first observation of strong coupling in ZnO-metal heterostructures. Because of the myriad nonlinear optical applications offered by strongly coupled systems, including electromagnetically induced transparency and hybridized plasmon-exciton states, the demonstration of strong coupling in the near- UV is of significant interest for practical applications in technology as well as for basic science.