Fast Electron Spectroscopy of Enhanced Plasmonic Nanoantenna Resonances
Day, Jared K.
Halas, Naomi J.
Doctor of Philosophy
Surface plasmons are elementary excitations of the collective and coherent oscillations of conductive band electrons coupled with photons at the surface of metals. Surface plasmons of metallic nanostructures can efficiently couple to light making them a new class of optical antennas that can confine and control light at nanometer scale dimensions. Nanoscale optical antennas can be used to enhance the energy transfer between nanoscale systems and freely-propagating radiation. Plasmonic nanoantennas have already been used to enhance single molecule detection, diagnosis and treat cancer, harvest solar energy, to create metamaterials with new optical properties and to enhance photo-chemical reactions. The applications for plasmonic nanoantennas are only limited by the fundamental understanding of their unique optical properties and the rational design of new coupled antenna systems. It is therefore necessary to interrogate and image the local electromagnetic response of nanoantenna systems to establish intuition between near-field coupling dynamics and far-field optical properties. This thesis focuses on the characterization and enhancement of the longitudinal multipolar plasmonic resonances of Au nanorod nanoantennas. To better understand these resonances fast electron spectroscopy is used to both visualize and probe the near- and far-field properties of multipolar resonances of individual nanorods and more complex nanorod systems through cathodoluminescence (CL). CL intensity maps show that coupled nanorod systems enhance and alter nanorod resonances away from ideal resonant behavior creating hybridized longitudinal modes that expand and relax at controllable locations along the nanorod. These measurements show that complex geometries can strengthen and alter the local density of optical states for nanoantenna designs with more functionality and better control of localized electromagnetic fields. Finally, the electron excitations are compared to plane wave optical stimulation both experimentally and through Finite Difference Time Domain simulations to begin to develop a qualitative picture of how the local density of optical states affects the far-field optical scattering properties of plasmonic nanoantennas.
Plasmonic nanoantenna; Cathodoluminescence; Fabry-Pérot plasmonic cavity