Optics at the Nanoscale: Light Emission in Plasmonic Nanocavities

Abstract

Nanophotonics has greatly benefited from the unique ability of surface plasmons to confine optical modes to volumes well below the diffraction limit of light. Plasmonics is an emerging area of research that opens the path for controlling light-matter interactions on the subwavelength scale, enabling truly nanophotonic technologies that are unattainable with conventional diffraction-limited optical components. Novel surface plasmon devices exploit electromagnetic waves confined to the interface between a metal and a dielectric and permit the researcher to shrink light to dimensions previously inaccessible with optics. The extremely high and localized fields in plasmonic nanocavities are finding applications in research areas such as single-molecule sensing, nano-lasers, and photothermal tumor ablation, among others. This thesis explores, both experimentally and theoretically, light emission in a number of plasmonic nanostructures. We present cathodoluminescence imaging spectroscopy as a new method of characterizing surface plasmons on metal films and localized in nanocavity resonators, with experimental observations supported by analytical calculations and electromagnetic simulation. This technique enables extremely localized surface plasmon excitation, a feature we exploit in both planar metal geometries and plasmonic nanocavities. We also study a specific nanocavity geometry, the plasmonic core-shell nanowire resonator, investigating both passive and active semiconductor core materials. This geometry allows precise control of the local density of optical states (LDOS), exhibiting the highest LDOS and smallest mode volumes in structures with dimensions as small as λ/50. Moreover, we discuss the Purcell effect as it applies to plasmonic nanocavities, and calculate enhancements in the radiative decay rate of more than 3000× in the smallest structures. These results demonstrate the promise of plasmonics to enable truly nanophotonic technologies and to manipulate light at the nanoscale.