Surface Plasmon Polaritons in Gold Nanostructures: Conversion, Coupling, and Confinement

Gold nanostructures have found use in molecular sensing, logic functions, and nanocircuitry through the propagation of surface plasmon polariton (SPP) modes. Despite SPP usefulness, the effects of defects in the supporting substrate, the effects of defects in the nanostructures, and the effects of the geometry of nanostructures on the modes, the mode shapes, and various mode characteristics (such as damping as measured by propagation length) has not been well understood. Other key issues include coupling between nanostructures and the interactions of multiple SPP modes launched simultaneously. This dissertation clarifies some of these fundamental properties as exhibited primarily in gold nanowires through the use of pump-probe spectroscopy and computational modeling using COMSOL Multiphysics. Two primary SPP modes are studied: the bound mode, which propagates at the nanostructure/substrate interface, and the leaky mode, which propagates primarily at the nanostructure/air interface. SPPs are launched using end-fire coupling in which a laser is focused on the end of the nanostructure. When the field of the laser matches that of the plasmon field, SPPs are launched. Often multiple modes are launched simultaneously during an experiment, despite only one mode being of interest in a given experiment. The modes often have different properties and are affected differently by geometry and other factors in the system. Discontinuities in the supporting substrate result in the elimination of the bound mode while the leaky mode is retained. Nanowires that are cut in several places suffer 60 – 80% attenuation when cut widths range from 20 – 100 nm. Counter-intuitively, greater losses at are sustained at smaller gap sizes due to coupling to localized surface plasmon resonances, demonstrating a fundamental loss mechanism. Additionally, it is observed that as the lateral size of the nanowires increases, damping decreases. This is due to the reduced confinement of the mode at larger sizes, resulting in proportionally more of the field outside of the nanowire.