Plasmonic Nanowire Photonics: Light Propagation and Field Enhancement

Abstract

For future generation optical components such as couplers, antennas and waveguides, we need nanoscale control and confinement of the electromagnetic fields. Over the years, metallic nanostructures have been studied intensely to confine light beyond the diffraction limit of light. Resonant oscillations of surface charge density in metallic nanostructures with incident laser field offer hugely enhanced and confined field at the surface of metal-dielectric interface. These oscillations are called ‘surface plasmon polaritons’ (SPPs). Of all nanostructures supporting SPPs, silver nanowire (AgNW) gains particular importance owing to the low loss SPP waveguiding leading to potential application as an optical antenna which forms the bedrock for on-chip photonic circuitry. In addition, uniform geometry, ease of synthesis, and wide range of resonance tunability ensure ease of implementation for various nanophotonic applications on any substrate. This thesis focuses on studying the light confinement and propagation in subwavelength plasmonic AgNW. We intensely explore the existence of transverse spin in SPPs propagating in AgNW and show the spin-momentum locking phenomenon due to evanescent nature of SPPs which can add to a more complete description of many effects due to SPPs such as directional coupling, and spin-Hall effect. Further, to overcome losses associated with light propagating in metals, we show that combining plasmonic and dielectric photonic structures can offer interesting opportunities in hybrid nanophotonics. We study subwavelength SPP propagation mediated via whispering gallery modes (WGMs) of dielectric microsphere (μS) for hybrid nanowiremicrosphere (AgNW-μS) system. From the surface of metallic nanostructures, field confinement has been pushed to few nanometers (nm) inside the nanogap between two metallic nanostructures by utilising metallic nanocavities. We study the tightly confined plasmon inside the nanogap supported by Nanowire over mirror (NWoM) geometry. Utilising the cavity plasmons, we probe spatial inhomogeneities of the nanogap which are often inaccessible in experimental characterisation. Furthermore, we observe local morphological changes on the surface of AgNW placed on a mirror forming NWoM geometry. We show that these changes are non-reversible and can be controlled on atomic scales using the optical excitation at room temperature. Presented findings could pave the way towards an atomic scale lithography for plasmonic nanostructures using metallic nanocavities.