Localized Optical fields and directional Far-field emissions from Plasmonic Nanojunctions

Abstract

The coupled oscillations of free electrons and the optical waves at the metal-dielectric interfaces are known as surface plasmons. This kind of light-matter interaction in plasmonic nanostructures has made it possible to realize the concept of optical antennae. These are the devices which can work as a transducer between free radiation and localized source. An optical antenna can be characterized by two main properties: a) ability to localize optical fields to sub-wavelength scales and b) the ability to spatially redirect the near-field emissions. Localization of fields is important for obtaining enhanced interactions with nearby molecules; whereas, defined directionality is useful to increase detection efficiency. Here in this thesis, we will present how the two fundamental excitations of surface plasmons i.e., (i) localized surface plasmon (LSP) and (ii) propagating surface plasmon polaritons (SPP) can be utilized to obtain field localization and well defined directional emission at plasmonic nanojunctions, which in turn work as optical antennae. First, we shall present how the LSP mediated field localization at the junction of a Palladium (Pd) bridged gold (Au) nanocylinder dimer can be tuned by varying the geometrical parameters of the system and how this geometry can be used for efficient hydrogen detection. Next, we will present how the propagating SPP in silver (Ag) nanowire (NW), generated due to terminal optical excitation of the wire; can lead to field localization at a serially-coupled Ag NW- dimer junction or a NW-nanoparticle junction which is spatially separated from the excitation point. We will also show that the efficient SPP waveguiding through the NW results in directional scattering of light at the coupled NW dimer junction. Furthermore, we will show how these phenomena can be employed to remotely excite molecules placed at the junctions and influence directional emission from them.