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 dielectricinterfaces are known as surface plasmons This kind of light matter interaction in plasmonicnanostructures 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 localizedsource 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 spatiallyredirect the near field emissions Localization of fields is important for obtaining enhancedinteractions with nearby molecules whereas defined directionality is useful toincrease detection efficiency Here in this thesis we will present how the two fundamentalexcitations of surface plasmons i e i localized surface plasmon LSP and ii propagating surface plasmon polaritons SPP can be utilized to obtain field localizationand well defined directional emission at plasmonic nanojunctions which in turn workas optical antennae First we shall present how the LSP mediated field localization at the junction ofa Palladium Pd bridged gold Au nanocylinder dimer can be tuned by varying thegeometrical parameters of the system and how this geometry can be used for efficienthydrogen detection Next we will present how the propagating SPP in silver Ag nanowire NW generateddue to terminal optical excitation of the wire can lead to field localization at aserially coupled Ag NW dimer junction or a NW nanoparticle junction which is spatiallyseparated from the excitation point We will also show that the efficient SPPwaveguiding through the NW results in directional scattering of light at the coupledNW dimer junction Furthermore we will show how these phenomena can be employedto remotely excite molecules placed at the junctions and influence directionalemission from them newline newline