Study on light transmission in metallic nanostructures through charge redistribution mechanism

Abstract

This dissertation studies on light transmission through metallic nanostructures using induced charge distribution mechanism. When light passes through metallic nanostructures, charges in a metallic nanostructure are induced and relocated with regard to the shape, dimension, and incident polarization of state. Induced charge redistribution is mainly investigated because it directly correlates to the electric field distribution and surface current flow at a metal nanostructure. Localized surface plasmons (LSP) at a metal nanostructure are observed in an optical regime, which mean the (confined) collective electron excitations at the interface between metallic nanostructure and surrounding dielectric. Fumbling induced charge distribution at a metal nanostructure in optical regime can lead to manipulation of the optical properties such as enhanced transmission, concentration and confinement. In case of coupled metallic nanorods where they produce localized electromagnetic field strength at the nanogap nestled between coupled metallic nanorods

enhanced local fields originate from local capacitive coupling. It results from the accumulated charges at the edge of each nanorod. Derived from this structure, plasmonic faced folded metallic nanorods are proposed by the folding and facing of metallic nanorods. Its physical dimension is designed on the basis of . This proposed structure presents unique induced charge redistribution models with regard to incident polarization state of light via numerical calculations. Furthermore, distinct surface current flow, called semi-circular current flow, is introduced, resulting from induced charge redistribution. Assuming that semi-circular current flow contributes to the optical properties of near- and far-field transmission, transmission characteristics passing through faced folded metallic nanorods are experimentally observed using three-dimensional holographic microscopy. This proposed structure is numerically and experimentally demonstrated. Novel route of surface current in a metallic nanostructure leads to the significant increase of degree of field intensity. It is verified with the numerical calculations based on finite difference time domain (FDTD) method, presenting the comparisons with simplified geometries. Whole beam path emanating from the proposed structure can be observed via three-dimensional holographic microscopy. Light intensity passing through the proposed structure is enhanced and more concentrated than light passing through the bare aperture, as a reference. As a result, we establish that faced folded metallic nanorods can be functionalized as a compact light concentrator accompanying with field enhancement. In addition, a current issue in plasmonics can be resolved by the use of metallic nanostructures. A metallic nanoslit suffers from weak transmission due to the diffraction limit. To overcome this limit, metallic nanoislands are embedded inside a metallic nanoslit. It is expected that embedded metallic nanoislands act as a platform to instigate plasmonic mode. Plasmonic modes in a metallic nanoslit with the embedded metallic nanoislands are numerically observed with regard to the incident polarization state of light. It can be attributed to enhancing the intensity of transmitted light passing through a metallic nanoslit in spite of diffraction limit. Additionally, embedded metallic nanoislands can play a critical role to adjust the location of resonant wavelength in its spectrum. As a consequence, transmission enhancement passing through a metallic nanoslit with the embedded metallic nanoislands can be experimentally demonstrated.