Gold Nanorod Dimers as Nanoscale Optical Antennas

Abstract

How to control and manipulate light at nanoscale by controlling the geometrical features of metallic nanostructures is the broad question we are trying to address in this thesis. The nanostructures we are interested in are single-crystalline gold nanorod monomers and dimers arranged in side-by-side coupled fashion, end-to-end connected manner and with specific inter-rod angles. Gold was chosen due to its biocompatibility as well as its spectral position in the visible region of the electromagnetic spectrum. Rod structure was considered due to its inherent anisotropy. We have synthesized gold nanorods of an average aspect ratio of 3 (60 nm length and 20 nm diameter) by a bottom-up method which results in single crystalline nanostructures and utilized UV-Vis absorption spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and dark field optical microscopy for characterization. A commercial software ‘Lumerical FDTD (finite-difference time domain) Solutions’ was used to perform electrodynamical simulations from which near-field and far-field characteristics were extracted for individual nanorods and dimers. The role of parameters like gap size, angle between dimers, excitation wavelength and polarization were investigated. Nanorod dimers at specific inter-rod angles exhibited asymmetric Fano-type spectral line profiles which resulted in anomalous near-field and far-field emission properties, when excitation wavelength hit the Fano-dip. Moreover, the far-field response was found to be very sensitive to the angle between nanorods, even though it did not appear so in the scattering spectra. In a nutshell, the potential of gold nanorod dimers as nanoscale optical antennas was demonstrated, which will have applications in designing directional light sources and Raman optical antennas.