Plasmonic nano-antenna arrays for surface enhanced Raman spectroscopy and other applications

• Main Author: Doherty, Matthew David
• Published: Queen's University Belfast 2013
• Subjects: 539.2
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.601361

On sub-wavelength scales, photon-matter interactions are limited by diffraction. Electromagnetic radiation propagating in free space - or far-field radiation - can be coupled into the surface plasmonpolaritons of nanostructured metallic surfaces in order to overcome this limitation. The distribution of electromagnetic energy in the near-field of these structures can be controlled by altering their geometry, dielectric environment and composition. Hence, surface plasmon polaritons allow electromagnetic radiation to be effectively utilized and controlled on the nanoscale. In this thesis a detailed study of the complex relationship between the electromagnetic near-field and far-field responses of 'real' nanostructured metallic surfaces is presented. The near-field and far-field responses are specified in terms of surface enhanced Raman scattering enhancement factor (SERS EF) spectra and optical extinction respectively. First, it is shown that in the far-field gold nanorod and nanotube array substrates exhibit two distinct localized surface plasmon-polariton resonances (LSPRs): a longitudinal and transverse mode. These modes are demonstrated both experimentally and theoretically, and a potential application of gold nanorod substrates as ultrathin absorbers is outlined. The near-field properties of these arrays are then studied, revealing the existence of a third type of LSPR: the cavity mode. The presence of this mode is confirmed using a combination of SERS EF spectra, electron microscopy and electromagnetic modelling. The cavity mode simultaneously has the largest impact on the near-field behaviour (as observed through the SERS EF) and the weakest optical interaction: it has a "near-field type" character. Conversely, the transverse and longitudinal modes have a significant impact on the far-field behaviour, but very little impact on SERS: they have a "far-field type" character. Based on this understanding of the contrasting character of the three LSPRs there follows a clear illustration and explanation of the non-correlation between the SERS EF spectra and the optical response, and some key consequences of this are described and demonstrated