Nanophotonic split-ring resonators as dichroics for molecular spectroscopy

The unique optical properties of metallic nanostructures have enabled the creation of a new generation of ultra sensitive biosensors based on vibrational spectroscopy. Through strict engineering of structural morphology, a nanometal’s free electrons can be tuned to resonate at a particular frequency, resulting in amplification and confinement of the electromagnetic field around certain areas of the structure. Molecules situated within these areas experience a greater degree of polarisation due to the oscillating plasmon field, a phenomena which, when combined with resonance Raman spectroscopy, has been shown to enable single molecule detection.1, 2 This thesis describes the fabrication and plasmonic characterisation of Au and Ag circular nano split-ring resonators using a combination of electron beam lithography, finite difference time domain simulation and transmission spectroscopy. Through alteration of ring radius, arc length, wall width, metal thickness and metallic composition it is shown that the asymmetric split-ring structures exhibit a multi-modal, polarisation dependent plasmonic response that can be tuned over several microns. Such a response enables these geometries to be employed as novel multi-wavelength biosensors via surface enhanced Raman spectroscopy and surface enhanced resonance Raman spectroscopy. This work goes on to demonstrate that by using electron beam lithography to manipulate the nano-scale geometry of Ag split-ring resonators, their optical properties can be tuned such that the structures exhibit two independently addressable, high frequency plasmon resonance modes for SERRS. In a series of sensing experiments it is shown that this tailored, multi-modal, polarisation dependent activity enables the split-rings to act as discriminating sensors, with each resonance tuned for a particular sensing purpose. Ultimately the structures are used as multi-wavelength, multi-analyte DNA SERRS sensors, with each resonance tuned both to the absorption wavelength of a differently coloured Raman reporter molecule and its corresponding laser excitation wavelength. In doing so, the ability of each resonance to independently sense clinically relevant concentrations of single DNA strand types from within a mixed population on the sensor surface is demonstrated