The Role of Plasmonic Resonance in Enhancement of Interaction of Light and Silver Nanostructures in Fluorescence

Abstract

Plasmonic resonant coupling in noble metallic nanostructures to incident radiation and the related subject of localized behavior of electromagnetic waves are currently of great interest due to their potential application to biosensing, chemical sensing, optical transmission, and nanophotonic devices. In this thesis we present the results of experimental investigations and numerical calculations on nanometer-sized metallic structure enhanced fluorescence (nMEF). These investigations are carried out with silver nanostructures fabricated by two different methods: e-beam lithography (top down) and spray pyrolysis (bottom up). By fabrication of arrays of nanoparticles with varying shape, size, and inter-particle spacing, we determine the dependence of fluorescence enhancement on these critical dimensions and locate the optimum configuration for fluorescence enhancement for a given molecule and excitation wavelength. For practical applications, especially for rapid sensor fabrication, we then repeat the nMEF measurement with size-selected Ag nanoparticles produced with a spray pyrolysis instrument equipped with differential mobility analyzer (DMA), basing our choice of particle size and average spacing (<em>~coverage^(-0.5)</em>) on the findings from our results on electron-beam lithographically patterned structures. To explore the role of particle plasmons in fluorescence enhancement quantitatively we compared the plasmon resonance frequencies for rod-shaped nanoparticles vs. width-to-length aspect ratios, with the shape which produces the optimum enhancement for a particular excitation/fluorescent-emission-peak frequency/polarization combination. In further experiments we measured fluorescence from two dimensional metallic stripes, with systematically-varying widths and spacings, both with and without an underlying noble metallic film. Finally, to probe the effect of the substrate, we compared fluorescence measurements in the absence of metal nanoparticles over oxide-coated silicon substrates and oxide-coated noble-metallic films. In analyzing these results we applied both the discrete dipole approximation (DDA)and finite difference time domain (FDTD)methods, calculating the electromagnetic field response of our arrays to incident light. Comparison between these calculations and experiment provides physical insight as to the mechanisms of nano-metal particle-enhanced fluorescence: our results indicate local field enhancement from silver nanoparticles is crucial in nMEF.