Investigating electromagnetic field enhancements from gold nanostructured arrays for plasmon enhanced fluorescence

Metal nanostructures can favorably change the properties of fluorescent molecules, increasing quantum yield, excitation and emission rates in a process known as plasmon enhanced fluorescence (PEF). Interactions between the nanostructures and fluorescent molecules can be described by three PEF mechanisms; near-field enhancement (NFE), resonance energy transfer (RET) and radiative decay engineering (RDE). The effect of these mechanisms on fluorescence is distance dependent, with enhancement occurring for distances greater the ~5nm and quenching of the signal when the fluorescent molecule is in close proximity to the nanostructure. This work focusses on the near-field enhancements associated with a gold nanorod array surface to determine a suitable setup for PEF applications. Using a finite element method (FEM) model, various nanorod array setups were simulated and the resonance and maximum field enhancements, E/E0 determined for each. Field enhancements occurred at different wavelengths than resonance as the enhancement was dominated by the existence of hot-spots. The maximum field enhancement of 7.88 occurred for an array of nanorods with 50nm diameter, 150nm height and center-to-center spacing of 60nm. The enhancement was due to hot-spots within the narrow gaps between nanorods, therefore this setup was not experimentally as fluorescent molecules would be unable to fit into the gaps. Nanorods with 50nm diameter and 100nm height in an array with 100nm periodicity provided an alternative setup, with maximum field enhancement of 6.37 due to a hot-spot at the top of the nanorod. Analysis showed that the field enhancement decreased rapidly with distance from the surface, but remained sufficiently strong for PEF applications.