A plasmonic hybrid nanostructure with controlled interaction strength

39 Abstract—We describe a novel plasmonic hybrid nanostructure based on a silver island film covered with a dielectric silica layer. The thickness of the silica layer is varied from 0 to approximately 46 nm on a single sample, thus allowing for continuous variation of the interaction strength between plasmon excitations in the metallic film and the excited states of pigments comprising photosynthetic complexes used to probe this interaction. While the largest separation between the silver film and photosynthetic complexes provides fluorescence featuring mono-exponential decay, thinner silica spacer distances show bi-exponential decay. The intensity of the fast component, which is attributed to the emission of photosynthetic complexes coupled to plasmon excitations, strongly decreases as a function of the spacer thickness. The interaction is stronger for excitation wavelengths resonant with plasmon absorption in the metallic layer. The interaction between plasmon excitations in metallic nanoparticles and fluorophores depends strongly upon the distance between these two nanostructures [1]. Typically, for distances larger than 40-50 nm, the interaction is very weak and leads to no measureable changes in the absorption or emission of fluorophores. When approaching a metallic nanoparticle, however, and reaching a distance between 10 nm and 25 nm, the emission intensity of fluorescence shows a gradual increase. This enhancement results from an increase in fluorophore absorption and/or emission rate due to the local influence of plasmon excitations in metallic nanoparticles. Lastly, for separations comparable or less than 5 nm, the emission exhibits quenching attributed to non-radiative energy transfer from a fluorophore to a metallic nanoparticle. Various nanostructures have been investigated from the point of view of plasmonic effects, including organic molecules [1, 2], semiconductor * nanocrystals [3], conjugated polymers [4], and multichromophoric photosynthetic complexes [5, 6]. Due to the strong sensitivity of fluorescence enhancement to the distance separating a metallic nanoparticle from a fluorophore, several strategies have been proposed to control and study this important facet of plasmonic enhancement. Metallic nanoparticles were covered with dielectric layers or functionalized with active groups that enable robust conjugation with fluorophores, and thin polymer layers have been applied in previous efforts. However, all these methods inhibit the fabrication of a structure in which the distance between the fluorophore and the metallic nanoparticle can be gradually varied, thus allowing the study of the distance dependence of the interaction strength. Perhaps the only realization of such a system concerns the metallic nanoparticle attached to an ultrasmall tip that can …