Enhanced dye fluorescence over silver island films: analysis of the distance dependence

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Abstract
Fluorescence intensity enhancement has been studied for several coverages (10-3 to 1 monolayer) of Rhodamine 6G separated from a silver island film by quartz spacer layers of varying thickness. When the dye film is brought into the proximity of the island film support, a first enhancement maximum is observed at ≈ 60 nm spacer thickness. For smaller distances, decrease in the enhancement is followed by a second maximum at 5–10 nm interlayer thickness. The magnitude of the latter maximum exhibits a pronounced dependence on dye coverage. The far-field enhancement maximum is due to an increase in fluorescence quantum yield induced by the presence of the island film substrate, combined with a resonant excitation phenomenon due to collective action of the islands. This effect is analysed in terms of stratified medium theory, in which the island film is represented by a suitable model for the effective dielectric constant. The short distance enhancement results from competition between the strongly amplified local field around the spheroidal metal particle and the radiationless loss of excitation energy by transfer to non-radiating particle surface plasmons.