ENHANCED DYE FLUORESCENCE OVER SILVER ISLAND FILMS - ANALYSIS OF THE DISTANCE DEPENDENCE

Fluorescence intensity enhancement has been studied for several covera ges (10(-3) to 1 monolayer) of Rhodamine 6G separated from a silver is land him by quartz spacer layers of varying thickness. When the dye fi lm is brought into the proximity of the island film support, a first e nhancement maximum is observed at approximate to 60 nm spacer thicknes s. For smaller distances, decrease in the enhancement is followed by a second maximum at 5-1O nm interlayer thickness. The magnitude of the latter maximum exhibits a pronounced dependence on dye coverage. The f ar-field enhancement maximum is due to an increase in fluorescence qua ntum yield induced by the presence of the island film substrate, combi ned with a resonant excitation phenomenon due to collective action of the islands. This effect is analysed in terms of stratified medium the ory, in which the island film is represented by a suitable model for t he effective dielectric constant. The short distance enhancement resul ts from competition between the strongly amplified local field around the spheroidal metal particle and the radiationless loss of excitation energy by transfer to nonradiating particle surface plasmons.