Nature of the enhanced optical absorption of dye-coated Ag island films.

This paper reports the results of a calculation of the optical response of a hexagonal lattice of Ag spheroids coated with confocal layers of dye. The layered spheroid polarizability and the local field, which includes the effects of interactions with the quartz substrate, are calculated in the electrostatic limit. Using experimental data for a 60-A mass thick annealed Ag island film to determine the Ag spheroid aspect ratio and assuming a Lorentz oscillator response for the dye, it is shown that the absorption spectrum can exhibit three or more resonances depending on the strengths of the dye dispersion and the particle interactions. By comparing the power absorbed in the dye layer to the peak power absorbed by an equal volume of dye laid on a quartz substrate, it is shown that an absorption enhancement of the order of 10 can be obtained even when the molecular absorption energy is separated from the Ag spheroid plasma resonance by an electron volt or more. The enhancement is due to the amplification by the Ag core of the electric fields associated with the driven modes of the dye layer.