Localized plasmons seen by propagating surface plasmons: Unique determination of their dielectric response

The plasmonic resonance that occurs when metal spheres interact with a metal surface over a thin gap is investigated by multiwavelength surface plasmon spectroscopy in Kretschmann configuration. A strategy is proposed to determine the polarizability per unit area from those experiments, and the limitations of this analysis are discussed. This approach completely avoids the ambiguity in the determination of thickness and dielectric constant of thin films in surface plasmon spectroscopy. Since it is intimately connected with the density of objects on the surface, the polarizability per unit area represents a very intuitive physical quantity that can be used to characterize the optical response of a layer. We show that results from standard extinction experiments are reproduced by this approach with higher information content and significantly increased signal-to-noise ratio.