Collective response of silver islands on a dielectric substrate when normally illuminated with electromagnetic radiation

Normal reflectance measurements of silver island films, for wavelengths between 250 and 1050 nm, are inverted to obtain the effective dielectric function of the inhomogeneous films. An effective medium model is applied to evaluate the dielectric function of the metallic islands, which are modelled in terms of oblate spheroidal particles whose average values for the semiaxes have been estimated from atomic force microscopy images. Effective depolarization factors, which account for substrate effects and dipolar interactions between islands, are used in the context of the effective medium model. The dielectric function of the system of dipoles assigned to the silver island film is also obtained from an approach based on evaluating renormalized polarizabilities of the metallic islands also taking into account dipolar interparticle interactions and substrate effects.