Retardation effects on the optical properties of ultrafine particles

The retarded polarization in an ensemble of ultrafine particles is incorporated in the Maxwell-Garnett theory for the average dielectric permeability of a system of randomly distributed particles. This theory is applied to the optical absorption of ultrafine silver particles in a glass matrix. For particles with mean diameters of 10 nm in samples with 20-vol% metal content the absorption may be enhanced by a factor of 2 in the near infrared due to retardation. For smaller particles, or for smaller metal contents, the retardation effects are diminished. The observability of the enhanced absorption is discussed.