Formulas are derived for the extinction cross section of small randomly oriented, two-layer spheroids and for the luminescence of small spheres coated with a luminescent layer. Numerical examples are presented showing the dependence of the extinction of dye-coated particles upon the particle size, the optical properties of the core (silver, gold, dielectric), the coating thickness, and the spheroid eccentricity. The interaction of metal and dye extinction bands, the splitting of these bands, and the enhancement of the extinction depend sensitively on these various parameters. The enhancement of the luminescence of dye layers coating metallic spheres is shown to be related to comparable enhancements in the extinction. The present model differs from earlier studies of surface-enhanced Raman scattering (SERS) in that the electric dipoles which simulate the dye molecules are embedded in a medium having the dielectric properties of the dye rather than dangling into the external medium. The present study utilizes air as the external medium. The enhancement effects would be much greater in water.

• Received 8 September 1981

DOI:https://doi.org/10.1103/PhysRevB.25.2433