Optical Response of a Sphere Coupled to a Metal Substrate

Optical response of a sphere placed above a flat Ag substrate is calculated. Local dielectric constants are assumed and exact account is taken of the retardation effect. For a Ag sphere above a Ag substrate, coupling of sphere and substrate plasmons results in a strong local field, whereby Raman scattering by molecules on the sphere surface is enhanced by a factor of 10 8 . The case of a dielectric sphere is also discussed. The mechanism is understood in terms of surface plasmon polariton excitation.