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A collective excitation of electrons in the surface of a metal. Surface plasmons can be excited optically and will radiate light when they encounter discontinuities (roughness) in the surface.

Metals contain a conduction band of electrons that are weakly bound. Electrons are spin 1/2, and so these conduction band electrons constitute a fermi gas. Plasmons are analogous to sound waves (periodic longitudinal air density fluctuations) in this fermi gas.

Surface plasmons may be useful in the future in optical electronics applications -- as very fast optical transistors or diodes for example.