A collective excitation
in the surface of a metal
. Surface plasmons can be excited optical
ly 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.