 Surface plasmon polariton

A transverse magnetic polarization electromagnetic wave bound to an interface through coupling to the material's collective charge response. Its fields propagate along the interface and decay away from it on both sides. In an ideal nonmagnetic lossless planar vacuum-material interface, a real negative relative permittivity below $-1$ supports the bound branch described by the lossless planar surface-plasmon dispersion. Material losses generally make the propagation constant complex.

Surface plasmon polaritons (SPPs) are electromagnetic waves that travel along the interface between a dielectric material and a conductor, typically metal. They arise from the coupling of light with the oscillations of free electrons at the surface of the metal. SPPs are characterized by their ability to propagate along the surface while being confined to a very small region near the interface, often on the scale of a fraction of the wavelength of light.