Usually guided electromagnetic waves require at least two plane interfaces. Nevertheless, guided electromagnetic wave propagation is also possible with a single plane interface between two media.
Surface polaritons are defined as modes that can propagate along an interface with the amplitude of the electromagnetic field decreasing along the direction that is normal to the interface. The polariton results from the interaction between the photons that constitute the electromagnetic field and the material polarization induced by this field. Plasmons are a special kind of surface polaritons where that interaction takes place between the photons and the free-electron gas of a metal at optical frequencies. Indeed surface polaritons can occur at the interface between a common isotropic medium (a doublepositive medium) and either a single-negative medium (as in a plasma medium operated below the plasma frequency) or a double-negative medium (or Veselago medium). However, the formation of surface polaritons does not require one of the constitutive parameters (the dielectric permittivity, the magnetic permeability, or both) to be negative. In fact it was shown by D’Yakonov that a surface polariton can also
propagate along the interface between lossless isotropic and uniaxial materials. Since then several types of surface polaritons have been analyzed at isotropic-uniaxial and isotropic-biaxial interfaces.