On surface plasmon-polariton waves guided by the interface of a metal and a rugate filter with a sinusoidal refractive-index profile

The canonical boundary-value problem of wave propagation guided by the planar interface of an isotropic homogeneous metal and a dielectric rugate filter with a refractive index that varies periodically normal to the interface may admit more than one solution, at a specific frequency. The different solutions indicate surface plasmon-polariton (SPP) waves that differ in phase speed, attenuation rate, linear polarization state, and field distribution. The multiplicity of SPP waves can only be attributed to the periodic nonhomogeneity of the rugate filter.