At the interface between the materials with negative and positive permittivities, which is the case for metal and dielectric layers, an incident electromagnetic (EM) radiation produces oscillations of the electron plasma on the metallic surface that in turn generate confined two-dimensional transverse EM waves at the interface. these waves, called surface plasmon polaritons (SPPs), are evanescent EM waves that decay exponentially in the direction perpendicular to the interface (z-axis) and practically vanish at both sides (±z-axes) of the interface. For more details on the principle, click: 10.3390/photonics5030015 or