Surface-polariton-like waves guided by thin, lossy metal films

The dispersion relations are solved for waves guided by a thin, lossy metal film surrounded by media of dielectric constant ${\epsilon }_1$ and ${\epsilon }_3$. For symmetric structures (${\epsilon }_1$=${\epsilon }_3$), there are the usual two Fano modes whose velocity and attenuation vary with film thickness. For very thin films, one of these modes can attain multicentimeter propagation distances when λ>1 μm. In addition, there are two leaky waves which correspond to waves localized at the ${\epsilon }_1$ (or ${\epsilon }_3$) dielectric-metal interface whose fields decay exponentially across the metal film and radiate an angular spectrum of plane waves into ${\epsilon }_3$ (or ${\epsilon }_1$, respectively). Both radiative waves can be interpreted as spatial transients, which could have physical significance near a transverse plane. When ${\epsilon }_1$≠${\epsilon }_3$, there are still four distinct solutions for a given film thickness, two radiative and two nonradiative. For lossy films, there are always two nonradiative solutions for thick enough films. As the thickness goes to infinity, the four solutions reduce to two waves, each radiative and nonradiative pair becoming degenerate. The physical interpretation of these solutions and their dependence on dielectric constant and wavelength are discussed.