Forbidden guided-wave plasmon polaritons in coupled thin films

We study the plasmon-polariton collective modes of a three-thin-film-on-a-substrate structure, where two of the thin films have free-charge carriers and are separated by a static dielectric. We find that this structure can support unusual guided-wave polaritons, which exist in a normally forbidden region of the surface polariton dispersion, and which are composed of coupled surface polariton modes of the individual films with a wavelike transverse electric-field amplitude in the spacer dielectric. The modes emerge from both bulk and surface collective mode bands and evolve into a ladder of solutions in the forbidden region. We also present the attenuated total reflection spectrum arising from the guided modes.