Optical Tamm states above the bulk plasma frequency at a Bragg stack/metal interface

We demonstrate theoretically that surface-plasmon polaritons, a form of\ud optical Tamm state, can occur at the interface between a metal and a\ud Bragg reflector at frequencies above the bulk plasma frequency of the\ud metal. The frequencies of the excitations are within the photonic band\ud gap of the Bragg reflector which provides the required evanescent decay\ud on that side of the interface. At finite in-plane wave vector, the low\ud value of the permittivity of the metal above its plasma frequency can\ud lead to an imaginary normal wave vector component in the metal, which\ud provides the localization on the other side of the interface. It is\ud proposed that the necessary conditions can be realized using a\ud GaAs/AlAs Bragg stack coated with a suitable conducting metal oxide\ud having a bulk plasma frequency of 1 eV, but the concept is valid for\ud other systems given an appropriate plasma frequency and photonic\ud band-gap structure. The dispersion relations of the plasmon polaritons\ud in the structures considered are calculated for both possible\ud polarizations, and it is shown how the excitations result in distinct\ud features in the predicted reflectivity spectra