In this study, we are interested in studying the refraction of a surface polariton by a vertical interface between two different surface active dielectric media. In this study, the interaction volume is limited by placing two shorting plasmons above and below the surface on which propagation take place. These planes are perfect conductors that force the tangential components of the electric field to vanish on their surfaces. The resulting structure is a closed waveguide that the electromagnetic modes of this waveguide are discrete.