Backpropagating modes of surface polaritons on a cross-negative interface.

We show that backpropagating modes of surface polaritons can exist at the interface between two semi-infinite cross-negative media, one with negative permittivity (e<0) and the other with negative permeability (µ<0). These single-interface modes that propagate along the surface of a cross-negative interface are physically of interest, since the single-negative requirements imposed on the material parameters can easily be achieved at terahertz and potentially optical frequencies by scaling the dimension of artificially structured planar materials. Conditions for material parameters that support a backpropagating mode of the surface polaritons are obtained by considering dispersion relation and energy flow density transported by surface polaritons and confirmed numerically by simulation of surface polariton propagation resonantly excited at a cross-negative interface by attenuated total reflection.