Rigorous surface-plasmon polaritons at a lossy interface

Making use of a rigorous treatment, we demonstrate that the approximate results that are customarily employed for the analysis of a plasmon field at a metal/dielectric boundary are incorrect even in some situations in which they are supposed to hold. We show further that the exact treatment implies the existence of a new type of backward-propagating polariton, analogous to the fields met in negative-index manmade structures. Such polariton is never encountered in the standard approximate analysis.