Plasmon polaritons with uniquely long free path

It is shown that the free path of plasmon polaritons propagating along the interface between a metal and a crystal may be greatly increased (up to several orders of magnitude) by the choice of a certain crystal orientation that provides complete delocalization of the polaritons. As an example, a theory is developed for the interface between a metal and a uniaxial crystal with its optical axis parallel to the interface, both considered as semi-infinite media.