  We study a surface plasmon polariton mode that is strongly confined in the
transverse direction and propagates along a periodically nanostructured
metal-dielectric interface. We show that the wavelength of this mode is
determined by the period of the structure, and may therefore, be orders of
magnitude smaller than the wavelength of a plasmon-polariton propagating along
a flat surface. This plasmon polariton exists in the frequency region in which
the sum of the real parts of the permittivities of the metal and dielectric is
positive, a frequency region in which surface plasmon polaritons do not exist
on a flat surface. The propagation length of the new mode can reach a several
dozen wavelengths. This mode can be observed in materials that are uncommon in
plasmonics, such as aluminum or sodium.
