Long-range surface plasmon polariton (LRSP) with lower attenuation and longer propagation length than conventional
surface plasmon polariton (SPP) on a single interface has been studied as new elements for integrated optical circuits.
LRSP exhibits on a metal film bounded by dielectric and can be expected to edge closer to practical use due to the long
propagation length. It is theoretically predicted that the propagation length of LRSP greatly increases as the lateral width
of a metal film decreases. Such mode propagating in a metal slab waveguide with extremely long propagation length is
named “super LRSP”. Here, we fabricate silver slab waveguide and observe the tendency of propagation properties of