Flat-top surface plasmon-polariton modes guided by double-electrode structures.

We characterize the frequency dependence of symmetrically-coupled long-range surface plasmon-polaritons (sc-LRSPPs) excited on double-electrode slab waveguides composed of five layers of insulator(I) and metal(M) stacked in order of IMIMI. When the core insulator has a refractive-index larger than the cladding ones, there is no cut-off core-thickness(D) for sc-LRSPP modes in all frequency range likely for modes in a conventional dielectric slab waveguide. At a specific frequency of ωc which depends on the index difference of insulator layers and the thickness of metal, the sc-LRSPP modes are non-dispersive at all for change in D. Furthermore, regardless of D alteration, the modes at ω = ω(c) consistently maintain a perfect flat-top profile in the core region and identical decay tails in the cladding. The sc-LRSPP modes with these prominent characteristics may excite an active medium sandwiched in between the metal layers very uniformly, therefore it will be interesting to implement such a non-dispersive flat-top mode for nonlinear applications of SPP waveguides.