Channel plasmon polaritons guided by graded gaps: closed-form solutions.

Using the effective-index approximation along with the explicit expression for the propagation constant of gap surface plasmons established recently [Opt. Express 16, 2676-2684 (2008)], we develop an analytic description of channel plasmon polariton (CPP) modes guided along V-grooves and gaps between two metal cylindrical surfaces. The results obtained for V-grooves are compared with those reported previously. Dispersion of the main CPP characteristics is calculated for air-gold configurations, allowing one to design graded-gap waveguides for the single-mode operation supporting well-confined fundamental CPP modes in a broad wavelength range.