Theoretical analysis of dielectric-loaded surface plasmon-polariton waveguides

Waveguiding of surface plasmon-polaritons (SPPs) by a dielectric ridge placed on a metal surface is analyzed using the effective-index method (EIM) and the finite element method (FEM). Main characteristics of these dielectric-loaded SPP waveguide (DLSPPW) structures, i.e., the mode effective index, confinement, and propagation length, are calculated at the telecom wavelength lambda=1.55 mu m for different widths and thicknesses of a polymer ridge (with the refractive index of 1.535) placed on a gold film surface. The condition for single-mode guiding is investigated using the EIM, and it is found that single-mode DLSPPW guiding can be realized for ridge thicknesses smaller than similar to 630 nm and widths below similar to 655 nm (when decreasing the ridge thickness, the ridge width suitable for single-mode guiding increases). It is also established that, in contrast to the usual trade-off, the DLSPPW mode lateral confinement can be improved simultaneously with the increase in the mode propagation length by choosing the appropriate ridge thickness. The accuracy of the EIM is evaluated by comparing the computed mode characteristics with those obtained with the FEM and found rather good for the modes being far from cutoff.