Theoretical analysis of dielectric-loaded surface plasmon-polariton waveguides

TLDR

The study investigates the conditions for single‑mode guiding in dielectric‑loaded surface plasmon‑polariton waveguides using the effective‑index method. Waveguiding of surface plasmon‑polaritons by a dielectric ridge on a metal surface is analyzed with the effective‑index method and finite element method, and the mode effective index, confinement, and propagation length are calculated at the telecom wavelength 1.55 µm for various ridge widths and thicknesses. The authors find that single‑mode DLSPPW guiding is achievable for ridge thicknesses below ~630 nm and widths below ~655 nm, that appropriate thickness can simultaneously enhance lateral confinement and propagation length, and that the effective‑index method accurately predicts mode characteristics for modes far from cutoff.

Abstract

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 $\ensuremath{\lambda}=1.55\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ 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 $\ensuremath{\sim}630\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and widths below $\ensuremath{\sim}655\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ (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.