I am trying to find the dispersion curves for the antisymmetric and symmetric modes of a metal-insulator-metal (MIM) plasmonic slab wave guide. I was able to find the correct dispersion curves for the asymmetric mode in Mode Solutions, using a antisymmetric boundary condition, finding the modes, and sweeping the frequency to find the effective index. However, when I changed the x boundary condition to symmetric, I was unable to find the symmetric modes with the expected dispersion curves. The effective index of these symmetric mode are not correct, so I am not finding the correct modes (I expect an index somewhere around 1.4, give or take, but I am instead getting results with the n_eff around 0.2). What can I do to calculate the correct symmetric mode?

(For reference for the expected dispersion, I am currently trying to replicate the dispersion curves in figure 3 of the following paper: J. A. Dionne, L. A. Sweatlock, and H. A. Atwater and A. Polman, “Plasmon slot waveguides: Towards chip-scale propagation with subwavelength-scale localization”, Phys. Rev. B 73, 035407 (2006).)

My Lumerical file is attached for reference. The waveguide has an SiO2 core of 35nm, and a silver cladding. I set up a mesh over the center of the waveguide. Since the plasmon should decay away from the interface, I used a wide simulation area with metal boundary conditions in x. The slab waveguide is infinite in y, so I used a periodic condition in +/- y. The modes I am looking for would propagate in z.

plasmon_WG_modes.lms (297.1 KB)