HFSS Eigenmode solver: Characterizing coupling between dielectric waveguides

I would like to model the dependence of the spacing between dielectric waveguides as a function of the waveguides' profile and spacing. These waveguides are dielectric - the modes and loosely confined in/around the dielectric and are largely degenerate hybrid modes. There is not metal around these waveguide. They are suspended above a substrate, so one can consider them to be entirely surrounded by free space. 

I would like to characterize the coupling (% power/length) for guides as I parametrically sweep the gap between then.

I have attempted to use the eigenmode solve with a very think slab (think essentially a 2D cross section of the parallel waveguides). My idea is that I should be able to find the spatial modes that corresponds to the guided modes of interest for a given delay between the master/slave boundary condition. The different between these Eigenmode frequencies can define the coupling coefficient.

In my attempt, the air box walls parallel to the waveguide longitudinal axis are PMC boundaries.

The issue I have run into is that my modes, when plotted, seem very unphysical. For example, all the E-Field energy is up in the order, near two PMC boundary conditions instead of concentrated around the dielectric waveguides.

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