Modal analysis of a center-symmetric line waveguide

The modal properties of a double layer, metal-dielectric line-wave waveguide are studied in a wide frequency range, both inside and outside the homogenization limit. Utilizing a sampling-and-fitting approach on the aperture plane, the generally complex longitudinal propagation constant is reconstructed in amplitude and phase. It is observed that, when raising the frequency above a certain threshold, because of intrinsic granularity of the device due to the periodic arrangement of unit cells, the dominant mode evolves to the form of a Bloch wave, which represents the solution of the wave equation in a periodic medium. The modal dispersion of the line wave is then compared to those pertaining to surface waves supported by the substrates, with a slow-wave, plasmonic propagation observed from 10 to 50 GHz.