  In this paper, we introduce and study parallel-plate waveguides formed by two
penetrable metasurfaces having arbitrary isotropic sheet impedances. We
investigate guided modes of this structure and derive the corresponding
dispersion relations. The conditions under which transverse magnetic and
transverse electric modes can exist are discussed, and different scenarios
including lossless (reactive) metasurfaces, gain-and-loss sheets and extreme
cases are under general consideration. Resonant and non-resonant dispersive
sheets and corresponding extreme cases are investigated. Our theoretical
results are confirmed with full-wave simulated results considering practical
realizations of the proposed parallel-plate waveguide which exploit two
frequency-selective surfaces. Finally, the obtained theoretical formulas are
applied to study the dispersion diagrams for waves along resonant sheets at
different distances between the two identical or different metasurfaces. We
hope that this study is useful for future applications at both microwave and
optical regimes.
