  We analyze planar electromagnetic waves confined by a slab waveguide formed
by two perfect electrical conductors. Remarkably, 2D Maxwell equations
describing transverse electromagnetic modes in such waveguides are exactly
mapped onto equations for acoustic waves in fluids or gases. We show that
interfaces between two slab waveguides with opposite-sign permeabilities
support 1D edge modes, analogous to surface acoustic plasmons at interfaces
with opposite-sign mass densities. We analyze this novel type of edge modes for
the cases of isotropic media and anisotropic media with tensor permeabilities
(including hyperbolic media). We also take into account `non-Hermitian' edge
modes with imaginary frequencies or/and propagation constants. Our theoretical
predictions are feasible for optical and microwave experiments involving 2D
metamaterials.
