Backward Zenneck wave along flat the media boundary

We consider the surface Zennek wave along the plane boundaries of the vacuum-medium interface with isotropic permittivity, and the vacuum-medium described by isotropic dielectric and magnetic permittivity. In the latter case, we also consider the H-wave and the possibility of negative real parts of the permeability. Forward and backward waves at the boundary of a material with an inhomogeneous permittivity profile are considered. The conditions for the existence of forward and backward surface and volume waves, as well as fast and slow surface waves, are studied.