  Bi-isotropic media, which include isotropic chiral media and Tellegen media
as special cases, are the most general form of linear isotropic media where the
electric displacement and the magnetic induction are related to both the
electric field and the magnetic intensity. In inhomogeneous bi-isotropic media,
electromagnetic waves of two different polarizations are coupled to each other.
In this paper, we develop a generalized version of the invariant imbedding
method for the study of wave propagation in arbitrarily-inhomogeneous
stratified bi-isotropic media, which can be used to solve the coupled wave
propagation problem accurately and efficiently. We verify the validity and
usefulness of the method by applying it to several examples, including the wave
propagation in a uniform chiral slab, the surface wave excitation in a bilayer
system made of a layer of Tellegen medium and a metal layer, and the mode
conversion of transverse electromagnetic waves into longitudinal plasma
oscillations in inhomogeneous Tellegen media. In contrast to the case of
ordinary isotropic media, we find that the surface wave excitation and the mode
conversion occur for both s and p waves in bi-isotropic media.
