  It has been found that in the media where the dielectric permittivity
$\epsilon$ or the magnetic permeability $\mu$ is near zero and in transition
metamaterials where $\epsilon$ or $\mu$ changes from positive to negative
values, there occur a strong absorption or amplification of the electromagnetic
wave energy in the presence of an infinitesimally small damping or gain and a
strong enhancement of the electromagnetic fields. We attribute these phenomena
to the mode conversion of transverse electromagnetic waves into longitudinal
plasma oscillations and its inverse process. In this paper, we study analogous
phenomena occurring in chiral media theoretically using the invariant imbedding
method. In uniform isotropic chiral media, right-circularly-polarized and
left-circularly-polarized waves are the eigenmodes of propagation with
different effective refractive indices $n_+$ and $n_-$, whereas in the chiral
media with a nonuniform impedance variation, they are no longer the eigenmodes
and are coupled to each other. We find that both in uniform chiral slabs where
either $n_+$ or $n_-$ is near zero and in chiral transition metamaterials where
$n_+$ or $n_-$ changes from positive to negative values, a strong absorption or
amplification of circularly-polarized waves occurs in the presence of an
infinitesimally small damping or gain. We present detailed calculations of the
mode conversion coefficient, which measures the fraction of the electromagnetic
wave energy absorbed into the medium, for various configurations of $\epsilon$
and $\mu$ with an emphasis on the influence of a nonuniform impedance. We
propose possible applications of these phenomena to linear and nonlinear
optical devices that react selectively to the helicity of the circular
polarization.
