  The chiral optical Tamm state (COTS) is a special localized state at the
interface of a handedness-preserving mirror and a structurally chiral medium
such as a cholesteric liquid crystal or a chiral sculptured thin film. The
spectral behavior of COTS, observed as reflection resonances, is described by
the temporal coupled-mode theory. Mode coupling is different for two circular
light polarizations because COTS has a helix structure replicating that of the
cholesteric. The mode coupling for co-handed circularly polarized light
exponentially attenuates with the cholesteric layer thickness since the COTS
frequency falls into the stop band. Cross-handed circularly polarized light
freely goes through the cholesteric layer and can excite COTS when reflected
from the handedness-preserving mirror. The coupling in this case is
proportional to anisotropy of the cholesteric and theoretically it is only
anisotropy of magnetic permittivity that can ultimately cancel this coupling.
These two couplings being equal results in a polarization crossover (the
Kopp--Genack effect) for which a linear polarization is optimal to excite COTS.
The corresponding cholesteric thickness and scattering matrix for COTS are
generally described by simple expressions.
