  A system comprising a $\Lambda$-type or V-type atom interacting with two
radiation fields exhibits, during its dynamical evolution, interesting optical
phenomena such as electromagnetically-induced transparency (EIT) and a variety
of nonclassical effects. Signatures of the latter are seen in the entanglement
dynamics of the atomic subsystem and in appropriate field observables. Some of
these effects have been experimentally detected, and have even been used to
change the nonlinear optical properties of certain atomic media. It is
therefore useful to investigate the roles played by specific initial states of
the radiation fields, detuning parameters, field nonlinearities and the nature
of field-atom couplings on EIT and on the entanglement between subsystems. We
investigate these aspects in the framework of a simple model that captures the
salient features of such tripartite entangled systems. Entanglement dynamics is
shown to be very sensitive to the intensity-dependent atom-field couplings.
Unexpected interesting features pertaining to the collapses and revivals of the
atomic subsystem von Neumann entropy appear. These features could, in
principle, be useful in enabling entanglement.
