  Nonlinear and nonlocal effects are discussed in the interaction of laser
fields with thermal Rydberg atoms in electromagnetically induced transparency
configuration. We show that under the crucial approximation that the time
variation in the dipole-dipole interactions due to atomic motions can be
neglected in an ensemble average, an analytical form can be obtained for the
nonlocal nonlinear atomic response of the thermal medium, and study it for
different parameter cases. We further propose a generalized model to describe
the modulational instability (MI) in absorptive nonlinear media, in order to
understand the propagation dynamics in the thermal Rydberg medium.
Interestingly, this model predicts that at short propagation distances, each
wave component exhibits the MI effect in absorptive nonlinear media, unlike in
the purely dispersive case.
