Spatial solitons and instabilities of light beams in a three-level atomic medium with a standing-wave control field

We consider propagation of a weakly nonlinear probe light beam in a resonant three-level atomic system where an optical lattice is induced by a standing-wave control field and conditions for electromagnetically induced transparency are created. We employ a unified theory to solve the respective nonlinear system of equations in various regimes and investigate formation of spatial optical solitons and instabilities, both analytically and numerically. We show that the effect of the lattice is twofold. On the one hand it allows one to reduce the probe beam intensity necessary for observation of nonlinear effects until the level of one-photon energy, and on the other hand the lattice allows for implementation of different dynamical regimes either by simple manipulation of its parameters or by varying one- and two-photon detunings or by changing the geometry of the incident probe beam.