Slow dark optical solitons caused by cross talk among optical transitions

We study the formation of slow dark optical solitons in a two-photon resonance Raman scheme. For a four-level atomic medium with two ground and two closely separated excited states, two transitions between one ground state and two excited states are coupled simultaneously by a strong cw laser field. This cross talk among optical transitions can modify the linear and nonlinear responses of another weak pulse probing the transition between the other ground state and one excited state. Under two-photon resonance condition and with appropriate one-photon detuning, we can obtain cancellation of the linear absorption, enhancement of Kerr nonlinearity, and slow group velocity propagation of the weak probe pulse. As a consequence, slow dark optical solitons can be formed.