Two-color ultraslow optical solitons via four-wave mixing in cold-atom media

We analyze a lifetime-broadened four-state, ladder-type four-wave mixing (FWM) scheme in the context of optical soliton formation. We show that a pulsed probe field and a pulsed FWM field of considerably different frequency can evolve into a pair of matched solitons with the same temporal shape and ultraslow group velocity (V-g/c similar to 10(-3)), i.e., two-color ultraslow optical solitons. In addition, we show regimes where two-color superluminal (V-g/c < 0) optical soliton propagation may occur.