Phase and amplitude control of the group velocity in a two-level system with shelving state

We study the propagation of two weak light pulses with the same temporal shape, symmetrically detuned from a stronger cw pump, in a two-level system which decays via a shelving state. We show that the group velocity of the weak pulses can be controlled, with small distortion, by modifying either the relative amplitude of the weak pulses or the total phase Φ. The changes in the local group velocity on propagation are due to pump absorption, and to changes in the total phase on propagation which occur for all cases apart from the case where the amplitudes of the weak fields are equal and Φ=0 or π. Slow light occurs when the initial total phase Φ=0,π/2 but not when Φ=π; when one pulse is less intense than the other, it experiences the greater slowing down, and the effect is greater than in the case of equal intensities.