There is considerable interest in controlling the group velocity of light, for a wide range of applications, such as quantum computers, high speed optical switches and communication systems. There are several mechanisms that enable the control of group velocity in dispersive media such as electromagnetically induced transparence (EIT) and coherent population oscillations (CPO) due to nonlinear interaction of the optical fields and the media.

We investigate the control of the group velocity of a probe in the presence of a pump and seeded FWM in a two-level system, decaying via an intermediate metastable state. We demonstrate that it is possible to switch between subluminal to superluminal propagation of the probe by changing the phase of the seeded FWM.

We also demonstrate that the magnitude of the group velocity of the probe is affected by changing the amplitude of the seeded FWM. These results can be explained by constructive or destructive interference between the coherent population oscillations (CPO) created by interaction between the pump and probe or FWM. We also show that the group velocities of the probe and FWM change on propagation length and do not reach a steady state value, due to absorption of the pump.