Subluminal and Superluminal Phenomena in a Four-Level Atom

In a four-level atomic system, we investigate the light pulsepropagation properties interacting with only one laser field. It isshown that in the steady state, the group velocity of the lightpulse can be changed from subluminal to superluminal by varying thefield detuning. Meanwhile, the effects of the field intensity on thegroup velocity are also shown. At last, with special parameters, theanalytical solution for the group index is also obtained.