Influence of the signal light on the transient optical properties of a   four-level EIT medium
  General formulae for the transient evolution of the susceptibility (absorption) induced by the quantum interference effect in a four-level N-type EIT medium is presented. The influence of the signal light on the transient susceptibility for the probe beam is studied for two typical cases when the strength of the coupling beam is much greater or less than that of the signal field. An interesting level reciprocity relationship between these two cases is found. 
Jian-Qi Shen, Zhi-Chao Ruan and Sailing He