The optical phenomena in multiple-photon interactions (I)
  In the recent experiment, the phenomena of superluminal and slow-light propagation in dispersive medium were found, and there are various explanation in theory. We find the phenomenon can be explained by multiple-photon interaction. Otherwise, the multiple-photon interaction can also appear other optical phenomenon: doubling frequency, sum frequency, difference frequency which are found in nonlinear optics. 
Xiangyao Wu, Pi-Feng Gong, Zhi-Yong Yi, Chang-Rui Wu, Xiao-Yan Gu and   Zong-Hua Shi