Observation of optical filtering effects with four-wave mixing in a cold   atomic ensemble
  We observe an optical filtering effect in four-wave mixing (FWM) process based on a cold atomic gas. The side peaks appear at the edges of pulse of generated optical field, and they propagate through the atomic media without absorption. The theoretical analysis shows that these side peaks corresponded to the high frequency part of pulse of generated signal, which means the atoms cannot response to the rapid change of the electromagnetic field in time. On the contrary, the low frequency components of generated signal are absorbed during the transmission through the atoms. In addition, we experimentally demonstrate that the backward side peak could be stored by using Raman transition in atomic ensemble and retrieved later. 
Dong-Sheng Ding, Yun Kun Jiang, Wei Zhang, Zhi-Yuan Zhou, Bao-Sen Shi,   and Guang-Can Guo
quant-ph physics.optics