  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.
