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Light pulse in $Λ$-type cold atomic gases

We investigate the behavior of the light pulse in $Lambda$-type cold atomic gases with two counterpropagating control lights with equal strength by directly simulating the dynamic equations and exploring the dispersion relation. Our analysis shows that, depending on the length $L_0$ of the stored wave packet and the decay rate $γ$ of ground-spin coherence, the recreated light can behave differently. For long $L_0$ and/or large $γ$, a stationary light pulse is produced, while two propagating light pulses appear for short $L_0$ and/or small $γ$. In the $γ\to 0$ limit, the light always splits into two propagating pulses for sufficiently long time. This scenario agrees with a recent experiment [Y.-W.Lin, et al., Phys. Rev. Lett. 102, 213601(2009)] where two propagating light pulses are generated in laser-cooled cold atomic ensembles.