Large cross-phase-modulation between two slow pulses by coupled double dark resonances

We present a scheme to realize a large cross-phase-modulation between two slow optical pulses by using the atomic configurations of coupled double dark resonances. In this scheme slow group velocities for these optical pulses can be automatically equalized in terms of the inherent symmetric atomic configuration, and the various absorptions can be overcome by using three-photon hyper-Raman amplification. Additionally, the switching between cross-type nonlinear amplification and absorption could be realized by just changing the sign of the difference between two pump detunings. This is useful for the implementations of all-optical switches. The linearity and cross nonlinearity in the Doppler-broadened medium with different laser propagating configurations have also been investigated.