Highly nonlinear atomic medium with steep and sign-reversible dispersion

Experimental investigation of the optical properties of atomic media with bright and dark long-lived light-induced Zeeman coherence is reported. We have obtained a small negative value for the group velocity of light pulses in Cs vapour (Vg simeq-c/6000), which agrees with direct anomalous dispersion measurements. The intensity dependent components of the refractive index of an atomic vapour with long-lived Zeeman coherence has been estimated. We have shown that such coherence can significantly enhance four-wave mixing, leading to the appearance of a multi-frequency comb-like spectrum at relatively low light intensity