Light propagation in an atomic medium with steep and sign-reversible dispersion

We show that ground-state Zeeman coherence prepared by two-photon Raman transitions in alkali atoms results in steep controllable and sign-reversible dispersion. Pulse propagation with small negative as well as positive group velocity of light (–c/5100 and c/41 000) in a Cs vapor cell is reported. Energy exchange between copropagating light components through long-lived Zeeman coherence with enhanced absorption or transmission has been observed.