Hysteresis loop with controllable shape and direction in an optical ring cavity

We have experimentally observed a ``backward'' (clockwise rotating) hysteresis cycle in the system of an optical ring cavity containing three-level (Λ-type configuration) rubidium atoms. The shape and direction of the observed hysteresis cycles can be easily controlled with experimental parameters. Such an interesting phenomenon is caused by the greatly modified absorption, dispersion, and nonlinear optical properties of the three-level atomic medium, due to the induced atomic coherence.