We propose a model to manipulate group velocity of a multi-frequency probe light in an electromagnetically induced transparency medium consisting of five-level cascade-type atoms associated with a giant self-Kerr nonlinearity. An analytic expression of group index for the probe light is derived as a function of parameters of the probe and coupling fields, atomic density, and lifetimes of excited atomic states. In the presence of the self-Kerr, both probe and controlling fields can be used as knobs to manipulate the probe light between the subluminal and superluminal propagation modes in three separated frequency regions. The theoretical model agrees with experimental observation, and it is helpful to find the optimized parameters and related applications. Furthermore, by using such a cascade excitation scheme, it could be possible to choose the uppermost excited electronic states having long lifetimes, as Rydberg states, to slow the light down to a few mm/s.
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