Maximal atomic coherence via selective trapping of dressed states

We consider strong-field index enhancement in three-level systems in which there is a strong spontaneous decay from an auxiliary level into either of two states that the probe field couples. A control field is detuned resonant with the transition between the auxiliary level and one of the dressed states produced by the strong probe field. It is shown that it is possible to achieve maximal atomic coherence, which characterizes an ultralarge index of refraction and vanishing absorption. This scheme is based on selective dressed population trapping, which is established by relying on the control field and the strong decay to transfer population from one dressed state to another through the auxiliary level. An advantage of the present scheme is that the conditions for its realization are accessible experimentally.