Propagation of laser pulses and coherent population transfer in dissipative three-level systems: An adiabatic dressed-state picture

The interaction of a pair of copropagating pulses with three-level Λ-type atoms is discussed in terms of time-dependent coupled and decoupled superpositions ‖±〉 of the lower levels. Due to the explicit time dependence of these states there is a nonadiabatic coupling between the ‘‘bright’’ state ‖+〉 and the ‘‘dark’’ state ‖-〉 in addition to the strong coupling between ‖+〉 and the upper level ‖a〉 . We show that under quasiadiabatic conditions and in the presence of decay from the upper level this coupling can be treated perturbatively and the Maxwell-Bloch equations can be solved analytically. With the help of such a perturbation approach, coherent population transfer and formstable laser pulse propagation are studied. © 1996 The American Physical Society.