Considers phase conjugation through degenerate four-wave mixing in a Λ-type three-level system under certain simplifying assumptions, the most restrictive among them being the assumption of symmetrical detuning of the pump from the two lower lying sublevels. When compared to an absorbing resonant two-level system, it is found that significantly higher values of the phase-conjugate reflectivity are possible at pump intensities well below saturation provided the sublevel splitting is less than a homogeneous linewidth. The physical mechanism responsible for phase conjugation is coherent population trapping arising from two-photon-induced atomic coherence between the two sublevels.
The effect of Zeeman coherence on phase conjugation is investigated in the saturation regime by using a nonperturbative analysis of resonant degenerate four-wave mixing with orthogonally polarized pump beams in a A-type homogeneously broadened three-level system. The dependence of the phase-conjugation efficiency on phase-interrupting collisions and pump imbalance is particularly studied. Significant phase-conjugate reflectivities can be obtained at pump intensities much below those required for a two-level system.