Coherent population trapping under bichromatic fields

Abstract

We explore extended $\ensuremath{\Lambda}$ systems driven by two resonant laser fields in a strong regime characterized by the fact that the Rabi frequencies are much bigger than the difference between the two field frequencies. We show that a specific coherent superposition of the lower states is trapped during the dynamics. This extended trapped state is different from the trapped state usually encountered in the rotating-wave-approximation regime. We show the robustness of this trapping in the sense that it is stable with respect to significant deviations of the field parameters (frequencies and amplitudes).