Phase control of maximal atomic coherence

The atomic coherence in a three-level I > atom is studied, in which each optical transition is driven by a coherent field and the metastable states are coupled to each other via a microwave field. It's shown that the atomic coherence crucially depends on the relative phase delay between the envelopes of the amplitudes of the three coupling fields. In particular, when the phase delay is adjusted to 0 or pi, the maximal atomic coherence arises, while the maximal atomic coherence doesn't occur once the phase delay is changed to pi/2. The maximal atomic coherence is attributed to the trapping of the population in the lower sublevels.