Effect of quantum interference on a three-level atom driven by two laser fields

We study the effect of quantum interference on the population distribution and absorptive properties of a V-type three-level atom driven by two lasers of unequal intensities and different angular frequencies. Three coupling c onfigurations of the lasers to the atom are analysed: (a) both lasers coupl ed to the same atomic transition, (b) each laser coupled to different atomi c transition and (c) each laser coupled to both atomic transitions. Dressed stales for the three coupling configurations are identified, and the popul ation distribution and absorptive properties of the weaker field are interp reted in terms of transition dipole moments and transition frequencies amon g these dressed states. In particular, we find that in the first two cases there is no population inversion between the bare atomic states, but the po pulation can be trapped in a superposition of the dressed states induced by quantum interference and the stronger held. We show that the trapping of t he population, which results from the cancellation of transition dipole mom ents, does not prevent the weaker field to be coupled to the cancelled (dar k) transitions. As a result, the weaker field can be strongly amplified on transparent transitions. In the case of each laser coupled to both atomic t ransitions the population can be trapped in a linear superposition of the e xcited bare atomic states leaving the ground state unpopulated in the stead y state. Moreover, we find that the absorption rate of the weaker field dep ends on the detuning of the strong field from the atomic resonances and the splitting between the atomic excited states. When the strong held is reson ant to one of the atomic transitions a quasi-trapping effect appears in one of the dressed states. In the quasi-trapping situation all the transition dipole moments are different from zero, which allows the weaker field to be amplified on the inverted transitions. When the strong field is tuned half way between the atomic excited states, the population is completely trapped in one of the dressed states and no amplification is found for the weaker field.