We show that continuous-wave, two color excitation of three-level atoms opens new channels of irreversible population transfer between atomic levels. We point out that dynamically induced irreversibility leads to unanticipated phenomena such as the inversion of atomic transitions in cases where the upper atomic level has no conventional source of irreversible population pumping. The widely held view that coherent excitation is reversible is called into question.PACS numbers: 32.80. Wr, 42.50.Hz Transitions between atomic states can arise in two ways: They can be stimulated by an external perturbation such as a laser field, or they can occur "spontaneously" as a consequence of the atom's coupling to the vacuum, Transitions stimulated by an electromagnetic field are reversible; that is, the probabilities of photon absorption and emission are equal. Spontaneous transitions, on the other hand, are irreversible, since they occur only from states of higher energy to states of lower energy.The reversibility of stimulated transitions has important and well-known consequences, such as the impossibility of achieving steady-state population inversion in a driven two-level atom [1] [see Fig. 1(a)]. Optically sustained inversions involving the ground state can, however, be produced in multilevel atomic systems where population transfer channels mediated by irreversible spontaneous decay come into play. A three-level "V" system exhibiting spontaneous emission on all transitions is shown in Fig. 1(b). For certain ratios of spontaneous emission rates, a laser field applied to the 1-3 transition will create a steady-state population inversion on the 2-3 transition [2]. This inversion is made possible through the presence of the irreversible spontaneous decay channel that feeds population from level 1 into level 2. We are interested in three-level "V" systems that have no spontaneous emission coupling between the excited states [see Fig. 1 (c)] and hence have no intrinsic irreversible process whereby either of these states can be populated. We will show that two laser fields, each nearly but not quite resonant with one of the two allowed atomic transitions, act together to create new dynamically irreversible pathways of population transfer that can lead to population inversion relative to the ground state. These processes arise in single atoms unperturbed by incoherent processes such as collisions. Inasmuch as the irreversible processes discussed here, and their generalizations, may lead to the appearance of population inversion in unconventional situations, these processes must be carefully accounted for in the analysis of systems believed to display lasing without inversion [3]. We note that other means of controlling population transfer to excited states have recently been explored and include temporally sequenced pulses [4], colored vacuum [5], and coherent trapping [6].We first define the system of concern and outline our means of analysis. Consider a closed, V-type, three-level atomic system as shown in Fig. 2. Two stron...

No abstract

No abstract