The Control Of Atomic Coherence Effects By Incoherent Field

In this thesis we propose to utilize an incoherent field to achieve the efficient control of atomic coherence effects. These include dynamically induced irreversibility and the strong-field index enhancement. The common physical mechanism to these two coherent effects is the selective trapping or selective excitation of either of dressed states. The different conditions for these effects are off-resonant or resonant coupling of the coherent field with the atom.Consider a there-level V system in which there are two dipole transitions, of which one dipole transition is weak and the other is strong. One strong coherent field is applied to the weak transition and one incoherent field drives strong transition. For such an arrangement, the coherent field creates dressed states, and the incoherent field is tuned to be resonant with one of two dressed transitions. As a result, the incoherent pump and the decay pathway constitute a successive two-step channel transfer population from one dressed state to the other. So selective dressed-population trapping is established. When the coherent field is off-resonant with the weak dipole transition, dressed population trapping leads to population inversion, which corresponds to almost complete inversion for the ideal condition. When the coherent field is resonant with the weak dipole transition, dressed population trapping leads to maximal atomic coherence (0.5), which corresponds to ultralarge index of refraction and vanishing absorption for the strong coherent field. Similarly, a there-level A system in which there are two dipole transitions, of which the weak dipole transition is driven by a strong coherent field, and the other strong dipole transition is driven by an incoherent field. When the coupling of coherent field with weak dipole transition is off-resonant, the selective dressed population excitation by the incoherent field leads to population inversion. The inversion is 0.5 under the ideal condition. On the resonance selective dressed excitation leads to half of maximal atomic coherence (0.25).The paper shows that the efficient control of the atomic coherent effects isachieved, in principle, by employing an incoherent field to transfer selectively dressed state population. In days that lasers are so widely used, we do not necessarily use the incoherent field as a drive. However, it is well known that a laser generally experiences free phase diffusion and has a finite bandwidth. Hence the use of partially coherent (incoherent field as the limiting case) field shows practical advantage. Our results show that population inversion is obtained in a wider central frequency range than for the coherent field case, and it is more advantageous in practical applications. For atomic barium, the states 6s 1S0, 6s6p 1P1, 6s6p 3P1 constitute those levels of the V system, the states 6P1, 650 and 6D2 constitute those of the A system. Such conditions of their realization are accessible to experiment.