| There appear some novel phenomena, such as EIT(electromagnetically induced transparency) and LWI(lasing without inversion) and so on, in the interaction of a radiation field with an atom due to the quantum interference among the atomic levels induced by coherent fields. These phenomena display attractively academic and applied values in many fields such as quantum memories for light, nonlinear optics and the generation of higher-frequency lasers and so on.This thesis mainly discusses the physics mechanism of lasing without inversion and proposed a closed four-level atomic model for the lasing without inversion. Compared to typical closed three-level atomic model, the four-level atomic model is no longer incoherently pumped on the transition at which lasing occurs. The four-level system can overcome some limits in the typicalΛ-type and V-type three-level systems, furthermore it is possible to achieve lasing even in the absence of the inversion condition for the thermal radiation fields.We find that when the detuning of probe field is equal to that of coherent coupling field, there is a peak value along with the increase of the detuning. To explain it, we bring forward that there are adiabatic and nonadiabatic stimulated Raman processes when a three-level atomic system interacts with two coherent fields, so there are two mechanisms influencing the change of probe field. We believe that it can explain more general lasing without inversion processes because of quantum interference induced by coherent fields.Finally,we discuss theΛ-type open three-level atomic system driven by a strong coherent field when both inherent pumping and decay are considered generally, we obtain the conditions under which theΛ-type open three-level atomic system can exhibit lasing without inversion . We also explain the quantum interference process of three-level system which exhibits lasing without inversion by analytically solving the Schr?dinger equation of atoms on appropriate conditions.
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