Quantum Coherence In Λ Three-level System Driven By Two Fields

This thesis presents a theoretical study of the quantum coherence in a ?- three-level system with bichromatic coupling field and a weak probe field and with two coupling fields and a weak field. The law of the probe absorption spectrum with the frequency and the strength of coupling field were analysed and discussed.To the ?- three-level system driven by bichromatic coupling field and a weak probe field, when one component of the strong bichromatic coupling field is resonant with corresponding transition and the other is detuning with an integer fraction of the Rabi frequency of the resonant field, the absorption spectrum exhibits a series of doublets and the number of peaks changes regularly with the variation of detuning of the coupling field. When both components of the bichromatic coupling field are symmetrically detuned from the transition, the spectrum also displays a series of symmetrical doublets, the number of absorption peaks and the strength of absorption peaks change regularly with intensity of the coupling field. In addition, the absorption peaks will disappear alternantly with intensity of the coupling field increase.To the ?- three-level system driven by two coupling fields and a weak probe field, when the two coupling fields are weak, The probe absorption spectrum exhibits two electromagnetically induced transparency (EIT). The position of EITs change with frequency of the two coupling fields. The two EITs will superpose when the two coupling fields are exactly resonant with corresponding transitions. The interval of two EITs is decided by the detuning of coupling fields, when one of the coupling fields is detuning with corresponding transition. On the other hand, the property of probe absorption spectrum is dicussed when one of the coupling fields is most stronger than the other. The absorption spectrum exhibits two electromagnetically induced absorptions (EIA) and a Mollow. With the increasing of the Rabi frequency of the stronger coupling field, the position of two EIAs will move to two sides; the strength of two EIAs and the gain of Mollow will decrease correspondly and the split extent of Mollow will increase.