The Studies Of Atomtic Coherence Effects

The atomtic coherence effects has one of the important forward subjects in laser physics and quantum optics, recent studies has shown that the quantum coherence and interference has led to a lot of new effects such as electromag-netically induced transparency, coherent population trapping, switching from subluminal propagation to superluminal propagation, etc. These new effects have great potential for the control of the coherence properties of a medium, frequency conversion of lasers,the information storage and propagation.In this paper we have studies two kinds of new phenomena of atomtic coherence effects. One is switching from subluminal into superluminal propagation in a three-level V system driven by a single coherent field and the other is efficient parametric amplification without coherent population trapping in driven A system and cascade system.First, we consider the switching of velocity. The significant feature of the system model is that a single coherent field drive the near-degenerate V system. The driving field is tuned resonance with the average frequency of two dipole-allowed transitions. Increasing the driving field intensity, the medium's dispersion changes from normal to anomalous, which leads to the switching from subluminal to superluminal light pulse propagation.Next we consider the quantum coherent and interference of four incident fields can produce efficient parametric amplification. The models are A system and cascade system. Contrary to conventional wisdom, we find that in the A system, the parametric signal is stronger without CPT than with CPT. Compared driven V system with driven cascade system, we show that the driven cascadesystem has the advantage of realizing efficient parametric amplification: (i)with the same parameter, parametric signal takes place stronger in driven cascade system than in driven A system with CPT. (ii)within a certain range, control one or two pump-atoms detuning 5 in driven cascade system, we can enhance parametric amplification efficiently, (iii) under the same intensities of the pump fields, the generation of signal is stronger in driven cascade when we are away from resonance of the pump fields than we are resonance of the pump fields. That is relaxed restrictions on the resonance between pump fields and atoms.