Coherent Population Trapping And Optical Bistability By Modulated Fields

The dispersion and absorption are a pair of fundamental properties of optical medium. The use of the dispersive properties is often limited by the absorption. The dispersion-absorption relations of the two-level atom system tell us that at or near the resonance frequency, the index of refraction is large but absorption is equally large. Far from resonance, the two-level medium becomes transparent but the index of refraction is almost the same as that in a vacuum. However, the dispersion-absorption relations of the media can be dramatically modified by employing coherent fields to drive it. Quantum coherence and interference have led to the occurrence of many new effects and techniques. Examples include Coherent Population Trapping(CPT), Electromagnetically Induced Transparency(EIT), Lasing Without Inversion, and Enhancement of the Index of Refraction Without Absorption. These new effects have many potential applications in fundamental physics and applied physics. In this paper, we investigate manipulation of the quantum interference in the case of modulated fields in a multilevel atomic system(three -level A system) from different point of view.Firstly, we examine the effects of cross correlated phase fluctuations on CPT induced by a pair of phase-modulated fields with equal modulation frequencies in a three-level A system. The maximal coherence of -0.5, which appears when CPT occurs for equal modulation indices, is preserved in the present of the critically cross-correlated fluctuations. Unexpectedly, the non-maximal coherence, which is established when CPT is obtained for different modulation indices, is significantly enhanced due to the critically cross-correlated fluctuations.Secondly, We present sideband control of optical bistability and multistability based on trichromatic electromagnetic-field induced transparency and quantum interference. Appearance or disappearance of the bistability and multistability, manipulation of the hysteresis hoop widths, and switching between bistability and trista-bility are achieved simply by varying the sideband amplitudes or the sum of relative phases of the sidebands to the central component. And, it should be noted that in our scheme, the optical bistabiltiy and multistability are affected by cooperativity parameter, whose threshold value need be significantly larger than the two-level case. In spite of this, the threshold parameters we present are reasonable for experimental realization.