Studies Of Absorption And Dispersion Properties Of The Three-level Cascade Medium

The dispersion and absorption properties of optical medium have been extensively applied in quantum optics and nonlinear optics. Recently great interest has been directed to the modification of dispersion and absorption properties via atomic coherence induced by coherent fields. In fact, the use of the dispersive properties is often limited by the absorption. The dispersive properties can be effectively used when the absorption or gain of the medium is small. Based on the effect of quantum coherence and interference, the dispersion-absorption relations can be rapidly modified by the application of a strong coherent field. The resonant absorption was eliminated and the dispersive properties of the medium can be enhanced. Manipulation of light speed in an optical medium can be realized by changing the dispersive properties of the medium. These have led to the observation of many new effects and new techniques in quantum optics and atomic physics. Examples include Electromagnetically Induced Transparency, Lasing Without Inversion, the subluminal and superluminal light propagation, and Enhancement of the Index of Refraction Without Absorption.We suggest that a three-level cascade medium is used to achieve the switching from normal to anomalous dispersion. In the cascade medium, two dipole-allowed transitions are coupled by two different coherent fields, respectively. A probe field is applied either to the upper transition or to the lower transition. The equations for density matrix elements are transformed into the set of equations with time-independent coefficients at the steady state via harmonic expansion, which are solved by matrix inversion. By the solutions we obtain the absorption and dispersion spectra. It is shown that for each case, the mediumexhibits either normal or anomalous dispersion at the same frequency regime. The switching from normal to anomalous dispersion is obtained by varying the intensity of one of two driving fields. The two differences lie between the proposed cascade system and the A and V systems. Firstly, near degenerate levels are required for the A and V systems when the probe field is simultaneously coupled to the two transitions to achieve dispersion switching. The present scheme is based on an equivalent realization of near degeneracy in the dressed picture, which is controlled by varying the intensity of the corresponding driving field. Secondly, dispersion switching does not occur in the A and V systems when the probe field is only coupled to either of two transitions and the decoupled state plays a dominant role. In contrast, the present scheme does not involve the decoupled state.On the other hand, 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 gets transparent while the index is not desirable. In this paper, we investigate the dispersive properties in the same three-level cascade medium. We found that in the three-level atomic mediums, the refractive-index enhancement of two probe fields without absorption can be obtained by controlling the intensity of driving fields.