Studies Of Electromagnetically Induced Transparency And Enhancement Of The Index Of Refraction Without Absorption

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 in a vacuum. Without exception, the behavior of negative dispersion (i.e., rapid decrease of index of refraction with increasing frequency, which leads to superluminal light propagation) in the central spectrum is destroyed by the large absorption. 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 observation of many new effects and new techniques in quantum optics and atomic physics. Examples include Coherent Population Trapping, Electromagnetically Induced Transparency, 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 the dispersive and absorptive properties of a system of three-level cascade atoms driven by a strong coherent field. Three characteristic features are found. First, for the same set of atom-light interaction parameters, the indices of refraction are large at three different frequencies where the absorption vanishes. These three frequencies are determined by the resonance transition frequencies between dressed states produced by the strongdriving field. Second, negative dispersion without absorption, which leads to superluminal light propagation, is achievable in the central resonance structure of the dispersion spectrum. The negative dispersion is slightly weakened as the spacings between dressed states increase. Third, the whole absorption spectrum displays, in general, three pairs of absorption peaks and three pairs of gain peaks. Due to the coherences between dressed states, these peaks are not exactly at the resonant resonance frequencies between dressed states, but in the vicinity of them. The separability of the out adjacent gain peaks is limited by the minimal spacing between dressed states. The outer adjacent gain peaks are well separated from each other when the minimal spacing between dressed states is far larger than the atomic decay rates. Finally, These features have been analyzed in terms of dressed states produced by the driving field.It is well known that Electromagnetically Induced Transparency(EIT) occurs because of the absorption cancellation by atomic coherence and interference. In a three-level A system, the condition of establishing EIT is the exact two-photon resonance. As long as the exact two-photon resonance is satisfied, and no matter what the one-photon detuning is, EIT can be achieved. In this paper we investigate the one-photon detuning tolerance when the strong nonlinear optical effects take place in the system based on perturbed EIT. Using the semiclassical theory, We calculate the correction of the detuning to the absorption and dispersion, and we investigate the impact of the one-photon detuning under the exact two-photon resonance to large Kerr nonlinearity and photon switching based EIT. The result is that the one-photon detuning will be limited for obtaining the expected nonlinear optical effects in the system based on perturbed EIT. This limited condition not only depends on the Rabi frequenciesof the coupling field and the signal field, but also depends on the detuning or the decay rate of the additional transition. In detail, one-photon detuning for large Kerr nonlinearity and photon switching via EIT should be far less than the product of the ratio of intensity of the coupling field to the intensity of the signal field, and the detuning or the decay rate of the additional transition.