We show how the application of a coupling field connecting the two lower metastable states of a Λ-system can produce a variety of new results on the propagation of a weak electromagnetic pulse. In principle the light propagation can be changed from subluminal to superluminal. The negative group index results from the regions of anomalous dispersion and gain in susceptibility. PACS number(s): 42.50. Gy, 42.25.Bs, 42.25.Kb

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We investigate whether it is possible to store and retrieve the intense probe pulse from a Λ-type homogeneous medium of cold atoms. Through numerical simulations we show that it is possible to store and retrieve the probe pulse which are not necessarily weak. As the intensity of the probe pulse increases, the retrieved pulse remains a replica of the original pulse, however there is overall broadening and loss of the intensity. These effects can be understood in terms of the dependence of absorption on the intensity of the probe. We include the dynamics of the control field, which becomes especially important as the intensity of the probe pulse increases. We use the theory of adiabatons [Grobe et al. Phys. Rev. Lett. 73, 3183 (1994)] to understand the storage and retrieval of light pulses at moderate powers.

We show that the propagation of light in a Doppler broadened medium can be slowed down considerably eventhough such medium exhibits very flat dispersion. The slowing down is achieved by the application of a saturating counter propagating beam that produces a hole in the inhomogeneous line shape. In atomic vapors, we calculate group indices of the order of 10 3 . The calculations include all coherence effects. PACS number(s): 42.50. Gy, It is now well understood that slow light can be produced by using the electromagnetically induced transparency (EIT) [1,2]. Many experiments have been reported in a variety of atomic and condensed media [3][4][5][6][7][8]. Such experiments reveal that the group velocity of the light pulses depends on the parameters of the control field, which produces EIT. Various applications of slow light have been proposed and realised [9][10][11][12][13][14]. Recently, Bigelow et al. [15] showed that one can produce slow light in systems like Ruby, without the need for applying a control field. They made a hole in homogeneous line in systems, where the transverse and longitudinal relaxation times are of very different order.In this paper, we consider the possibility of producing slow light in a Doppler broadened system. This is somewhat counterintuitive as one would think that Doppler broadening would make the dispersion, or more precisely, the derivative of the susceptibility, rather negligible. We, however, suggest the use of the method of saturation absorption spectroscopy [16][17][18][19][20] to produce a hole of the order of the homogeneous width in the Doppler broadened line. The application of a counter propagating saturated beam can result in considerable reduction in absorption, and adequate normal dispersion to produce slow light. We calculate group index of the order of 10 3 . We illustrate our results using the case of the atomic vapors. However, similar or even more remarkable results on slowing of light can be obtained for inhomogeneously broadened solid state systems, where the densities are large.Consider the geometry as shown in the Fig. 1. Here a modulated pulse of light propagates in the directionẑ in a medium of two level atoms. For simplicity we consider the incident pulse of the formHere m and ν are the modulation index and frequency respectively. A counter propagating cw pump field, E c (t), is used for producing saturationThe effective linear susceptibility χ(ω) of the two level atomic systems which is interacting with the field Eeand E c (t), can be calculated to all orders in the counter propagating field (2). The effective susceptibility χ(ω) is well known from the work of Mollow [21]where ∆ = ω c − ω 1g and δ = ω − ω c are represents the detuning of the pump and probe field respectively. For an atom moving with velocity v, we replace ω c by (ω c + kv). and ω by (ω − kv). The Rabi frequency of the pump is given in terms of the dipole moment matrix element, d 1g , byThe T 1 and T 2 are, respectively, the longitudinal and transverse relaxation times and N is the densi...

We develop models for the propagation of intense pulses in solid state media which can have either saturated absorption or reverse absorption. We model subluminal propagation in ruby and superluminal propagation in alexandrite as three and four level systems, respectively, coupled to Maxwell's equations. We present results well beyond the traditional pump-probe approach and explain the experiments of Bigelow et al. [Phys. Rev. Lett. 90, 113903 (2003)]Science 301, 200 (2003)]] on solid state materials.