We investigate the dispersion and the absorption properties of a weak probe field in a three-level pump-probe atomic system. It is shown that the slope of dispersion changes from positive to negative just with the intensity of the coherent or indirect incoherent pumping fields. It is demonstrated that the absorption free superluminal light propagation is appeared in this system. Keywords: quantum coherence, susceptibility, group velocity Propagation of electromagnetic pulse in a dispersive medium has been a topic of recent study in the field of quantum coherence and interference [1][2][3][4]. Lord Rayleigh discussed the phenomenon of anomalous dispersion of a wave group in 1899 [5]. It is well known that the group velocity of a light pulse can be slowed down, or it can become greater than c (the speed of light in vacuum) or even become negative in a transparent medium [6]. The effect of superluminality is that the emerging pulse has essentially the same shape and width as that of the incident wave packet, but its peak travels with a velocity higher than c and even exits the medium before the incident pulse enters. This processes can be understood in terms of superposition and interference of traveling plane waves that have a distribution of frequencies and add up to form a narrow-band light pulse [7,8]. A rather simple mathematical proof shows that fast light behavior is completely consistent with Maxwell's equations describing pulse propagation through a dispersive material and hence does not violate Einstein's special theory of relativity (the special theory of relativity is based on Maxwell's equations) [9]. Intriguing question arise whether one can have a controlling parameter in a single experiment for switching from the normal to anomalous dispersion. Various schemes have been proposed to switching from subluminal to superluminal light propagation in an atomic medium. The most important key to successful experiments on subluminal and superluminal light propagation lies in its ability to control the optical properties of a medium by coherent or

Transmission and reflection of an electromagnetic pulse through a dielectric slab doped with the quantum dot molecules are investigated. It is shown that the transmission and reflection coefficients depend on the inter-dot tunneling effect and can be simply controlled by applying a gate voltage without any changing in the refractive index or thickness of the slab. Such simple controlling prepares an active beam splitter which can be used in all optical switching, optical limiting, and other optical systems.

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Abstract. The propagation of a weak probe field in a four-level N-type quantum system in the presence of spontaneously generated coherence (SGC) is theoretically investigated. The optical properties of the system are studied and it is shown that the group velocity of light pulse can be controlled by relative phase of applied fields. By changing the relative phase of applied fields, the group velocity of light pulse changes from transparent subluminal to the transparent superluminal light propagation. Thus, the phase-controlled absorption-free superluminal light propagation is obtained without applying an incoherent laser fields to the system. The propagation of a weak probe light pulse is studied by solving the Maxwell's wave equation on numerical grid in space and time. Moreover, we study the third order self-and cross-Kerr susceptibility of probe field and calculate the nonlinear cross-phase shift for different values of intensity of applied fields. In addition, we take into account the effect of Doppler broadening on the light pulse propagation and it is found that a suitable choice of laser propagation directions allows us to preserve our results even in the presence of Doppler effect. It is demonstrated that by increasing the Doppler width of distribution to the room temperature, the dispersion changes from transparent subluminal to transparent superluminal light propagation which is our major motivation for this work.

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