Electric susceptibility of a laser-dressed atomic medium is calculated for a model Λ-like system including two lower states and a continuum structured by a presence of an autoionizing state or a continuum with a laser-induced structure. Depending on the strength of a control field it is possible to obtain a significant reduction of the light velocity in a narrow frequency window in the conditions of a small absorption. A smooth transition is shown between the case of a flat continuum and that of a discrete state serving as the upper state of a Λ system. PACS numbers: 42.50.Gy, 32.80.-t * Electronic address: raczyn@phys.uni.torun.pl 2 It has been known for a long time that atoms in the Λ configuration irradiated by two laser fields may exhibit peculiar dynamical properties. In particular the population may be trapped in the so-called dark state, being a combination of the two lower states. It constitutes a basis for subtle coherent dynamical manipulations like an efficient population transfer (stimulated Raman adiabatic passage -STIRAP), robust against any broadening of the upper state [1,2]. Effects of such a type are present also in the case in which the upper state of a Λ configuration has been replaced by a continuum [3,4,5] or, in strong laser fields, by a set of coupled continua [6].Dynamical effects in particular atoms are reflected in unusual light propagation effects in atomic media. One of the most important propagation effects in a medium of atoms in the Λ configuration is the electromagnetically induced transparency (EIT), which consists in making the medium transparent for a weak probe laser beam by irradiating it by a strong copropagating control beam [7,8]. Instead of an absorption line there appears a transparency window and a normal dispersion. By switching the control field off when the pulse is inside the sample one can slow the light pulse down and finally stop or store it [9,10,11]. The photon energy of the signal is transferred to the control beam while the information about the pulse is written down in the form of a coherence (nondiagonal element of the density matrix) between the two lower states. Switching the signal on again results in reading the information out: the pulse is reconstructed in a coherent way.A natural question arises whether similar propagation effects may occur in the case of a continuum serving as an upper state. Some answer has been given by van Enk et al. [12,13], who considered a general case of two beams propagating inside the medium. Their conclusion was that a necessary condition for EIT was that the asymmetry parameter in the continuum model was zero. In their case the losses due to photoionization are of second order with respect to the probe field. Thus, in the linear approximation with respect to the probe beam and neglecting propagation effects for the strong control beam, one still may apply the standard approach in which the probe propagation is discussed in terms of an atomic susceptibility. Below we give analytic expressions for the susceptibility in the...

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