We study theoretically the phenomenon of enhanced backscattering ͑EBS͒ from a bulk disordered gain medium due to the presence of a dye and a pump beam. We show that the presence of the gain, in combination with the effect of saturation, can dramatically sharpen the EBS peak function. In the particular case of a point source incidence for the probe beam along with the presence of a plane-wave pump beam, the EBS approaches a ␦ function in angular space.The phenomenon of enhanced backscattering ͑EBS͒ of light from a bulk disordered medium constitutes perhaps the clearest example of the importance of phase coherence in optical propagation in the multiple-scattering regime. Following its first experimental observation in the early 1980s, 1 this effect has received much attention in the optics community.2,3 The scientific importance of the EBS is further enhanced by its direct analogy to the quantum correction of conductance in disordered metals at low temperatures due to weak localization, 4 as both effects originate from the perfect phase coherence of time-reversed interfering multiplescattering paths in the backscattering direction. More recently, the phenomenon of EBS has been analyzed in the context of light scattering from random rough surfaces, 5,6 and it is hoped that the EBS peak function can be used to extract useful information about the geometrical and scattering properties of the random surfaces under study.The study of EBS has been mainly confined to various random systems in linear optics. Recently, several authors started to address the issue of what would happen in the nonlinear optical regime. In particular, Kravtsov, Agranovich, and Grigorishin 7 showed that in the case of secondharmonic generation ͑due to 2 nonlinear susceptibility 8 ͒ the EBS peak height for second-harmonic light is very much reduced from the usual factor of 2, due to the restriction that only second-harmonic light generated near the sample surface contributes to the EBS signal, whereas those generated anywhere in the bulk contribute to the diffuse background. Later Agranovich and Kravtsov 9 predicted that when the 3 nonlinear susceptibility 8 is taken into account, the EBS peak function of linear optics must be modified by the appearance of a narrow ''dip'' very close to the backscattering direction, although this result has recently been challenged by Heiderich, Maynard, and van Tiggelen. 10In this paper, we study how the EBS peak is modified in a disordered multiple-scattering medium by a different source of nonlinearity, namely when the probe light can gain intensity due to stimulated emission. Initial treatment of this problem was provided by Zyuzin recently, 11 who, however, did not take into account the important effect of saturation. We shall devise a simple model in the present paper to address this physical effect.A concrete, experimentally relevant, system we have in mind is a dye-dissolved fluid containing a high concentration of scatterers ͑e.g., polystyrene spheres͒. For simplicity, we model the dye as a three-level ...
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