Radiative atom-atom interactions in optically dense media: Quantum corrections to the Lorentz-Lorenz formula

Generalized single-atom Maxwell-Bloch equations for optically dense media a re derived taking into account noncooperative radiative atom-atom interacti ons. Applying a Gaussian approximation and formally eliminating the degrees of freedom of the quantized:radiation field and of all but a probe atom le ads to an effective time-evolution operator for the probe atom. The mean co herent amplitude of the local held seen by the atom is shown to be given by the classical Lorentz-Lorenz relation. The second-order correlations of th e field lead to terms that describe relaxation or pump processes and level shifts due to multiple scattering or reabsorption of spontaneously emitted photons. In the Markov limit a nonlinear and nonlocal single-atom density m atrix equation is derived. To illustrate the effects of the quantum correct ions, we discuss amplified spontaneous emission and radiation trapping in a dense ensemble of initially inverted two-level atoms and the effects of ra diative interactions on intrinsic optical bistability in coherently driven systems. [S1050-2947(99)05703-0].