Local-field effects and configurational disorder in nonlinear optical systems with coherence

We argue that in nonlinear optical systems with randomly distributed atoms one should go beyond the Clausius-Mossoti limit in order to take into account the effect of local-field fluctuations induced by configurational disorder in atom positions. This effect is analyzed by means of a random local-mean-field approach with the neglect of the correlations between dipole moments of different atoms. The formalism is applied to three-level Λ-type systems with quantum coherence possessing an absorptionless index of refraction and lasing without inversion. We show that the effect of configurational fluctuations results in the significant suppression of the atom susceptibility compared to the predictions based on the Clausious-Mossoti equation.