Local-field effects in a dense ensemble of resonant atoms: Model of a generalized two-level system

The local-field effects are considered which are feasible in a dense ensemb le of resonant atoms modeled by multilevel quantum systems. Our approach is based on the generalized two-level system [V.S. Butylkin, A.E. Kaplan, and Yu.G. Khronopulo, Zh. Eksp. Teor. Fit. 59, 921 (1970)]. Making use of this model, we take account of the nonresonant polarization and Stark shift of the absorption line due to the difference in linear polarizabilities of ato ms in ground and excited states. With near dipole-dipole interaction and qu adratic Stark effect included simultaneously, new features of the hysteresi s behavior of the population difference as a function of the external-field intensity are predicted. [S1050-2947(49)01504-8].