Figure 1: (a) Vapor cell: Dilute atomic vapors near resonance can be modeled as two-level systems in a vacuum. At low densities, the interaction between atoms is negligible, and it is sufficient to consider the interaction of a single particle with the field when developing equations of motion. (b) Dense medium: At high densities, the particles interact via the electromagnetic field. The Lorentz local-field condition (LLFC) leads to the Clausius - Mossotti - Lorentz - Lorenz (CMLL) relation for linearly polarizable particles and to near dipole-dipole interaction for two-level systems. (c) Multicomponent medium with dilute nonlinear component: We consider optically nonlinear condensed mater comprised of two polarizable components. If the density of nonlinear particles is sufficiently low then interaction between the nonlinear particles is negligible. (d) Dense nonlinear multicomponent media: When near dipole-dipole interactions are significant due to a high density of two-level atoms, local-field effe associated with the presence of an optically linear material component lead to local-field enhancement of the NDD interaction, local cooperative decays, and coherence exchange processes. [Media 1] [Media 2] [Media 3] [Media 4]

Figure 2: Calculation of the near field: Individual dipoles are used in the calculation of E _near. In cubic symmetry E _near = 0, but here the symmetry of the lattice is altered by the presence of a second species. [Media 5]

Figure 3: Transition from microscopic to macroscopic variables: The transition from microscopic to macroscopic variables is obtained by integrating over a large number of particles. For a dense multi-component medium, all species of particles are included in the volume of integration. [Media 6]