Local field effects that occur due to dipole-dipole interactions have been vigorously investigated since the initial works of Lorentz and Lorenz. Linear interactions of light in condensed matter and dense gases are manifest in the Clausius-Mossotti relations, and nonlinear effects have been known for some time. Generalized Maxwell-Bloch formulation incorporating local field effects[5, 6] has stimulated considerable theoretical development and predictions, but to date few experiments have been performed. Several experiments, however, have addressed key issues stemming from various theoretical developments. Boyd and coworkers have observed local field induced spectral red shifts in rubidium vapor, and Rand and coworkers have observed intrinsic bistability due to local field effects in a crystal lattice highly doped with rare earth ions. Most recently, Eliel and coworkers have studied local field effects in very high density metal vapors, and find novel cooperative effects and excitation dependent line narrowing. The papers presented in this Focus Issue are indicative of the currently increasing interest and activity in this field.
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