Introduction

Local field effects that occur due to dipole-dipole interactions have been vigorously investigated since the initial works of Lorentz[1] and Lorenz[2]. Linear interactions of light in condensed matter and dense gases are manifest in the Clausius-Mossotti relations[3], and nonlinear effects have been known for some time[4]. 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[7], and Rand and coworkers have observed intrinsic bistability due to local field effects in a crystal lattice highly doped with rare earth ions[8]. Most recently, Eliel and coworkers have studied local field effects in very high density metal vapors[9], 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.

The paper by Yelin and Fleishhauer treats, for the first time, near dipole-dipole interaction effects from the framework of a fully quantum mechanical Greenís function and evolution operator approach. They predict macroscopic quantum contributions which give rise to atomic excitation dependent incoherent pumping-like contributions and spectral shifts. The work of Crenshaw, Sullivan, and Bowden predicts atomic excitation dependent line narrowing due to combined dipole-dipole interactions among a dense collection of two-level atoms, and their reciprocal interactions with a dielectric host medium. These coherence effects, as well as the quantum incoherent effects predicted by Yelin and Fleischhauer may serve direct interpretation in relation to results of the university of Leiden experiments[9].

Dynamical Lorentz shifts in an optically dense superradiant amplifier are analyzed by Manassah and Gross. They demonstrate amplifier length dependent spectral shifts in the transmitted and reflected signals which are modified by the dynamical Lorentz shift. Vugmeister, Bulatov, and Rabitz show departure from Clausius-Mossotti relations due to combination of configurational disorder and local field effects. An earlier paper, but one strongly related to this collection of contributions, is that by G. S. Agarwal, Macroscopic Approach to Coherent Population Trapping State and its Relaxation in a Dense Medium, in which he considers the effects of local fields on population trapping in a two-field, three level lambda system. The effects of spontaneous decay on the translational dynamics of atoms incident, or localized in a cavity have been analyzed by Japha, Kurizki, and Akulin.

The collection of papers compiled in this focus issue address a broad range of topics in relation to the nonlinear and quantum optics and dynamics of atoms involving local field interactions. It is anticipated that this collection of theoretical contributions will manifest significance in stimulating further theoretical and experimental investigations, and facilitate crucial interpretation of recent and current experimental investigations.

We, as Special Focus Issue Coordinators, take this opportunity to express much appreciation to the authors of the papers that constitute this Special Focus Section. Each of the solicitations for papers was made consistent with recent contributions to the field by the respective authors. It has been our pleasure to serve this effort.