On Electromagnetic Wave Interaction with Dense Resonant Atom Medium
  The excitation of atomic levels due to interaction with electromagnetic waves became of interest in accelerator physics in relation to high efficiency charge exchange injection into rings for high beam power applications. Usually, the beam density is so small that its influence on the wave is completely neglected. Here we consider the case of dense beams - the beam dimensions are large as compared to light reflection length. This paper shows that the waves can be trapped in the medium under these conditions. Moreover, the atoms with induced dipole moments start to interact strongly with each other, leading to possibility to create some atomic patterns when the medium is relatively cold. 
V. Danilov Ornl