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Optical gratings in the collective interaction between radiation and atoms, including recoil and collisions

The introduction of collisions and of a thermal distribution for the atomic momentum in the model for the Collective Atomic Recoil Laser (CARL) is at the origin of important modifications in the interpretation of the mechanisms that give rise to the amplification of the backreflected wave. It is shown that the atomic density grating, considered to be the cause of gain in CARL, disappears in the presence of collisions, while other gratings—in population and polarization phase—survive. While the population grating appears to be merely a consequence of the collective interaction, the latter is the likely cause for the instability. Finally, simulations show that models that make use of an exponential relaxation mechanism for the atomic momentum,rather than accounting for collisions explicitly, largely overestimate the strength of the interaction.