Reduced model for the description of radiation-matter interaction including atomic recoil

We show that a model for the collective atomic recoil laser, previously introduced to include collisions with an external buffer gas, can be reduced to a single dynamical equation for the probe amplitude. This is the result of a clever adiabatic elimination of the atomic variables and of the assumption of a negligible effect of the probe field onto the atomic motion. This reduced model provides a fairly accurate description of the phase diagram of the original set of equations and allows for the investigation of more realistic regimes, where the direct simulation of the full model would be otherwise unfeasible. As a result, we find that the onset of a coherent field can be either described by a second- or first-order transition, the former scenario being observable only below a given temperature. Moreover, the first-order transition is accompanied by an intrinsic optical bistability region.