Polarisation selection rules and disorienting collision effects in resonant degenerate four-wave mixing

Various polarisation selection rules guiding phase conjugate emission via degenerate four-wave mixing are discussed. It is shown that, in resonant gas media, depolarising collisions may induce phase conjugation in cases when this non-linear process in otherwise forbidden. However, Doppler broadening often limits the efficiency of non-collinear collision-induced four-wave mixing and introduces a very rapid decrease of the reflectivity with increasing angular separations between pump and probe beams.