Nondegenerate parametric down conversion in coherently prepared   two-level atomic gas
  We describe parametric down conversion process in a two-level atomic gas, where the atoms are in a superposition state of relevant energy levels. This superposition results in splitting of the phase matching condition into three different conditions. Another, more important, peculiarity of the system under discussion is the nonsaturability of amplification coefficients with increasing pump wave intensity, under "sideband" generation conditions. 
Gevorg Muradyan, Atom Zh. Muradyan