Distinguishing nonlinear processes in atomic media via orbital angular   momentum transfer
  We suggest a technique based on the transfer of topological charge from applied laser radiation to directional and coherent optical fields generated in ladder-type excited atomic media to identify the major processes responsible for their appearance. As an illustration, in Rb vapours we analyse transverse intensity and phase profiles of the forward-directed collimated blue and near-IR light using self-interference and astigmatic transformation techniques when either or both of two resonant laser beams carry orbital angular momentum. Our observations unambiguously demonstrate that emission at 1.37 {\mu}m is the result of a parametric four-wave mixing process involving only one of the two applied laser fields. 
Alexander M. Akulshin, Russell J. McLean, Eugeniy E. Mikhailov, and   Irina Novikova
quant-ph physics.atom-ph physics.optics