Enhanced four-wave mixing via elimination of inhomogeneous broadening by   coherent driving of quantum transition with control fields
  We show that atoms from wide velocity interval can be concurrently involved in Doppler-free two-photon resonant far from frequency degenerate four-wave mixing with the aid of auxiliary electromagnetic field. This gives rise to substantial enhancement of the output radiation generated in optically thick medium. Numerical illustrations addressed to typical experimental conditions are given. 
Alexander K. Popov and Alexander S. Bayev