2 MECHANISMS FOR INVERSIONLESS AMPLIFICATION IN 4-LEVEL ATOMS WITH RAMAN PUMPING

We analyze the physical origin of the inversionless amplification in f our-level atoms driven by a Raman field and identify the existence of two different mechanisms for the predicted gain. The first mechanism i s connected with the appearance of a population inversion between the atomic states dressed by the coherent pump, while the second is linked to the buildup of coherence among these states. Proper selection of t he relaxation parameters can make either the first or the second mecha nism dominant over the other although usually both coexist.