Amplitude and phase control of gain without inversion in a four-level   atomic system using loop-transitio
  Amplitude and phase control of gain without inversion is investigated in a four level loop-structure atomic system. Two features are presented. One is that gain without inversion can be obtained through the amplitude control of the applied fields. The other is that gain without inversion show a phase-dependence on the relative phase between the fields applied on the two two-photon transitions. Gain and phase-dependence originate from interference induced by such a loop-transition structure. 
Jinhua Zou, Dahai Xu and Huafeng Zhang
physics.optics physics.atom-ph