Amplification without inversion in a medium with collisional dephasing

Abstract

Amplification without inversion in the presence of collisions in V and \ensuremath{\Lambda} systems is studied within the framework of density-matrix formalism. We show that while it is possible to achieve amplification without inversion close to Raman resonance in both systems, it is affected by collisions in a different way. In the V system, collisions enhance quantum interference, whereas in the \ensuremath{\Lambda} system they quench it. This effect depends crucially on the collision characteristics, essentially on the relative values of the collisional relaxation rates for the Raman coherence and for the optical coherences. For the \ensuremath{\Lambda} system, different incoherent pumping processes are studied: pumping via the upper level and direct pumping between the two low-energy levels. The direct pumping process is interesting for the frequency up-conversion of the coherent and incoherent pump fields. Furthermore, with direct pumping the gain always occurs without inversion.