Amplification without population inversion
Additional Document Info
Three- and four-level schemes of inversionless amplification with different types of coherent pumping is reviewed. Amplification without inversion between the 'bare' states is a result of population inversion between eigenstates of the density matrix. In particular, it can be achieved due to the coherent population trapping, i.e., effective removal from the interaction with the field a fraction of atoms at lower levels.