Two mechanisms for population inversion via two-colour coherent excitation inthree-level systems

Abstract

We analyse the physical origin of population inversion via continuous wave two-colourcoherent excitation in three-level systems by dressing the inverted transition. Twodifferent mechanisms are identified as being responsible for the population inversion. ForV-configured systemsand cascade (Ξ)configured systems with inversion on the lower transition, theresponsible mechanism is the selective trapping of dressed states, andthe population inversion approaches the ideal value of 1. For Λ-configured systemsand Ξ-configuredsystems with inversion on the upper transition, population inversion is based onthe selective excitation of dressed states, with the population inversion tendingtowards 0.5. As the essential difference between these two mechanisms, theselective trapping of dressed states occurs in systems with strong decay intodressed states while the selective excitation appears in systems with strong decayout of dressed states.