High refractive index and lasing without inversion in an open four-level atomic system

In this letter, a novel atomic scheme is proposed to study the transient evolution of the atomic response with applications to lasing with and without population inversion. We introduce an open four-level atomic medium and compare its transient properties with the corresponding closed system. The impact of cavity parameters i.e. the atomic exit rate from cavity and atomic injection rates on transient response of weak probe field of open system are investigated. It is realized that existence of cavity parameters leads to some interesting results such as large amplification, high refractive index without absorption as well as lasing with and without inversion. These results cannot be obtained in corresponding closed system, due to lack of atomic exit and injection rates. This extra controllability and flexibility, makes open four-level system much more practical than its counterpart closed one.