Abstract

Considerable discussion has recently taken place concerning the possibility of obtaining stimulated emission without population inversion. In 1989, Harris^[1] developed a theory to show that, when two upper levels of a four-level system are purely lifetime broadened and decay spontaneously to the same continuum, there will be a destructive interference in the absorption profile of lower-level atoms, which is not present in the stimulated emission profile of upper level atoms, thus resulting in laser gain without inversion. In this process, the upper levels are coupled by spontaneous decay to the same final continuum or discrete level. More recently, Harris, Field and Imamoglu^[2] extended this concept to nonlinear processes, and showed that application of strong electromagnetic-field coupling, between metastable and upper states, can resonantly enhance the nonlinear susceptibility of the medium, and at the same time induce transparency and a zero in the contribution of the resonance transition to the refractive index. Moreover, they predicted that many orders of improvement in conversion efficiency could be achieved because of the induced transparency.

© 1992 IQEC