FEASIBILITY OF LASING WITHOUT INVERSION IN THE 2-LEVEL SYSTEM WITH SPLIT UPPER LEVELS COUPLED BY MICROWAVE-RADIATION

Zhu, Cresser, and Fearn [preceding Comment, Phys. Rev. A 43, 5170 (1991)] give an erroneous formal proof that lasing without inversion in the system with two upper optically identical levels is impossible. Their approach has little to do with the case of two close but different, well-separated split levels, considered in our previous paper [Blok and Krochik, Phys. Rev. A 41, 1517 (1990)]. It is evident even without laborious calculation that lasing without inversion is impossible in the system of indistinguishable levels, considered by Zhu, Cresser, and Fearn, because one cannot provide the interference among identical transitions. Their formal proof is, in addition, incorrect because they apply a steady-state approximation to the conditions where it is not valid; the transient response of the medium must be calculated instead. We also used the steady-state approximation in our previous paper, but very cautiously, in a domain where it is valid to obtain some analytical formulas, which proved to be qualitatively correct under the transient response approach [Blok and Krochik (unpublished)].