Theoretical explanation of the first experimentally observed laser without inversion in a two-level scheme

The theory of lasing in an ensemble of driven two-level atoms is shown to agree satisfactorily with the experimental observation of lasing without inversion by Grandcl\'ement, Grynberg, and Pinard [Phys. Rev. Lett. 59, 40 (1987); 59, 44 (1987)]. It is demonstrated that the lasing is dominated by the multiphoton amplification processes induced in the medium.