Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://link.aps.org/doi/10.1103/PhysRevA.48.3196 (Publisher version)
Keitel, C. H., Kocharovskaya, O. A., Narducci, L. M., Scully, M. O., Zhu, S. Y., & Doss, H. M. (1993). Two mechanisms for inversionless amplification in four-level atoms with Raman pumping. Physical Review. A, 48(4), 3196-3202. doi:10.1103/PhysRevA.48.3196.