Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://www.sciencedirect.com/science/article/pii/003040189290479B (Publisher version)
Fearn, H., Keitel, C. H., Scully, M. O., & Zhu, S. Y. (1992). Lasing without inversion in a simple model of a three-level laser with microwave coupling. Optics Communications, 87(5-6), 323-330. doi:10.1016/0030-4018(92)90479-B.