1. L. Allen, J.H. Eberly, Optical Resonance and Two-Level Atoms (Wiley, New York, 1975)
2. A. Haug, Festkörperprobleme XII, 411 (1975)
3. V.S. Letokhov, V.P. Chebotayev, Nonlinear Laser Spectroscopy, Springer Series in Optical Sciences, vol. 4 (Springer, Berlin, 1977)
4. R.L. Shoemaker, in Coherence and Laser Spectroscopy. ed. by J.I. Steinfeld (Plenum, New York, 1978), p.197
5. A. Haug, J. Lumin. 20, 173 (1979). ((and references therein))