1. For a nice survey see for example, L. Allen and J.H. Eberly Optical Resonance and Two-level Atoms, John Wiley and Sons (New York), 1976.
2. A very extensive literature is available on two-photon processes. For a critical comparison of different theoretical approaches and additional references see for example: D. Grischkowski, in Laser Applications to Optics and Spectroscopy, Vol. II of Physics of Quantum Electronics Eds. S.F. Jacobs, M. Sargent III, J.F. Scott, M.O. Scully (Addison-Wesley 1975), D. Grischkowski, M.M.T. Loy, P.F. Liao, Phys. Rev. A12, 2514 (1975).
3. F. Biraben, B. Cagnac, G. Grynberg, Phys. Rev. Lett. 32 643 (1974), M.D. Levenson, N. Bloembergen, Phys. Rev. Lett. 32, 645 (1974).
4. E.M. Belenov, I.A. Poluektov, Sov. Phys. JETP 29, 754 (1969). M. Takatsuji, Physica 51, 265 (1971); Phys. Rev. B2, 340 (1970). N. Tan-no, K. Yokoto, H. Inaba, Phys. Rev. Lett. 29, 1211 (1972).
5. M. Takatsuji, Phys. Rev. All, 619 (1975). F.H.M. Faisal, J. Phys. B., 10, 2003 (1977). J.C. Diels, Opt. Quant. Electron 8 513 (1976). H.P. Yuen, F.Y.F. Chu, Proceedings of the Fourth Rochester Conference on Coherence and Quantum Optics, Eds. L. Mandel and E. Wolf (to be published).