1. See, for example, Y.R. Shen: Surf. Sci. 299/300, 551 (1994)
2. Y.R. Shen: The Principles of Nonlinear Optics (J Wiley, New York 1984) Chap. 25
3. Y.R. Shen: In Fundamental Systems in Quantum Optics, ed. by J. Dalibard, J.M. Raimond, J. Zinn-Justin (North Holland, Amsterdam 1992) Course 15
4. Y.R. Shen: In Frontiers in Laser Spectroscopy, Proc. Int'l School of Physics ?Enrico Fermi,? ed. by T.W. Hänsch (Academic, New York 1994)
5. N. Bloembergen, R.K. Chang, S.S. Jha, C.H. Lee: Phys. Rev. 174, 813 (1968) and references therein