1. See for example,Multiphoton Processes, Ed. by J. H. Eberly and P. Lambropoulos, (J. Wiley, NY, 1978).
2. See for example, J. E. Bjorkholm in ?Nonlinear Spectroscopy: Course LXIV Proc. of the Int. School of Physics ?Enrico Fermi??, Ed. by N. Bloembergen (North Holland, Amsterdam, 1977).
3. A. Javan, Phys. Rev.107, 1579 (1957).
4. R. L. Panock and R. J. Temkin, IEEE J. Quan. Electron.QE-13, 425 (1977).
5. M. Takami, Japan J. Appl. Phys.15, 1063 (1976).