1. A. Einstein, On the Quantum Theory of Radiation, Physikalische Zeit schrift 18, 121 (1917); this paper has been reprinted in the book by D. Ter Haar, The Old Quantum Theory, Pergamon Press, Oxford (1967).
2. R. Loudon, The Quantum Theory of Light, Clarendon Press, Oxford (1973).
3. K. Thyagarajan and A.K. Ghatak, Lasers: Theory and Applications, Plenum Press, New York (1981).
4. A.K. Ghatak, Optics, Tata McGraw-Hill, New Delhi, (1992).
5. W.R. Salzman, Time evolution of simple quantum mechanical systems:II. Two state system in intense fields, Phys. Rev. Lett. 26, 220 (1971).