1. B. DiBartolo: Optical Interactions in Solids (Wiley, New York 1968), Chap. 15; also S. Hüfner: Optical Spectroscopy of Transparent Rare-Earth Compounds (Academic Press, New York 1978)
2. A.L. Schawlow: In Advances in Quantum Electronics, ed. by J.R. Singer, (Columbia University Press, New York 1962) p. 50
3. D.E. McCumber, M.D. Sturge: J. Appl. Phys. 34, 1682 (1963)
4. G.F. Imbusch, W.M. Yen, A.L. Schawlow, D.E. McCumber, M.D. Sturge: Phys. Rev. 133, A1029 (1964)
5. S. Geschwind, G.E. Devlin, R.L. Cohen, S.R. Chinn: Phys. Rev. 137, A1087 (1965)