1. L. F. Mollenauer and D. H. Olson J. Appl. Phys. 46, 3109 (1975).
2. W. Gellermann, K. P. Koch and F. Lüty, Laser Focus, April 1982.
3. G. Baldacchini, Cryst. Latt. Def. Amorph. Mat. 18, 43 (1989).
4. W. Gellermann, J. Phys. Chem. Solids 52, 249 (1991).
5. F. Lüty, Proc. Symp. on Optical Properties of Solids and Physics of Surfaces, Mexico, 1979.