1. Ashcroft, N. W.; Mermin, N. D. (1976): Solid State Physics. Holt, Rinehart & Winston, N. Y.
2. Baer, T. M. (1986): Laser Focus 22, June 86, p. 82
3. Beigang, R.; Litfin, G.; Schneider, R. (1979): Phys. Rev. A20, 229
4. Beyer, E.; Märten, O.; Behler, K.; Weick, J. M. (1985): Laser und Optoelektronik 3, 282
5. Bohren, A.; Sigrist, M. W. (1997): Infrared Phys. Technol. 38,423