1. S.L. Shapiro: (ed.) Ultrashort Light Pulses, Topics Appl. Phys. Vol. 18 ( Springer, Berlin, Heidelberg 1977 )
2. J.J. Hopfield: Phys. Rev. 112, 1555 (1958)
3. M.A. Lampert: Phys. Rev. Lett. 1, 450 (1958); S.A. Moskalenko: Optics and Spectroscopy 5, 147 (1958)
4. See, e.g. Excitons at High Densities, ed. by H. Haken, S. Nikitine, Springer Tracts Mod. Phys. Vol. 73 ( Springer, Berlin, Heidelberg 1975 )
5. N.F. Mott: Metal-Insulator Transitions ( Taylor and Francis, London 1974 )