1. J. J. Hopfield:Phys. Rev.,112, 1555 (1958).
2. S. I. Pekar:Sov. Phys. JETP,33(6), 785 (1958);38(11), 1286 (1960).
3. V. M. Agranovich:Sov. Phys. JETP,37(10), 307 (1960).
4. See, for instance,S. I. Pekar:Crystal Optics and Additional Light Waves (Benjamin-Cummings, Melno Park, 1983);I. Broser, R. Broser, E. Beckmann andE. Birkicht:Solid State Commun.,39, 1209 (1981);
5. M. V. Lebedev, M. I. Strashnikova, V. B. Timofeev andV. V. Chernyi:Sov. Phys. JETP Lett.,39, 440 (1984);