1. Yu. K. Danileiko, A. A. Manenkov, A. M. Prokhorov, and V. Ya. Khaimov-Mal'kov, Fiz. Tverd. Tela (Leningrad),19, 2738 (1968).
2. V. S. Doladugina and E. E. Berezina, Opt. Mekh. Prom.,6, 35 (1964).
3. V. S. Doladugina and E. E. Berezina, Opt. Mekh. Prom.,4, 6 (1965).
4. N. M. Melankholin and N. G. Martynova, Kristallografiya,10, 214 (1965).
5. G. W. Dueker and J. Q. Atwood, Appl. Opt.,4, 109 (1965).