1. B. P. Stoicheff, Phys. Rev. Lett.,7, 168 (1963).
2. E. Garmire, F. Pandarese, and C. H. Townes, Phys. Rev. Lett.,11, 160 (1963).
3. R. W. Terhune, Bull. Amer. Phys. Soc.,8, 359 (1963); Solid State Design,4, 38 (1963).
4. R. Chiao and B. P. Stoicheff, Phys. Rev. Lett.,12, 290 (1964).
5. K. Shimoda, Jap. J. Appl. Phys.,5, 33 (1966).