1. For a review see: A. Javan, Fundamental and Applied Laser Physics, Proceedings of the Esfahan Symposium, edited by M. S. Feld, A. Javan, and N. A. Kurnit, Wiley-Interscience, 1973, Page 295.
2. See S. Ezekiel, this volume.
3. See A. Mooradian, this volume.
4. L. O. Hocker, D. R. Sokoloff, V. Daneu, A. Szöke, and A. Javan, Appl. Phys. Letters 12, 401 (1968).
5. V. Daneu, D. R. Sokoloff, A. Sanchez, and A. Javan, Appl. Phys. Letters 15, 398 (1969).