1. A preliminary account of the work was given at the X International Quantum Electronics Conference, May 29 - June 2, 1978, Atlanta, Ga.
2. A. V. Nurmikko and G. W. Pratt Jr., Appl. Phys. Lett. 27, 83 (1975).
3. A. V. Nurmikko, Optics Comm. 16, 265 (1976).
4. J. C. Campbell et al., Appl. Phys. Lett. 27, 202 (1975).
5. P. Ho, L. A. Glasser, H. A. Haus, and E. P. Ippen, paper presented at the X International Quantum Electronics Conference, ibid.