1. D. A. Leonard, Laser Focus,3, 26 (1967).
2. E. L. Patterson, G. B. Gerardo, and A. W. Johnson, Appl. Phys. Lett.,21, 293 (1972).
3. Yu. I. Bychkov, V. F. Losev, V. V. Savin, and V. F. Tarasenko, Kvantovaya Élektron.,2, 2047 (1975).
4. V. V. Savin, V. F. Tarasenko, and Yu. I. Bychkov, Zh. Tekh. Fiz.,46, 198 (1976).
5. E. L. Patterson, J. Appl. Phys.,44, 3193 (1973).