1. Contribution from the Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan, NTT Opto-electronics Laboratories, Nippon Telegraph and Telephone Corporation, Naka-gun, Ibaraki 319-11, Japan, Fuculty of Engineering, Gunma University, Tenjincho, Kiryu 376, Japan, and School of Material Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-12, Japan