1. Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan; NTT Optoelectronics Laboratories, Nippon Telegraph and Telephone Corporation, Naka-gun, Ibaraki 319-11, Japan; School of Material Science, Japan Advanced Institute of Science and Technology, 15 Asahidai, Tatsunokuchi, Ishikawa 923-12, Japan; and Faculty of Engineering, Gunma University, Tenjincho, Kiryu 370, Japan