1. Research Institute of Electrical Communication, Tohoku University, Sendai 980, Japan, and Exploratory Research for Advanced Technology, Nishizawa Perfect Crystal Project, Research Development Corporation of Japan, c/o Semiconductor Research Institute, Kawauchi, Sendai 980, Japan
2. Research Institute of Electrical Communication, Tohoku University, Sendai 980, Japan
3. Exploratory Research for Advanced Technology, Nishizawa Perfect Crystal Project, Research Development Corporation of Japan, c/o Semiconductor Research Institute, Kawauchi, Sendai 980, Japan