1. Department of Electronics, Tokai University, Hiratsuka 259‐1292, Japan
2. Institute for Metal Research, Tohoku University, Katahira, Sendai 980‐8577, Japan
3. Deptartment of Electrical & Electronics Engineering, Fukui University, Bunkyo, Fukui 910‐8507, Japan
4. Department of Photonics, Ritsumeikan University, Kusatsu, Shiga 525‐8577, Japan