1. Department of Electrical and Electronic Engineering, Oita University, Oita 870-1192, Japan
2. Department of Childhood Education, Beppu University Junior College, Beppu, Oita 874-8501, Japan
3. Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 819-0395, Japan
4. Division of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan