1. Department of Physics, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, South Korea
2. Department of Electrical and Communication Engineering, HangYang University, Ansan, 425-791, South Korea
3. Department of Information and Communications, Joongbu University, Chungnam, 132-940, South Korea
4. Department of Electronic Engineering, Korea University, Seoul, 136-075, South Korea