1. Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, South Korea
2. Division of Quantum Phases and Devices, Department of Physics, Konkuk University, Seoul 143-791, South Korea
3. Department of Physics and Division of Energy System Research, Ajou University, Suwon 443-749, South Korea
4. Department of Physics and Astronomy and Center for Subwavelength Optics, Seoul National University, Seoul 151-747, South Korea
5. Department of Physics, Sungkyunkwan University, Suwon 440-746, South Korea
6. Department of Physics, Ewha Womans University, Seoul 120-750, South Korea
7. Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
8. Department of Material Science and Engineering, Seoul National University, Seoul 151-747, South Korea