1. Contribution from the BK21 Physics Division, Institute of Basic Science, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, Korea, Department of Physics, University of Seoul, Seoul 130-743, Korea, and Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea