1. Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
2. Institute of Photonics Technology, Jinan University, Guangzhou 510632, P. R. China
3. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
4. Department of Condensed Matter Physics, Graduate School of Science, Hokkaido University, Sapporo 060-8010, Japan