1. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
3. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
4. Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
5. Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan