1. Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
2. Division of Scientific Research, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
3. Department of Photonics & Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan
4. Department of Materials Science & Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
5. Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
6. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan