1. Physical Science and Engineering Division, King Abdullah University of Science & Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
2. Institute of Electronics Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
3. Department of Physics, National Chung Hsing University, Taichung 402, Taiwan
4. SulfurScience Technology Co. Ltd, Taipei 106, Taiwan