1. Department of Physics, National Taiwan University, Taipei 106, Taiwan
2. Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 974, Taiwan
3. Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei 106, Taiwan
4. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
5. Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan