1. Department of Physics, Tamkang University, New-Taipei 251, Taiwan, Republic of China
2. Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan, Republic of China
3. Department of Microstructure and Characterization, Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsin-Chu 310, Taiwan, Republic of China
4. Graduate Institute in Electro-Optical Engineering, Tatung University, Taipei 104, Taiwan, Republic of China