1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
2. Department of Applied Science, R. O. C. Naval Academy, Kaohsiung 813, Taiwan and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
3. Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan and the Center of Crystal Research, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
4. Department of Photonics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
5. Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, 804, Taiwan
6. Department of Electronics Engineering and Institute of Electronics, National Chiao-Tung University, Hsinchu 300, Taiwan