1. Department of Physics, National Kaohsiung Normal University, Kaohsiung 824, Taiwan
2. Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan
3. Molecular Science and Technology, Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan
4. Department of Physics, National Central University, Chungli 320, Taiwan
5. Department of Physics, National Cheng-Kung University, Tainan 701, Taiwan