1. Department of Applied Physics, National Pingtung University of Education, Pingtung 900, Taiwan
2. Graduate Institute of Science Education, National Kaohsiung Normal University, Kaohsiung 824, Taiwan
3. Department of Physics, National Sun Yet-Sen University, Kaohsiung 804, Taiwan
4. Department of Physics, National Kaohsiung Normal University, Kaohsiung 824, Taiwan