1. Department of Physics, National Chung Cheng University, Ming Hsiung, Chia Yi 621, Taiwan
2. Center for High Technology Development, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
3. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
4. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan