1. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
2. Department of Physics, National Taiwan University, Taipei 106, Taiwan
3. Graduate Institute of Applied Physics, National Taiwan University, Taipei 106, Taiwan
4. Institute of Physics, Academia Sinica, Taipei 115, Taiwan
5. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan