1. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
2. Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan
3. Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
4. Quantum-Phase Electronics Center and Department of Applied Physics, School of Engineering, The University of Tokyo, Hongo, Bunkyo, Tokyo 113-8654, Japan
5. Department of Medical Research, China Medical University Hospital, Taichung, Taiwan