1. Applied Physics Program University of Michigan Ann Arbor MI 48109 USA
2. Department of Chemistry National Taiwan University Taipei 10617 Taiwan
3. Department of Physics and Astronomy and Collaborative Innovation Center of IFSA (CICIFSA) Shanghai Jiaotong University Shanghai 200240 P. R. China
4. Institute of Atomic and Molecular Science Academia Sinica Taipei 10617 Taiwan
5. Department of Physics and Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor MI 48109 USA