1. Graduate
Institute of Applied Physics, National Taiwan University, Taipei 10617, Taiwan
2. Research
Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan
3. Institute
of Opto-electronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan
4. Department
of Physics, University of Massachusetts Boston, Boston, Massachusetts 02125, United States
5. Department
of Physics, National Taiwan University, Taipei 10617, Taiwan
6. School
of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore