1. Department
of Electrical Engineering, Yale University, New Haven, Connecticut 06511, United States
2. Department
of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
3. Department
of Applied Physics, Yale University, New Haven, Connecticut 06511, United States
4. Department
of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan