1. Beijing
National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. State
Key Laboratory on Integrated Optoelectronics, College of Electronic
Science and Engineering, Jilin University, Changchun 130012, China
3. Department
of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy,New York 12180, United States