1. Institute of Microelectronics of Chinese Academy of Sciences Beijing 100029 P. R. China
2. University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. State Key Laboratory of Electronic Thin Films and Integrated DevicesUniversity of Electronic Science and Technology of China Chengdu 610054 P. R. China
4. State Key Laboratory of Infrared PhysicsShanghai Institute of Technical PhysicsChinese Academy of Sciences Shanghai 200083 P. R. China
5. Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic SystemsSchool of PhysicsBeijing Institute of Technology Beijing 100081 P. R. China