1. Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic SystemsSchool of Materials Science & EngineeringBeijing Institute of Technology Haidian District Beijing 100081 P. R. China
2. East China Institute of Optoelectronic Integrated Device Suzhou 215163 P. R. China
3. School of Optics and PhotonicsBeijing Institute of Technology Haidian District Beijing 100081 P. R. China