1. State Key Laboratory of Information Photonics and Optical CommunicationsSchool of ScienceBeijing University of Posts and Telecommunications Beijing 100876 China
2. State Key Laboratory of Superlattices and MicrostructuresInstitute of SemiconductorsChinese Academy of Sciences & Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100083 China
3. Shanghai Key Laboratory of Electrical Insulation and Thermal AgingSchool of Chemistry and Chemical EngineeringShanghai Jiao Tong University Shanghai 200240 China