1. Department of Electrical and Computer EngineeringNational University of Singapore 4 Engineering Drive 3 Singapore 117576 Singapore
2. Key Laboratory of Optoelectronic Materials Chemistry and PhysicsFujian Institute of Research on the Structure of MatterChinese Academy of Sciences Fuzhou 350002 China
3. Institute of Laser EngineeringBeijing University of Technology 100 Ping Le Yuan, Chaoyang District Beijing 100124 China