1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
2 State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
3 University of Chinese Academy of Sciences, Beijing 100049, China
4 Key Laboratory of Space Active Opto-electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
5 e-mail: zmhuang@mail.sitp.ac.cn
6 e-mail: EDHZHANG@ntu.edu.sg