1. Department of Opto-electronics and Information Engineering; School of Precision Instruments and Opto-electronics Engineering; Tianjin University; Tianjin 300072 China
2. Key Laboratory of Optoelectronic Information Technology; Tianjin University; Ministry of Education; Tianjin 300072 China
3. Department of Electronic Engineering; City University of Hong Kong; 83 Tat Chee Avenue, Kowloon Hong Kong China