1. Key Laboratory of Optoelectronic Technology and Systems, Ministry of Education, Department of Optoelectronic Engineering, Chongqing University, Chongqing 400044, P. R. China
2. College of Electronic Information Engineering, Changchun 130012, P. R. China
3. College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 400044, P. R. China