1. Department of Opto-electronics and Information Engineering, College of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, P. R. China, Department of Physics, Nankai University, Tianjin 300071, P. R. China, and Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, P. R. China