1. Department of Electrical and Electronic Engineering, Southern University of Science and Technology, No. 1088, Xueyuan Rd., Xili, Nanshan
District, Shenzhen, Guangdong 518055, P. R. China
2. School of Physics, Nankai University, Tianjin 300071, P. R. China
3. State Key Laboratory on Advanced Displays and Optoelectronics Technologies, Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China