1. School of Information Science and Engineering, Dalian Polytechnic University, Dalian 116034, P. R. China
2. College of Microelectronics and Key Laboratory of Optoelectronics Technology, Faculty of Information Technology, Beijing University of Technology, Beijing 100124, P. R. China
3. Department of Electrical Engineering and State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Hong Kong Special Administrative Region 999077, P. R. China