1. Research Institute of Photonics, Dalian Polytechnic University, Dalian 116034, P. R. China
2. Liaoning Research Institute of Light Industry, Shenyang 110036, 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