1. School of Physics and Electronic Engineering, School of Physics and Electronic Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang, Jiangsu Province 212013, P. R. China
2. Key Laboratory of Terahertz Solid-State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, P. R. China
3. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China