1 School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen 518172, China
2 University of Chinese Academy of Sciences, Beijing 100049, China
3 Key Laboratory of Terahertz Solid-State Technology, Chinese Academy of Sciences, Shanghai Institute of Microsystem and Information Technology, CAS, Shanghai 200050, China
4 Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China
5 Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden
6 e-mail: shumin@mail.sim.ac.cn
7 e-mail: zhangzy@cuhk.edu.cn