1. Department of Physics Capital Normal University Beijing Key Laboratory of Metamaterials and Devices Key Laboratory of Terahertz Optoelectronics Ministry of Education, and Beijing Advanced Innovation Center for Imaging Technology Beijing 100048 China
2. Shanghai Institute of Optics and Fine Mechanics Chinese Academy of Sciences Shanghai 201800 China
3. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 China
4. Center for Integrated Nanotechnologies Los Alamos National Laboratory Los Alamos NM 87545 USA