1. Department of Physics, Huaiyin Institute of Technology, Jiangsu 223001, China
2. National Laboratory for infrared physics, Shanghai institute of technical physics, Chinese academy of sciences, 224502, China
3. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing, 210093, China
4. Department of Physics, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China
5. Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA