1. College of Physics, Qingdao University, Qingdao 266071, China
2. College of Electronic Information, Qingdao University, Qingdao 266071, China
3. Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, USA
4. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China