1. School of Science, Nanjing University of Posts and Telecommunications (NJUPT), Nanjing 210023, P. R. China
2. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, P. R. China
3. CAS Key Laboratory of Nanophotonic Materials and Devices & Key Laboratory of Nanodevices and Applications, i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, P. R. China
4. Department of Basic, Dalian Naval Academy, Dalian 116018, P. R. China
5. School of Physics, Southeast University, Nanjing 211189, P. R. China