1. Department of Optics and Optical Engineering , University of Science and Technology of China , Hefei , Anhui 230026 , China
2. School of Optical and Electronic Information & Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology , Wuhan , Hubei 430074 , China
3. Physical Measurement Laboratory, National Institute of Standards and Technology , Gaithersburg , MD 20877 , USA
4. Maryland NanoCenter, University of Maryland , College Park , MD 20742 , USA
5. National Laboratory of Solid State Microstructures & College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University , Nanjing , Jiangsu 210093 , China