1. Department of Applied Physics, Zhejiang University of Technology, Hangzhou 310023, P. R. China
2. School of Physics, National Laboratory of Solid State Microstructures, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China
3. Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, California 90089, United States