1. Department of Applied Physics Zhejiang University of Technology Hangzhou 310023 P. R. China
2. Ming Hsieh Department of Electrical and Computer Engineering University of Southern California Los Angeles CA 90089 USA
3. School of Physics National Laboratory of Solid‐State Microstructures Nanjing University Nanjing 210093 P. R. China
4. Department of Physics and Astronomy University of Southern California Los Angeles CA 90089 USA