1. Department of Physics Colorado State University Fort Collins CO 80523 USA
2. School of Optical and Electronic Information Huazhong University of Science and Technology Wuhan Hubei 430074 China
3. National Laboratory of Solid State Microstructures and Department of Physics Nanjing University Nanjing Jiangsu 210093 China
4. Department of Physics and Astronomy University of Wyoming Laramie Wyoming 82071 USA
5. School of Advanced Materials Discovery Colorado State University Fort Collins CO 80523 USA