1. School of Mathematics and Physics, Nanyang Institute of Technology, Nanyang 473004, China
2. School of Integrated Circuits, Huazhong University of Science and Technology, Wuhan 430074, China
3. Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China
4. Ames Laboratory, U. S. Department of Energy and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA