1. State Key Laboratory of Optoelectronic Materials and Technologies/MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, P. R. China, Zeiss Center of Excellence, California Institute for Communication and Information Technology, University of California, Irvine, California 92697-2800, and Department of Physics, Hong Kong University of Science and Technology, Kowloon, Hong Kong, P. R. China