1. Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 P.R. China
2. Leibniz Institute of Photonic Technology Albert‐Einstein‐Str. 9 07745 Jena Germany
3. Division of Bionanophotonics Department of Applied Physics Chalmers University of Technology Gothenburg SE‐412 96 Sweden
4. School of Science Xi'an Jiaotong University Xi'an 710049 China
5. Material Science and Engineering University of California San Diego CA 92093 USA