1. College of Chemistry; Liaoning University; Shenyang 110036 China
2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics; Chinese Academy of Sciences; PO Box 603-146 Beijing 100190 China
3. School of Physics and Optoelectronic Technology; Dalian University of Technology; Dalian 116024 China
4. IPHT-Leibniz Institute of Photonic Technology; Albert-Einstein-Str. 9 D-07745 Jena Germany
5. School of Chemistry and Chemical Engineering; Shandong University; Jinan 250100 China
6. Division of Solid State Physics; Lund University; Lund 22100 Sweden