1. Laboratory of Soft Matter Physics, Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Box 603-146, 100190, Beijing, China, Laboratory for Nanophotonics, Department of Physics and Astronomy, Rice University, Houston, Texas 77005-1892, School of Science, Central University for Nationalities, Beijing 100081, China, and Division of Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, S-221 00, Lund, Sweden