1. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Box 603-146, 100190, Beijing, China
2. Department of Physics and Astronomy, Department of Electrical and Computer Engineering, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005
3. Division of Solid State Physics/The Nanometer Consortium, Lund University, Box 118, S-22100, Lund, Sweden