1. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Box 603-146, Beijing 100190, China
2. Centro Mixto de Física de Materiales CSIC-UPV/EHU and Donostia International Physics Center DIPC, San Sebastián 20018, Spain
3. Department of Physics and Astronomy, Department of Electrical and Computer Engineering, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005
4. Division of Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, S-22100 Lund, Sweden