1. School of Science, Department of Optoelectronic Science, Harbin Institute of Technology at Weihai, Weihai 264209, P. R. China
2. Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, Duisburg 47057, Germany
3. Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China