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, Lotharstr. 1, Duisburg 47057, Germany
3. School of Marine Science and Technology, Harbin Institute of Technology at Weihai, Weihai, Shandong 264209, P. R. China