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