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