1. Hunan Key Laboratory of Nanophotonics and Devices, School of Physics and Electronics, Central South University, 410083 Changsha, P. R. China
2. School of Materials Science and Engineering, Central South University, 410083 Changsha, P. R. China
3. NanoSYD, Mads Clausen Institute and DIAS Danish Institute for Advanced Study, University of Southern Denmark, Alsion 2, DK-6400 Sonderborg, Denmark