1. School of Optoelectronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, P. R. China
2. School of Physics, Southeast University, Nanjing 211189, P. R. China
3. New Energy Technology Engineering Laboratory of Jiangsu Province & School of Science, Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China