1. The Atomic, Molecular & Materials Physics Group, Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, P. R. China
2. College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong 518060, 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