1. School of Physics and Optoelectric Engineering, Guangzhou Higher Education Mega Center, Guangdong University of Technology, Guangzhou 510006, P. R. China
2. School of Advanced Materials and Nanotechnology, Xidian University, Xian 710126, China
3. School of Automation, Guangdong University of Technology, Guangzhou 510006, China
4. Department of Physics, The University of Hong Kong, Hong Kong 999077, P. R. China