1. School of Physics and Optoelectronic Engineering, Guangdong University of Technology, WaiHuan Xi Road, No. 100, Guangzhou 510006, P. R. China
2. School of Electronics and Communications, Guangdong Mechanical & Electronical College of Technology, Guangzhou 510515, P. R. China
3. School of Applied Physics and Materials, Wuyi University, Jiangmen 529020, P. R. China