1. State Key Laboratory of Optoelectronic, Materials and Technologies, School of Physics and Engineering; Sun Yat-sen University; Guangzhou 510275 P.R. China
2. Department of Applied Physics, College of Science; South China Agricultural University; Guangzhou 510642 P.R. China
3. Department of Physics; Guangdong University of Education; Guangzhou 510303 P.R. China