1. Department of Physics, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon 999077, Hong Kong
2. Research Center for Advanced Optics and Photo Electronics, Department of Physics, College of Science, Shantou University, Shantou, Guangdong 515063, China
3. Department of Material Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon 999077, Hong Kong