1. State Key Laboratory of Luminescent Materials and Devices; Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices; School of Materials Science and Engineering; South China University of Technology; Guangzhou 510640 China
2. Department of Physics; South China University of Technology; Guangzhou 510640 China
3. Department of Applied Physics; The Hong Kong Polytechnic University; Hong Kong China
4. College of Optical Science and Engineering; Zhejiang University; Hangzhou 310027 China