1. Department of Physics and Materials Science; City University of Hong Kong; 83 Tat Chee Avenue, Kowloon Hong Kong Hong Kong S.A.R.
2. Centre for Functional Photonics (CFP); City University of Hong Kong; 83 Tat Chee Avenue Kowloon Hong Kong Hong Kong S.A.R.
3. Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems; School of Materials Science & Engineering; Beijing Institute of Technology; Beijing 100081 China