1. Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong, P. R. China
2. Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
3. State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Tat Chee Avenue, Hong Kong, P. R. China
4. Center of Functional Photonics, City University of Hong Kong, Tat Chee Avenue, Hong Kong, P. R. China