1. Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong, P. R. China
2. State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Tat Chee Avenue, Hong Kong, P. R. China
3. Center of Functional Photonics, City University of Hong Kong, Tat Chee Avenue, Hong Kong, P. R. China