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