1. College of Electronic Science and TechnologyShenzhen University Shenzhen 518060 P. R. China
2. State Key Laboratory of Terahertz and Millimeter Waves and Department of Materials Science and EngineeringCity University of Hong Kong Hong Kong SAR 999077 P. R. China
3. Institute for Advanced StudyShenzhen University Shenzhen 518060 P. R. China
4. Department of ChemistrySouth University of Science and Technology of China No 1088, Xueyuan Road, Xili, Nanshan District Shenzhen Guangdong 518055 P. R. China