1. Department of Physics, Shanghai University, Shanghai 200444, People's Republic of China
2. Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
3. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, Key Laboratory of Optoelectronics Information and Technology (Ministry of Education), Tianjin University, Tianjin 300072, People's Republic of China
4. Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia