1. Institute of Molecular Plus, Department of Physics, School of Science, Tianjin University, Tianjin 300072. China
2. Department of Physics and Center for Optoelectronics and Photonics Paderborn (CeOPP), Universität Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany
3. Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, China
4. Wyant College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA