1. School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China
2. Henan Key Laboratory of Infrared Materials & Spectrum Measures and Applications, Henan Normal University, Xinxiang, Henan 453007, P. R. China
3. School of Physics, National Demonstration Center for Experimental Physics Education, Shandong University, Jinan, Shandong 250100, P. R. China
4. ARC Centre of Excellence in Exciton Science, School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia