1. School of PhysicsState Key Laboratory of Crystal MaterialsShandong University Jinan Shandong 250100 P. R. China
2. Henan Key Laboratory of Infrared Materials and Spectrum Measures and ApplicationsHenan Normal University Xinxiang Henan 453007 P. R. China
3. Department of Physics and Institute of Advanced MaterialsHong Kong Baptist University Kowloon Tong Hong Kong 999077 P. R. China
4. ARC Centre of Excellence in Exciton ScienceSchool of ChemistryThe University of Melbourne Parkville Victoria 3010 Australia