1. School of PhysicsState Key Laboratory of Crystal MaterialsShandong University Jinan Shandong 250100 China
2. Henan Key Laboratory of Infrared Materials & Spectrum Measures and ApplicationsHenan Normal University Xinxiang 453007 China
3. ARC Centre of Excellence in Exciton ScienceSchool of ChemistryThe University of Melbourne Parkville Victoria 3010 Australia