1. Centre for Advanced
Optoelectronic Functional Materials Research and Key Laboratory for
UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China
2. State Key Laboratory of Applied Optics, Changchun Institute
of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
3. Department
of Physics, Hong Kong University of Science and Technology, Clear
Water Bay, Kowloon, Hong Kong