1. The China−Germany Research Center for Photonic Materials and Devices, The State Key Laboratory of Luminescent Materials and Devices, and Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, The School of Materials Science and Engineering and The School of Physics, South China University of Technology, Guangzhou 510640, P. R. China
2. School of Applied Physics and Materials, Wuyi University, Jiangmen 529000, P. R. China