1. Shanghai Institute of Ceramics, Chinese Academy of Sciences, 585 He-Shuo Road, Shanghai 201899, P.R. China
2. Shanghai Normal University, 100 Guilin Road, China, Shanghai 200234, P. R. China
3. Institute of Physics of the Czech Academy of Sciences, Cukrovarnicka 10/112, Prague 16200, Czech Republic
4. Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei Province 430074, China