1. Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, P. R. China
2. College of Physical Science and Technology, Institute of Optoelectronic Technology, Yangzhou University, Yangzhou 225002, P. R. China
3. School of Physics and Optoelectronic Technology, Baoji University of Arts and Sciences, Baoji 721007, P. R. China