1. School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, P. R. China
2. School of Physics, Shandong University, Jinan 250100, P. R. China
3. Department of Physics, Jining University, Qufu 273155, P. R. China
4. College of Integrated Circuits, Ludong University, Yantai 264025, P. R. China