1. Department
of Applied Physics, Chongqing University, Chongqing 400044, P. R. China
2. Chongqing
Engineering Laboratory for Detection, Control and Integrated System, Chongqing Technology and Business University, Chongqing 400067, P. R. China
3. School
of Physics, Shandong University, Jinan 250100, P. R. China