1. Key Lab of Advanced Transducers and Intelligent Control System of Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, P. R. China
2. Department of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, P. R. China
3. Department of Physics, Wuhan University, Wuhan 430072, P. R. China