1 School of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China
2 Key Laboratory of Opto-electronics Information Technology, Tianjin University, Ministry of Education, Tianjin 300072, China
3 Institute of Neurosurgery, Southwest Hospital, Third Military Medical University, Chongqing 400038, China
4 Department of Applied Physics, the Hong Kong Polytechnic University, Hong Kong, China
5 e-mail: yuyewang@tju.edu.cn
6 e-mail: xudegang@tju.edu.cn