1. Institute of Modern Optics and Center of Single-Molecule Science, Tianjin Key Laboratory of Micro-scale Optical Information Science and Technology, Nankai University, Tianjin 300350, China
2. Key Laboratory of Medical Physics and Image Processing of Shandong Province, School of Physics and Electronics, Shandong Normal University, Jinan, Shandong 250358, China
3. Department of Physics, University of Konstanz, Konstanz 78457, Germany