1. Institute of Atomic and Molecular Physics, Sichuan University, 610065 Sichuan, P. R. China
2. School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, U.K.
3. National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada
4. Engineering Research Center in Biomaterials, Sichuan University, 610065 Sichuan, P. R. China
5. State Key Laboratory of Polymer Materials Engineering, Sichuan University, 610065 Sichuan, P. R. China