1. Research Institution for Biomimetics and Soft Matter, Fujian Key Provincial Laboratory for Soft Functional Materials Research, College of Materials and College of Physical Science and Technology, Xiamen University, Xiamen 361005, P. R. China
2. Key Laboratory of Optoelectronic Technology and Systems, Chongqing University, Ministry of Education, Chongqing 400044, China
3. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore