1. College of Materials Science and Engineering; Nanjing Tech University; Nanjing 210009 P. R. China
2. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 21 Nanyang Link Singapore 637371 Singapore
3. Institute of Chemical and Engineering Sciences; A*STAR; 1 Pesek Road Jurong Island 627833 Singapore
4. College of Optical and Electronic Technology; China Jiliang University; Hangzhou 310018 P. R. China
5. CINTRA CNRS/NTU/Thales; UMI 3288, 50 Nanyang Drive Singapore 637553 Singapore