1. Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore
2. The Photonics Institute and Centre for Disruptive Photonic Technologies Nanyang Technological University Singapore 637371 Singapore
3. School of Mathematical and Physical Sciences University of Technology Sydney Ultimo NSW 2007 Australia
4. School of Materials Science and Engineering Harbin Institute of Technology Harbin 150001 P. R. China