1. Nanophotonic Laboratory, Department of Physics, Nanjing University of Science and Technology, Nanjing 210094, China
2. Centre for Disruptive Photonic Technologies, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
3. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602
4. School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
5. Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372