1. Division of Physics and Applied PhysicsSchool of Physical and Mathematical SciencesNanyang Technological University Singapore 637371 Singapore
2. Centre for Disruptive Photonic TechnologiesThe Photonics InstituteNanyang Technological University Singapore 637371 Singapore
3. Centre for Photonic Metamaterials & Optoelectronics Research CentreZepler InstituteUniversity of Southampton Southampton SO17 1BJ UK