1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 21 Nanyang Link Singapore 637371 Singapore
2. Center for Disruptive Photonic Technologies; The Photonics Institute; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
3. Optoelectronics Research Centre and Centre for Photonic Metamaterials; University of Southampton; Southampton SO17 1BJ UK