1. Optoelectronics Research Centre & Centre for Photonic Metamaterials, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom
2. School of Mathematics and Physics, Anqing Normal University, Anqing 246133, People's Republic of China
3. Centre for Disruptive Photonic Technologies & The Photonics Institute, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371