1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, Singapore.
2. The Photonics Institute and Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore, Singapore.
3. State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, Guangzhou 510275, China.
4. Optoelectronics Research Centre, University of Southampton, Southampton, UK.