1. Centre for Disruptive Photonic Technologies, School of Physical and Mathematical Science, Nanyang Technological University, Singapore 637371, Singapore
2. School of Physical and Mathematical Science, Nanyang Technological University, Singapore 637371, Singapore
3. Department of Physics, University of Warwick, Coventry CV4 7AL, U.K.
4. School of Chemistry, Chemical Engineering and Biotechnology, Nanyang Technological University, Singapore 637371, Singapore
5. MATES Lab, Harish-Chandra Research Institute, A CI of Homi Bhabha National Institute, Allahabad 211019, India
6. School of Materials Science & Technology, Nanyang Technological University, Singapore 639798, Singapore