1. Centre for Disruptive Photonic Technologies, TPI, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore
2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore
3. Center for Nano Science and Technology, Istituto Italiano di Tecnologia, via Pascoli 70/3, 20133 Milan, Italy
4. Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, 28 Medical Drive, 117456 Singapore
5. Department of Physics, Politecnico di Milano and Center for Nano Science and Technology, Istituto Italiano di Tecnologia, 20133 Milan, Italy