1. Division of Physics and Applied Physics, School of Physical
and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
2. Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China
3. State Key Laboratory of Mechanics and Control
of Mechanical Structures, College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210085, China
4. Centre for Disruptive Photonic Technologies
(CDPT), Nanyang Technological University, Singapore 637371, Singapore