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