1. Centre for Disruptive Photonic Technologies; Nanyang Technological University; 637371 Singapore
2. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 637371 Singapore
3. National Laboratory for Infrared Physics; Shanghai Institute of Technical Physics; Chinese Academy of Sciences; Shanghai 200083 China
4. School of Materials Science and Engineering; Nanyang Technological University; 639798 Singapore