1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
2. Centre for Disruptive Photonic Technologies; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
3. School of Electrical and Computer Engineering; Oklahoma State University; Stillwater OK 87078 USA
4. Department of Physics; Engineering Physics and Astronomy; Queen's University; Kingston Ontario K7L 3N6 Canada