1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
2. State Key Laboratory of Modern Optical Instrumentation, and College of Information Science and Electronic Engineering; Zhejiang University; Hangzhou 310027 China
3. School of Electrical and Electronic Engineering; Nanyang Technological University; Nanyang Avenue, Singapore 639798 Singapore
4. Centre for Disruptive Photonic Technologies; Nanyang Technological University; Singapore 637371 Singapore