1. Key Laboratory of Optoelectronics Technology Ministry of Education Faculty of Information Technology Beijing University of Technology Beijing 100124 P. R. China
2. Key Laboratory of Advanced Functional Materials Ministry of Education College of Materials Science and Engineering Beijing University of Technology Beijing 100124 P. R. China
3. Centre for OptoElectronics and Biophotonics School of Electrical and Electronic Engineering Nanyang Technological University Singapore 639798 Singapore
4. Centre for Disruptive Photonic Technologies Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore