1. Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
2. School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China
3. Jiangsu Key Laboratory of Thin Films, School of Physical Science and Technology, Soochow University, Suzhou 215006, P. R. China
4. CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, Singapore 637553, Singapore