1. School of Materials Science & Engineering Nanyang Technological University Singapore 639798 Singapore
2. School of Physics Beijing Institute of Technology Beijing 100081 China
3. CINTRA CNRS/NTU/THALES UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6 Singapore 637553 Singapore
4. School of Electrical and Electronic Engineering Nanyang Technological University Singapore 639798 Singapore