1. Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic TechniqueXi'an Jiaotong University Xi'an 710049 P. R. China
2. School of Electronic and Information EngineeringXi'an Jiaotong University Xi'an 710049 P. R. China
3. Institute of Materials Research and EngineeringA*STAR (Agency for Science, Technology and Research) 2 Fusionopolis Way, #08‐03, Innovis Singapore 138634 Singapore
4. Department of Electrical and Electronic EngineeringThe University of Hong Kong Hong Kong P. R. China
5. School of Biomedical SciencesThe University of Hong Kong Hong Kong P. R. China