1. School of Electronic Engineering, Xidian University, Xi'an, China
2. Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, Espoo, Finland
3. Key Laboratory for Physical Electronics and Devices of the Ministry of Education and Shaanxi Key Lab of Information Photonic Technique, School of Electronic Science and Engineering, Faculty of Electronic and Information Engineering, Xi'an Jiaotong University, Xi’an, China