1. Shandong Technology Center of Nanodevices and Integration, School of Microelectronics , Shandong University , Jinan , Shandong , China
2. Department of Opto-electronics and Information Engineering, School of Precision Instruments and Opto-electronics Engineering, Tianjin University , Tianjin , China
3. School of Electrical and Electronic Engineering, University of Manchester , Manchester, M13 9PL , United Kingdom