1. Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China
2. Ministry of Education Nanophotonics and Advanced Instrument Engineering Research Center, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China
3. Shanghai Institute of Intelligent Electronics and Systems, Fudan University, 220 Handan Road, Shanghai 200433, China
4. Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan