1. Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum MaterialsSouth China Academy of Advanced OptoelectronicsSouth China Normal University Guangzhou 510006 P. R. China
2. Institute of Electronic Paper DisplaysSouth China Academy of Advanced OptoelectronicsSouth China Normal University Guangzhou 510006 P. R. China
3. Shenzhen Guohua Optoelectronics Tech. Co. Ltd. Shenzhen 518110 P. R. China
4. Laboratory of Solid State Microstructures and Innovative Center of Advanced MicrostructuresNanjing University Nanjing 210093 P. R. China
5. Department of PhysicsBoston College Chestnut Hill MA 02467 USA