1. Department of Electronic and Information EngineeringThe Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong SAR P. R. China
2. Department of Physics and Institute of Advanced MaterialsHong Kong Baptist University Kowloon Tong Hong Kong SAR P. R. China
3. School of PhysicsState Key Laboratory of Crystal MaterialsShandong University Jinan Shandong 250100 P. R. China
4. Department of PhysicsThe Chinese University of Hong Kong Hong Kong SAR P. R. China