1. Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices, Ministry of Education, Wuhan University, Wuhan 430072, People's Republic of China, Fax: +86‐27‐6875‐2569
2. State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People's Republic of China