1. College of Electronic Information, Sichuan University, Chengdu 610064, China
2. Optical Nanostructures Laboratory, Columbia University, New York, New York 10027, USA
3. Mechanical Engineering, Columbia University, New York, New York 10027, USA
4. The Institute of Microelectronics, Singapore 117685, Singapore
5. North China Research Institute of Electro-Optics, Beijing 100015, China