1Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, Kowloon, Hong Kong SAR, China
2State
Key Laboratory on Integrated Optoelectronics and College of Electronic
Science and Engineering, Jilin University, Changchun 130012, China
3Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China