1Institute of Advanced Materials (IAM), and School of Materials Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210046, China
2Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China
3Department of Photonics Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
4Department of Electrical and Computer Engineering, University of Wisconsin, Madison, Wisconsin 53706, USA