1. School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, China
2. School of Optical and Electronic Information, National Engineering Laboratory for Next Generation Internet Access System, Huazhong University of Science and Technology, No. 1037 Luoyu Road, Wuhan, Hubei 430074, China
3. Center for OptoElectronics and Biophotonics, School of Electrical and Electronic Engineering, School of Physical and Mathematical Science, and Photonics Institute, Nanyang Technological University, Singapore 639798, Singapore