1. School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China
2. Shandong Key Laboratory of Optics and Photonic Devices, School of Physics and Electronics, Shandong Normal University, Jinan 250358, China
3. Department of Electrical and Computer Engineering, Dalhousie University, Halifax, NS B3J 2X4, Canada
4. School of Physics and Microelectronics, Key Laboratory of Materials Physics, Ministry of Education, Zhengzhou University, Zhengzhou, Henan 450052, China
5. Department of Information and Communication Engineering, Fukuoka Institute of Technology, Fukuoka 811-0295, Japan