1. School of Physics & Information Technology, Shaanxi Normal University, Xian 710119, China
2. College of Science, Northwest A & F University, Yangling 712100, China
3. Center for OptoElectronics and Biophotonics, School of Electrical and Electronic Engineering & The Photonics Institute, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore