1. School of Optoelectronic Science and Engineering University of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. Center for OptoElectronics and Biophotonics School of Electrical & Electronic Engineering Nanyang Technological University Singapore 639798 Singapore
3. School of Physical and Mathematical Science and Photonics Institute Nanyang Technological University Singapore 639798 Singapore
4. Yangtze Delta Region Institute (Huzhou) University of Electronic Science and Technology of China Huzhou 313001 P. R. China