1. Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices and Guangzhou Key Laboratory for Special Fiber Photonic Devices and Applications South China Normal University Guangzhou 510006 P. R. China
2. Photonics Research Institute Department of Electronic and Information Engineering The Hong Kong Polytechnic University Kowloon Hong Kong 999077 P. R. China
3. The State Key Laboratory of Terahertz and Millimeter Waves Centre for Biosystems, Neuroscience and Nanotechnology City University of Hong Kong Kowloon Hong Kong 999077 P. R. China
4. School of Physical Sciences Great Bay University Dongguan Guangdong Province 523000 P. R. China