1. Centre for Optical and Electromagnetic Research, National Engineering Research Center for Optical Instruments, College of Optical Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310058, People’s Republic of China
2. Ningbo Research Institute, Zhejiang University, Ningbo, Zhejiang 315100, People’s Republic of China
3. Sino-Swedish Joint Research Center of Photonics (JORCEP), School of Electrical Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden