Affiliation: | a Centre for Optical and Electromagnetic Research, JORCEP (Joint Research Center of Photonics of the Royal Institute of Technology (Sweden) and Zhejiang University), Zhejiang University, Hangzhou 310058, China b Department of Microelectronics and Applied Physics (MAP), Royal Institute of Technology (KTH), Kista Photonic Research Centre (KPRC), Electrum 229, 164 40 Kista, Sweden |