1. Centre for Optical and Electromagnetic Research; Zhejiang Provincial Key Laboratory for Sensing Technologies; JORCEP [Joint Research Center of Photonics of the Royal Institute of Technology; Lund University and Zhejiang University], Zijingang campus; Zhejiang University 310058 Hangzhou P. R. China
2. State Key Laboratory of Silicon Materials; Department of Materials Science and Engineering, Zhejiang University; Hangzhou 310027 P. R. China
3. Department of Electromagnetic Engineering; Royal Institute of Technology; 100 44 Stockholm Sweden