1. Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentations; Zhejiang University; Hangzhou 310058 People's Republic of China
2. Department of Physics; Zhejiang University; Hangzhou 310007 People's Republic of China
3. Department of Electrical and Computer Engineering, Graduate School of Science and Technology; Kumamoto University; Kumamoto 860-8555 Japan
4. Department of Electromagnetic Engineering, School of Electrical Engineering; Royal Institute of Technology; S-100 44 Stockholm Sweden