1. Ningbo Institute
of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
2. Department
of Physics, Faculty of Science, Ningbo University, Ningbo 315211, China
3. Quantum Photonic Science Research Center
and Department of Physics, Hanyang University, Seoul 133-791, South Korea
4. Department of Physics, Sungkyunkwan University, Suwon 440-746, South Korea
5. Faculty
of Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom