1. Jiangsu Key Laboratory of Meteorological Observation and Information Processing School of Electronic and Information Engineering Nanjing University of Information Science & Technology Nanjing 210044 P. R. China
2. Department of Physics and London Centre for Nanotechnology King's College London Strand London WC2R 2LS UK
3. Jiangsu Key Laboratory on Optoelectronic Technology School of Physical Science and Technology Nanjing Normal University Nanjing 210023 P. R. China