基金Supported by National High Technology Research and Development Program of China (863 Program) (2008AA040201), National Natural Science Foundation of China (90920302), National Science and Technology Pillar Program of China (2009BAH41B01), National Natural Science Foundation of China and Research Grants Council of Hong Kong (60931160443) The authors thank Michael T. Johnson in the Depart- ment of Electrical Engineering, Marquette University in USA for the experiments suggestion and helping to improve the English writing.