基金
The authors are grateful to the research group of Prof. Kenneth K. Y. Wong at The University of Hong Kong for their unselfish contribution to this paper. They are also grateful for the supports from the National Basic Research Program of China （973 Program） （No. 2010CB327606）
the National Nature Science Foundation of China （Grant No. 61108007） and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics.
