1. Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan 430 074 P. R. China
2. School of Optical and Electronic Information Huazhong University of Science and Technology Wuhan 430 074 P. R. China
3. Nonlinear Physics Centre Research School of Physics Australian National University Canberra 2601 Australia
4. Electrical Engineering Division School of Engineering University of Cambridge Cambridgeshire Cambridge CB3 1SE UK