This work was supported by grants from the National Basic Research Program of China (the 973 Program, No. 2013CB531303 to Dr. J.G.C.; No. 2014CB744601 to F.W.) and the National Natural Scientific Foundation of China (NSFC, No. 81302754 to P.F.W). It was also supported by PCSIRT (No. IRT13016), the National Key Scientific Instrument and Equipment Development Project of China (No. 2013YQ03092306) and Science Fund for Creative Research Groups of the Natural Science Foundation of Hubei Province (2015CFA020) to J.G.C.