This work is supported by the National Science Foundation of China (NSFC) under Grant #11575070 and Jefferson Science Associates, LLC under U.S. DOE Contract #DE-AC05-06OR23177 and U.S. DOE Grant #DE-FG02-97ER41028. I would like to thank F. Arleo, B. Duclou, J.-P. Lansberg, T. Lappi, H. Fujii, Y.-Q. Ma, J.-W. Qiu, R. Venugopalan, B.-W. Xiao, F. Yuan for interesting discussions and comments.