The formation process of laser induced periodic surface structure(LIPSS) on polymethylmethacrylate(PMMA) is investigated.The PMMA film is prepared on an indium tin oxide wafer and irradiated with a p-polarized XeCl excimer laser (15 ns,308 nm),assisted with an external electric field.The characteristics of the surface corrugations are determined by an atomic force microscopy.It is found that the corrugation spacing of nanometer scale can be achieved and remarkably decreased by changing the applied voltage f...