Corners of grating will become the scatting points of the incident light which was the source of GSPs. These scatting points can be divided into two kinds due to the geometric symmetry, which is A and B. Each scatting point will scatter into two GSP modes propagating in two directions (blue and green).
Yuan et al. Nanoscale Research Letters 2014 9:90 doi:10.1186/1556-276X-9-90