The picture shows a contour plot of the component of the Poynting vector perpendicular to a metal surface where there is one slit flanked by 10 grooves (periodically distributed) at each side (geometrical parameters: slit and gooves widths 40nm, groove depth: 100 nm, groove period: 500 nm, working wavelength: 560 nm). Image dimensions: 70 x 30 microns. In the calculation, the metal is assumed to be a perfect conductor (i.e. the fields do not penetrate into the metal).

Image courtesy of Dr. Luis Martin-Moreno, Universidad de Zaragoza, Spain.