Comparative study of surface plasmon scattering by shallow ridges and grooves

We revisit the scattering of surface plasmons by shallowsurface defects for both protrusions and indentations of various lengths, which are deemed infinite in one dimension parallel to the surface. Subwavelength protrusions and indentations of equal shape present different scattering coefficients when their height and width are comparable. In this case, a protrusion scatters plasmons like a vertical point dipole on a metal surface, while an indentation scatters like a horizontal point dipole on a metal surface. We corroborate that long and shallow asymmetrically shaped surface defects have very similar scattering coefficients, as already found with approximate methods. In the transition from short shallow scatterers to long shallow scatterers, the radiation can be understood in terms of interference between a vertical and a horizontal dipole. The results attained numerically are exact and accounted for with analytical models.