We demonstrate experimentally that deviations of a nearly sinusoidal grating from the exact sine profile strongly influence the shape of the surface plasmon resonance anomalies in higher diffraction orders, especially in the first order. It is shown that this effect can be applied to an accurate determination of the actual surface corrugation of nearly sinusoidal gratings.
E.-H. Rosengart and I. Pockrand, "Influence of higher harmonics of a grating on the intensity profile of the diffraction orders via surface plasmons," Opt. Lett. 1, 194-195 (1977)