Recent investigations into high-aspect-ratio short-pitch metal grating structures have shown that it is possible to excite surface plasmon polaritons (SPPs) even in the zero-order region of the spectrum. The predominant reason this is possible is that extremely large bandgaps occur in the SPP dispersion curves, which are caused by the large depths, and heights, of the structures. The form of the resultant dispersion curves has also been found to be highly dependent on the shape of the grating profile. We present an extension to a previously published paper that described the nature of the SPPs excited on narrow-ridged short-pitch metal gratings in the classical mount by considering the case in which the radiation is incident at nonzero azimuthal angles (the conical mount). In particular, we consider the case of 90° and 45° azimuthal angles and discuss the coupling to the SPP modes and the way in which polarization conversion is evident on such structures.
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