  Spoof surface modes on nanostructured metallic surfaces are known to have
tailorable dispersion dependent on the geometric characteristics of the
periodic pattern. Here we examine the spoof plasmon dispersion on an isolated
grating and a grating-planar mirror cavity configuration. The spoof polariton
dispersion in the cavity is obtained using the scattering matrix approach, and
the related differential modal density of states is introduced to obtain the
mode dispersion and classify the cavity polariton modes. The grating-mirror
cavity geometry is an example of periodically nanostructured metals above a
planar ground plane. The properties discussed here are relevant for
applications ranging from thin electromagnetic perfect absorbers to near-field
radiative heat transfer.
