  We study plasmonic resonances in electrostatically gated graphene nanoribbons
on silicon dioxide substrates. Absorption spectra are measured in the mid-far
infrared and reveal multiple peaks, with width-dependent resonant frequencies.
We calculate the dielectric function within the random phase approximation and
show that the observed spectra can be explained by
surface-plasmon-phonon-polariton modes, which arise from coupling of the
graphene plasmon to three surface optical phonon modes in the silicon dioxide.
