Strong coupling in the far-infrared between graphene plasmons and the   surface optical phonons of silicon dioxide
  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. 
Isaac. J. Luxmoore, Choon How Gan, Peter Q. Liu, Federico Valmorra,   Penglei Li, Jerome Faist, and Geoffrey R. Nash
cond-mat.mes-hall