  We report mid-infrared photocurrent spectra of graphene nanoribbon arrays on
SiO2 dielectrics showing dual signatures of the substrate interaction. First,
hybrid polaritonic modes of graphene plasmons and dielectric surface polar
phonons produce a thermal photocurrent in graphene with spectral features that
are tunable by gate voltage, nanoribbon width, and light polarization.
Secondly, phonon-polaritons associated with the substrate are excited, which
indirectly heat up the graphene leading to a graphene photocurrent with fixed
spectral features. Models for other commonly used substrates show that the
responsivity of graphene infrared photodetectors can be tailored to specific
mid-IR frequency bands by the choice of the substrate.
