  The photoresponse of graphene at mid-infrared frequencies is of high
technological interest and is governed by fundamentally different underlying
physics than the photoresponse at visible frequencies, as the energy of the
photons and substrate phonons involved have comparable energies. Here we
perform a spectrally resolved study of the graphene photoresponse for
mid-infrared light by measuring spatially resolved photocurrent over a broad
frequency range (1000-1600 cm$^{-1}$). We unveil the different mechanisms that
give rise to photocurrent generation in graphene on a polar substrate. In
particular, we find an enhancement of the photoresponse when the light excites
bulk or surface phonons of the SiO$_2$ substrate. This work paves the way for
the development of graphene-based mid-infrared thermal sensing technology.
