  Mid-infrared (mid-IR) photo-detection has been recently growing in importance
because of its multiple applications, including vibrational spectroscopy and
thermal imaging. We propose and demonstrate a novel pyro-resistive
photo-detection platform that combines a ferroelectric substrate (a z-cut
LiNbO3 crystal) and a graphene layer transferred on top of its surface with
electrical connections. Upon strong light absorption in the LiNbO3 substrate
and the subsequent temperature increase, via the pyroelectric effect,
polarization (bound) charges form at the crystal surface. These causes doping
into graphene which in turn changes its carrier density and conductivity. In
this way, by monitoring the graphene electrical resistance one can measure the
incident optical power. . Detectivities of about 10^5 cm sqrt(Hz)/W in the 6 to
10 microns wavelength region are demonstrated.We explain the underlying
physical mechanism of the pyro-resistive photo-detection and propose a model
that reproduces accurately the experimental results. We also show that up to
two orders of magnitude larger detectivity can be achieved by optimising the
geometry and operating in vacuum, thus opening the path to a new class of
mid-IR photo-detectors that can challenge classical HgCdTe devices, especially
in real applications where cooling is to be avoided and low cost is crucial.
