  We present a graphene photodetector for telecom applications based on a
silicon photonic crystal defect waveguide. The photonic structure is used to
confine the propagating light in a narrow region in the graphene layer to
enhance light-matter interaction. Additionally, it is utilized as split-gate
electrode to create a pn-junction in the vicinity of the optical absorption
region. The photonic crystal defect waveguide allows for optimal
photo-thermoelectric conversion of the occurring temperature profile in
graphene into a photovoltage due to additional silicon slabs on both sides of
the waveguide, enhancing the device response as compared to a conventional slot
waveguide design. A photoresponsivity of 4.7 V/W and a (setup-limited)
electrical bandwidth of 18 GHz are achieved. Under a moderate bias of 0.4 V we
obtain a photoconductive responsivity of 0.17 A/W.
