  We report compact, scalable, high-performance, waveguide integrated
graphene-based photodetectors (GPDs) for telecom and datacom applications, not
affected by dark current. To exploit the photothermoelectric (PTE) effect, our
devices rely on a graphene-polymer-graphene stack with static top split gates.
The polymeric dielectric, poly(vinyl alcohol) (PVA), allows us to preserve
graphene quality and to generate a controllable p-n junction. Both graphene
layers are fabricated using aligned single-crystal graphene arrays grown by
chemical vapor deposition. The use of PVA yields a low charge inhomogeneity 8 x
10$^{10}$ $cm^{-2}$ at the charge neutrality point, and a large Seebeck
coefficient 140 ${\mu}$V K$^{-1}$, enhancing the PTE effect. Our devices are
the fastest GPDs operating with zero dark current, showing a flat frequency
response up to 67 GHz without roll-off. This performance is achieved on a
passive, low-cost, photonic platform, and does not rely on nanoscale plasmonic
structures. This, combined with scalability and ease of integration, makes our
GPDs a promising building block for next-generation optical communication
devices.
