  Controlling, detecting and generating propagating plasmons by all-electrical
means is at the heart of on-chip nano-optical processing. Graphene carries
long-lived plasmons that are extremely confined and controllable by
electrostatic fields, however electrical detection of propagating plasmons in
graphene has not yet been realized. Here, we present an all-graphene
mid-infrared plasmon detector, where a single graphene sheet serves
simultaneously as the plasmonic medium and detector. Rather than achieving
detection via added optoelectronic materials, as is typically done in other
plasmonic systems, our device converts the natural decay product of the
plasmon---electronic heat---directly into a voltage through the thermoelectric
effect. We employ two local gates to fully tune the thermoelectric and
plasmonic behaviour of the graphene. High-resolution real-space photocurrent
maps are used to investigate the plasmon propagation and interference, decay,
thermal diffusion, and thermoelectric generation.
