  The combination of plasmonic nanoparticles and graphene enhances the
responsivity and spectral selectivity of graphene-based photodetectors.
However, the small area of the metal-graphene junction, where the induced
electron-hole pairs separate, limits the photoactive region to sub-micron
length scales. Here, we couple graphene with a plasmonic grating and exploit
the resulting surface plasmon polaritons to deliver the collected photons to
the junction region of a metal-graphene-metal photodetector. This results into
a 400% enhancement of responsivity and a 1000% increase in photoactive length,
combined with tunable spectral selectivity. The interference between surface
plasmon polaritons and the incident wave introduces new functionalities, such
as light flux attraction or repulsion from the contact edges, enabling the
tailored design of the photodetector's spectral response. This architecture can
also be used for surface plasmon bio-sensing with direct-electric- readout,
eliminating the need of complicated optics.
