  Graphene is a very attractive material for broadband photodetection in
hyperspectral imaging and sensing systems. However, its potential use has been
hindered by tradeoffs between the responsivity, bandwidth, and operation speed
of existing graphene photodetectors. Here, we present engineered
photoconductive nanostructures based on gold-patched graphene nanoribbons,
which enable simultaneous broadband and ultrafast photodetection with high
responsivity. These nanostructures merge the advantages of broadband optical
absorption, ultrafast photocarrier transport, and carrier multiplication in
graphene nanoribbons with the ultrafast transport of photocarriers to the gold
patches before recombination. Through this approach, high-responsivity
operation is achieved without the use of bandwidth- and speed-limiting quantum
dots, defect states, or tunneling barriers. We demonstrate high-responsivity
photodetection from the visible to the infrared regime (0.6 A/W at 0.8 {\mu}m
and 11.5 A/W at 20 {\mu}m) with operation speeds exceeding 50 GHz. Our results
demonstrate an improvement of the response times by more than seven orders of
magnitude and an increase in bandwidths of one order of magnitude compared to
those of higher-responsivity graphene photodetectors based on quantum dots and
tunneling barriers.
