  Dynamic wavelength tunability has long been the holy grail of photodetector
technology. Because of its atomic thickness and unique properties, graphene
opens up new paradigms to realize this concept, but so far this has been
elusive experimentally. Here we employ detailed quantum transport modeling of
photocurrent in graphene field-effect transistors (including realistic
electromagnetic fields) to show that wavelength tunability is possible by
dynamically changing the gate voltage. We reveal the phenomena that govern the
behavior of this type of device and show significant departure from the simple
expectations based on vertical transitions. We find strong focusing of the
electromagnetic fields at the contact edges over the same length scale as the
band-bending. Both of these spatially-varying potentials lead to an enhancement
of non-vertical optical transitions, which dominate even the absence of phonon
or impurity scattering. We also show that the vanishing density of states near
the Dirac point leads to contact blocking and a gate-dependent modulation of
the photocurrent.
