Dynamic wavelength tunability has long been the holy grail of photodetector technology. Because of their atomic thickness and unique properties, carbon nanomaterials such as carbon nanotubes and graphene open up new paradigms to realize this concept, but so far this has been elusive experimentally. In this presentation, I will discuss the value behind tunable photodetectors, and present detailed quantum transport modeling of photocurrent in realistic devices, including full simulations of the electromagnetic fields. Our work shows that wavelength tunability is possible by dynamically changing the gate voltage, and we reveal the phenomena that govern the behavior of this type of device and show significant departure from the simple expectations based on vertical optical 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. We further highlight the challenges in realizing this technology by studying the impact of charge fluctuations on optical absorption.