  Graphene has extraordinary electronic and optical properties and holds great
promise for applications in photonics and optoelectronics. Demonstrations
including high-speed photodetectors, optical modulators, plasmonic devices, and
ultrafast lasers have now been reported. More advanced device concepts would
involve photonic elements such as cavities to control light-matter interaction
in graphene. Here we report the first monolithic integration of a graphene
transistor and a planar, optical microcavity. We find that the
microcavity-induced optical confinement controls the efficiency and spectral
selection of photocurrent generation in the integrated graphene device. A
twenty-fold enhancement of photocurrent is demonstrated. The optical cavity
also determines the spectral properties of the electrically excited thermal
radiation of graphene. Most interestingly, we find that the cavity confinement
modifies the electrical transport characteristics of the integrated graphene
transistor. Our experimental approach opens up a route towards cavity-quantum
electrodynamics on the nanometre scale with graphene as a current-carrying
intra-cavity medium of atomic thickness.
