  The monolithic integration of novel nanomaterials with mature and established
technologies has considerably widened the scope and potential of nanophotonics.
For example, the integration of single semiconductor quantum dots into photonic
crystals has enabled highly efficient single-photon sources. Recently, there
has also been an increasing interest in using graphene - a single atomic layer
of carbon - for optoelectronic devices. However, being an inherently weak
optical absorber (only 2.3 % absorption), graphene has to be incorporated into
a high-performance optical resonator or waveguide to increase the absorption
and take full advantage of its unique optical properties. Here, we demonstrate
that by monolithically integrating graphene with a Fabry-Perot microcavity, the
optical absorption is 26-fold enhanced, reaching values >60 %. We present a
graphene-based microcavity photodetector with record responsivity of 21 mA/W.
Our approach can be applied to a variety of other graphene devices, such as
electro-absorption modulators, variable optical attenuators, or light emitters,
and provides a new route to graphene photonics with the potential for
applications in communications, security, sensing and spectroscopy.
