  For optical communication, information is converted between optical and
electrical signal domains at a high rate. The devices to achieve such a
conversion are various types of electro-optical modulators and photodetectors.
These two types of optoelectronic devices, equally important, require different
materials and consequently it has been challenging to realize both using a
single material combination, especially in a way that can be integrated on the
ubiquitous silicon platform. Graphene, with its gapless band structure, stands
out as a unique optoelectronic material that allows both photodetection and
optical modulation. Here, we demonstrate a single graphene-based device that
simultaneously provides both efficient optical modulation and photodetection.
The graphene device is integrated on a silicon waveguide and is tunable with a
gate made from another layer of graphene to achieve near-infrared
photodetection responsivity of 57 mA/W and modulation depth of 64%. This novel
multifunctional device may lead to many unprecedented optoelectronic
applications.
