Several technologies,
including photodetection, imaging, and data
communication, could greatly benefit from the availability of fast
and controllable conversion of terahertz (THz) light to visible light.
Here, we demonstrate that the exceptional properties and dynamics
of electronic heat in graphene allow for a THz-to-visible conversion,
which is switchable at a sub-nanosecond time scale. We show a tunable
on/off ratio of more than 30 for the emitted visible light, achieved
through electrical gating using a gate voltage on the order of 1 V.
We also demonstrate that a grating-graphene metamaterial leads to
an increase in THz-induced emitted power in the visible range by 2
orders of magnitude. The experimental results are in agreement with
a thermodynamic model that describes blackbody radiation from the
electron system heated through intraband Drude absorption of THz light.
These results provide a promising route toward novel functionalities
of optoelectronic technologies in the THz regime.