  We demonstrate a free-space optical communication link with an optical
transmitter that harvests naturally occurring Planck radiation from a warm body
and modulates the emitted intensity. The transmitter exploits an
electro-thermo-optic effect in a multilayer graphene device that electrically
controls the surface emissivity of the device resulting in control of the
intensity of the emitted Planck radiation. We design an amplitude-modulated
optical communication scheme and provide a link budget for communications data
rate and range based on our experimental electro-optic characterization of the
transmitter. Finally, we present an experimental demonstration achieving
error-free communications at 100 bits per second over laboratory scales.
