  Under high electrical current, some materials can emit electromagnetic
radiation beyond incandescence. This phenomenon, referred to as
electroluminescence, leads to the efficient emission of visible photons and is
the basis of domestic lighting devices (e.g., light-emitting diodes). In
principle, electroluminescence can lead to mid-infrared (mid-IR) emission of
confined light-matter excitations called phonon-polaritons, resulting from the
coupling of photons with crystal lattice vibrations (optical phonons). In
particular, phonon-polaritons arising in the van der Waals crystal hexagonal
boron nitride (hBN) exhibit hyperbolic dispersion, which enhances light-matter
coupling. For this reason, electroluminescence of hyperbolic phonon-polaritons
(HPhPs) has been proposed as an explanation for the peculiar radiative energy
transfer within hBN-encapsulated graphene transistors. However, since HPhPs are
confined, they are inaccessible in the far-field, so that any hint of
electroluminescence is only based on indirect electronic signatures and needs
to be confirmed by direct observation. Here, we demonstrate far-field mid-IR
({\lambda} = 6.5 {\mu}m) electroluminescence of HPhPs excited by strongly
biased high-mobility graphene within a van der Waals heterostructure, and we
quantify the associated radiative energy transfer through the material. The
presence of HPhPs is revealed via far-field mid-IR spectroscopy due to their
elastic scattering at discontinuities in the heterostructure. The associated
radiative flux is quantified by mid-IR pyrometry of the substrate receiving the
energy. This radiative energy transfer is shown to be reduced in hBN with
nanoscale inhomogeneities, demonstrating the central role of the
electromagnetic environment in this process.
