  Above-threshold light emission from plasmonic tunnel junctions, when emitted
photons have energies significantly higher than the energy scale of the
incident electrons, has attracted much recent interest in nano-optics, while
the underlying physical mechanism remains elusive. We examine above-threshold
light emission in electromigrated tunnel junctions. Our measurements over a
large ensemble of devices demonstrate a giant material dependence of photon
yield (emitted photons per incident electrons), as large as four orders of
magnitude. This dramatic effect cannot be explained only by the radiative field
enhancement effect due to the localized plasmons in the tunneling gap. Emission
is well described by a Boltzmann spectrum with an effective temperature
exceeding 2000 K, coupled to a plasmon-modified photonic density of states. The
effective temperature is approximately linear in the applied bias, consistent
with a suggested theoretical model in which hot carriers are generated by
non-radiative decay of electrically excited localized plasmons. Electrically
driven hot-carrier generation and the associated non-traditional light emission
could open new possibilities for active photochemistry, optoelectronics and
quantum optics.
