  Atomic sized plasmonic tunnel junctions are of fundamental interest, with
great promise as the smallest on-chip light sources in various optoelectronic
applications. Several mechanisms of light emission in electrically driven
plasmonic tunnel junctions have been proposed, from single-electron or higher
order multi-electron inelastic tunneling to recombination from a steady-state
population of hot carriers. By progressively altering the tunneling conductance
of an aluminum junction, we tune the dominant light emission mechanism through
these possibilities for the first time, finding quantitative agreement with
theory in each regime. Improved plasmonic resonances in the energy range of
interest increase photon yields by two orders of magnitude. These results
demonstrate that the dominant emission mechanism is set by a combination of
tunneling rate, hot carrier relaxation timescales, and junction plasmonic
properties.
