Crossover from above-threshold to below-threshold plasmonic electroluminescent emission at the atomic scale

Light emission from biased metal tunnel junctions has been known for decades, with renewed research interest brought by the rise of the field of plasmonics. Controversies have existed regarding different possible underlying mechanisms that represent distinct physical processes in generating far field emission. Whether the photons come from inelastic processes involving the coherent tunneling of discrete electrons or from radiative recombination of plasmonically excited hot carriers still remains elusive. Here, leveraging planar, lithographically defined aluminum tunnel junctions, we report a crossover from above-threshold emission to below-threshold emission when enlarging the tunneling gap distance via electromigration. Analysis of spectra shows that different above-threshold mechanisms under hot debate can coexist during the light emission process, and particular mechanisms are respectively dominant at different conductance ranges. These experiments show that the light emission mechanism in metal tunnel junctions is really a question of current and voltage regimes and relative timescales.