Thermal origin of light emission in nonresonant and resonant nanojunctions

Abstract

Electron tunneling is associated with light emission. In order to elucidate its generating mechanism, we provide an experimental ansatz that employs fixed-distance epitaxial graphene as metallic electrodes. In contrast to previous experiments, this open geometry permits an unobscured light spread from the tunnel junction, enabling both a reliable calibration of the visible to infrared emission spectrum and a detailed analysis of the dependence of the parameters involved. In a nonresonant geometry, the emitted light is perfectly characterized by a Planck spectrum. In an electromagnetically resonant environment, resonant radiation is added to the thermal spectrum, both being strictly proportional in intensity. In full agreement with a simple heat conduction model, we provide evidence that in both cases the light emission stems from a hot electronic subsystem in interaction with its linear electromagnetic environment. In a long-running discussion whether the light is of thermal or electromagnetic origin, these results on graphene nanojunctions clearly favor the thermal picture.