Quantum fluctuations of the current in a tunnel junction at optical frequencies

We have investigated the mechanism at the origin of the infra-red radiation emitted by a biased tunnel junction by detecting photons at frequencies $\nu<eV/h$. To address this regime, the bias voltage $V$ exceeds one volt and the potential profile of the tunnel barrier is driven far from its equilibrium state. As a consequence, the $I(V)$ characteristic of the junction is strongly nonlinear. At optical frequencies, the transport through the junction cannot be simply expressed in term of the dc current and the current fluctuations are no longer described by the fluctuation-dissipation relation. Taking into account the energy and voltage dependence of the transmission of the tunnel junction in a Landauer-B\"{u}ttiker scattering approach, we experimentally demonstrate that the photon emission results from the fluctuations of the current inside the tunneling barrier.