Quantum current fluctuations in a tunnel junction at optical frequency*

We report the measurement of emission noise by a planar metallic tunnel junction driven far from its equilibrium state. To investigate this regime, we bias the junction at voltage V>1Volt, and measure the emitted photons at optical frequency (f < eV/h ∼ 10¹⁴Hz). This emission results from the scattering of surface-plasmon-polaritons, generated by high frequency current fluctuations inside the junction due to tunneling electrons (shot-noise). This allows noise measurement at timescales smaller than the RC time of the junction. We show that the emitted photon power P_f(V) at frequency f, which is proportionnal to the current spectral density S_II(f,V), isn't given by the usual fluctuation-relation which have been previously demonstrated in the GHz range for metallic and supraconductive linear tunnel junction. Our results are in agreement with a prediction based on the Landauer-Büttiker scattering approach accounting for the energy/voltage dependence of the transmission of the tunnel barrier. With a quantitative calculation of the emission efficiency, we demonstrate that the photon emission results from current fluctuations inside the barrier.