Light emission by tunneling Cooper pairs across a vacuum gap

We have observed visible light emission by tunneling Cooper pairs across the vacuum gap of a scanning tunneling microscope (STM) formed by a Nb tip and a Nb sample at 4.7 K. Besides the usual STM light emission that is excited by single electron tunneling, we have observed a new peak at the photon energy of ℏω=2eV0 for a bias voltage V0. This peak is observed only when the system is cooled below the superconducting transition temperature of niobium (9.3 K). We conclude that the new emission is excited by inelastic tunneling of Cooper pairs in superconducting Nb.