Infrared studies of induced superconductivity in indium-arsenide quantum wells

The describe experiments which probe the electrical and infrared optical properties of superconductor-semiconductor hybrid structures, specifically structures where superconducting Nb contacts the two-dimensional electron gas in an InAs quantum well. In order to study the supercurrent-carrying Andreev states in a Josephson junction, we conduct transport, experiments in Josephson field-effect transistors, where a voltage applied to an insulated gate modulates the density of electrons in and the supercurrent through an InAs-based weak link. The density-dependence of the critical current fits a model for the Andreev states in the junction in which quasiparticles travel ballistically across the quantum well between superconducting electrodes. Probing the dynamics of these states on the scale of the superconducting energy gap in Nb, we find robust signatures of the proximity effect in the InAs using far-infrared transmission spectroscopy. A model for the conductivity of superconducting thin films predicts transmission spectra which show similarities to the transmission spectra measured in our structures. From this fit we extract an effective energy gap in the InAs quantum well due to the proximity of superconducting Nb. We probe these structures in both superconductor/normal metal geometries—where the superconducting Nb lies directly on top of a layer of InAs—and superconductor/normal metal/superconductor geometries—where the an InAs quantum well connects superconducting Nb stripes. In both geometries we observe proximity effects in the far-infrared transmission spectra of the InAs.