Photoresponse and nonequilibrium properties of a superconductor-normal-metal point contact

We report measurements of the photoresponse of superconductor-normal-metal point contacts (SNPC's) between electrochemically etched W whiskers (tip radius 10-200 nm) and bulk superconducting Ta immersed in superfluid He at 1.8 K. These observations are shown to be well described by a model in which the incident light induces an increase in thr effective electron temperature of the irradiated W tip, and a nonequilibrium quasiparticle density in the Ta resulting in a decrease in the superconducting gap. The resulting optically induced current increment through the contact manifests itself as a photoresponse Voltage across the contact. and can be computed from the BTK conductance theory of ballistic low-barrier SNPC's. The effect is used in this work to probe the increase in effective electron temperature of the nanoscale metal tip induced by the incident light; this temperature is shown to vary with laser power as T-eff(3)-T-He(3) = aI(laser), . an effect that follow's from a previously unobserved heat conduction mechanism in which the nanoscale dimension of thr tip plays an important role. The photoresponse also provides a direct observation of destruction of superconductivity in the vicinity of the contact at sufficiently large currents through the contact.