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Non-conventional superconducting materials for Superconducting Tunnel Junctions as optical detectors

In my graduation thesis I described the results of a study of the near-equilibrium lifetimes of elementary excitation (quasiparticles) in superconductors not investigated in previous papers for the development of new possible Superconducting Tunnel Junction (STJ) configurations using these new materials as detectors for Ultraviolet, Optical and Near Infrared. The overcoming of some performance weaknesses of conventional optical detectors, as the limited quantum efficiency at short wavelenghts, the inability of photon counting and the possibility to have simultaneously high spatial and wavelenght resolution, can be achieved by using STJs detectors. The possibility to have more advantageous solutions by using new materials is part of researches by "Superconductivity" Group from the Faculty of Engineering of Naples and other international groups (E.S.A., Max Planck Institute, Yale University). These detectors' working bases itself on the measurement of the excess quasiparticles produced as a result of the photoabsorption process in a STJ. For a best measurement of the current pulse which is due to the flow of quasiparticles through the electrodes of the junction, it's important that the relaxation processes (no excitation) are very slow and the tunnel process is very fast. These relaxation processes can be related to the low frequency part of the superconductor's phonons density of states. In order to extend non-equilibrium superconductivity knowledge to new materials for potential superconducting detectors, I used an ad hoc assumption based on the Debye approximation for the phonon density of states and on the McMillan formula for weakly stong coupling superconductors which gives a good fit to experimental data for the critical temperature. This theory has been applied to conventional superconducting materials too (Al, Nb...). Then potential detectors have been developed using materials selected on the ground of high critical temperature and slow relaxation processes. For a large part of studied materials (quantum efficiency is 40-50 per cent), the simulated spettroscopic responses give resolutions in the range 10-44nm when a radiation of 500nm is absorbed. The configurations based on Mo and Mo-Re alloys have good behaviour as regards absorption efficiency, critical temperature and collected charge. For each model it's possible, in theory, to obtain best performance by modifying the geometry of the junction and the external circuit. These expedients are necessary for those materials (v. Nb, V, Ta...) with very low density of generated quasiparticles because the resolution is degraded by electronic noise. The possibility to realize such configurations employing chosen materials depends on all problems related to junction quality... An interesting material, for which technology has been developed for other applications, is the Titanium Nitride. For this material it's been obtained a resolution of 45nm at 500nm and 0.3K.

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