Superconducting tunnel junctions can be used as either thermal detectors or photon counters in the visible range. One of the main mechanisms responsible for the detector losses is the reflection occurring at the surface of the irradiated film. For niobium the reflectance is about 50% in the visible range. To enhance the responsivity we have considered the effect of antireflection coatings made with hydrogenated amorphous silicon-nitrogen alloys. Such materials, having a tunable refractive index, allow multilayers to be realized which reduce the reflectivity to a few per cent. A stochastic optimization algorithm has been used for the design of the multilayer; it adopts the maximum value of the reflectance in the wavelength range of interest as the figure of merit. Some designs are presented and discussed. To test the effect of the antireflection coatings at 4.2 K, an experimental set-up has been developed where the light spot can be moved by a piezoelectric micropositioner from a coated to an uncoated part of the junction. [References: 11]
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