Cryogenic refractometer for high accuracy measurements of the refractive index of materials

A prototype for a compact and relatively simple system capable to measure the refraction index of glasses, between 0.4 to 1.7 micron, at room and cryogenic temperatures (T=100-300 K), with an absolute precision of few parts of 100'000 has been developed at INAF facilities in Merate. It is based onto the measurement of the deviation angle of monochromatic light passing through a prism sample placed in the cryogenic chamber. The precision of the measurements depends on many factors, in particular the angle measurement and the thermal stability. Different subsystems have been studied to keep all these parameters under accurate control. One of the main issues has been the trade-off between the simplicity of the set-up and the precision of the refractive index measurement.