Cryogenic temperature-dependence of HRFZ Silicon refractive index from Mid- to Far-IR down to 5 K

High Resistivity Float-Zone (HRFZ) Silicon is a well-known material employed for optical components at far infrared wavelengths. For accurate design of those components its temperature-dependent refractive index is an important parameter. In this contribution we present the results of transmission measurements in a Fourier Transform Spectrometer of a HRFZ Silicon sample cooled down to 5K to estimate the refractive index at the passage from the mid to the far-IR down to 5K. From room-temperature to 5K the refractive index and absorption coefficient showed a decrease of about 0.83 % and of about 7-15 %, respectively.