Refractive index measurements of Ge

A program has been started at NIST to make high-accuracy measurements of the infrared (IR) index properties of technologically important IR materials, in order to provide the IR optics community with updated values for the highest quality materials now available. For this purpose, we designed and built a minimum-deviation-angle refractometry system enabling diffraction-limited index measurements for wavelengths from 0.12 μm to 14 μm. We discuss the apparatus and procedures that we use for IR measurements. First results are presented for germanium for the wavelength range from 2 μm to 14 μm, with standard uncertainties ranging from 2 × 10^-5 near 2 μm to 8 × 10^-5 near 14 μm. This is an improvement by about an order of magnitude of the uncertainty level for index data of germanium generally used for optic design. A Sellmeier formula fitting our data for this range is provided. An analysis of the uncertainty is presented in detail. These measurements are compared to previous measurements of Ge.