Silicon and germanium are important and popular, transmissive, mid-wave infrared (MWIR) and long-wave-infrared (LWIR) optical materials used in a very wide variety of applications including scientific instruments, thermal imaging, and defense infrared systems. Prior to the present study, accurate, cryogenic, refractive index measurements of these two materials using the Cryogenic High Accuracy Refraction Measuring System (CHARMS) at the Goddard Space Flight Center (GSFC) were last completed for NASA’s James Webb Space Telescope program and published around 2006. Numerous infrared optical systems have since been successfully designed and built based on these previous CHARMS index data. A new, more accurate study of cryogenic refractive indices of silicon and germanium has been carried out for sample materials produced in the 2019-2020 time period. This paper presents findings for about a half dozen samples of each material from different boules and, including results from 2006, represents the first significant study of interspecimen index variability of Si and Ge using CHARMS. These new measurements spanned the wavelength region from 1.11-5.43 μm for Si and 1.90-5.43 μm for Ge over the temperature range from 100-310 K. Updated measurement uncertainties for these materials in CHARMS are presented reflecting improvements made to CHARMS since 2006.
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