  Silicon and germanium are perhaps the two most well-understood semiconductor
materials in the context of solid state device technologies and more recently
micromachining and nanotechnology. Meanwhile, these two materials are also
important in the field of infrared lens design. Optical instruments designed
for the wavelength range where these two materials are transmissive achieve
best performance when cooled to cryogenic temperatures to enhance signal from
the scene over instrument background radiation. In order to enable high quality
lens designs using silicon and germanium at cryogenic temperatures, we have
measured the absolute refractive index of multiple prisms of these two
materials using the Cryogenic, High-Accuracy Refraction Measuring System
(CHARMS) at NASA Goddard Space Flight Center, as a function of both wavelength
and temperature. For silicon, we report absolute refractive index and
thermo-optic coefficient (dn/dT) at temperatures ranging from 20 to 300 K at
wavelengths from 1.1 to 5.6 microns, while for germanium, we cover temperatures
ranging from 20 to 300 K and wavelengths from 1.9 to 5.5 microns. We compare
our measurements with others in the literature and provide
temperature-dependent Sellmeier coefficients based on our data to allow
accurate interpolation of index to other wavelengths and temperatures. Citing
the wide variety of values for the refractive indices of these two materials
found in the literature, we reiterate the importance of measuring the
refractive index of a sample from the same batch of raw material from which
final optical components are cut when absolute accuracy greater than +/-5 x
10^-3 is desired.
