Our Cryogenic Optical Test facility has the capability for measuring the refractive index of materials with accuracies up to 10-6
at temperatures in the range of 35K - 400 K and wavelengths in the range of 250 nm - 25,000 nm, using different spectrophotometers:
Our method used for refractive index measurements is a variant of the “method of minimum deviation by a prism” in which a collimated beam interacts with the prism under an angle of incidence θ. The beam passes the front surface and is refracted into the prism towards the back surface where it is reflected back. Any wavelength for which the prism material is transparent corresponds with a value for θ for which the beam is exactly retro-reflected along the same optical path towards its opposite direction as shown in the following picture:
The refractive index as a function of wavelength is measured by setting the angle of incidence and scanning the wavelength range of the instrument to find the wavelength to which the refractive index corresponds, causing a local peak in the spectrum.
Typical results are shown in the following graphs (which have the abscissa in wavenumbers):
Below is an example of results we obtained on a Germanium sample: