LETTERS Near- to mid-IR refractive index of 28Si, 29Si and 30Si monoisotopic single crystals

We have prepared ²⁸Si, ²⁹Si and ³⁰Si single crystals with enrichments above 99.9 at % and a silicon single crystal of natural isotopic composition. The oxygen and carbon concentrations in all the crystals are within 5×10¹⁵ cm^-3, and the content of metal impurities is 0.01 — 0.1 ppma. The refractive index of the crystals has been determined in the range 1.05-25.5 μm using interference refractometry, and its dispersion and material dispersion have been determined.