A company with a lot of information on the optical properties of silicon is EEV (formerly English Electric Valve) in Chelmesford, U.K. They may be able to give you information versus temperature (at least for room temperature and 77 K - they are in the business of making scientific CCD's). They also have data like refractive index versus wavelength versus temperature. People you might contact there include Dr. Peter Poole or Mr. Ian Flint. I don't have their e-mail adreses but EEV is on the Web. John West