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