FREQUENCY AND TEMPERATURE-DEPENDENCE OF THE REFRACTIVE-INDEX OF SAPPHIRE

Because the index of refraction is temperature dependent, a temperatur e gradient across a window causes image blur and bore sight error. Pri or to this paper, there have been no direct temperature-dependent meas urements reported on the mid-infrared refractive index for sapphire, a popular infrared window material of high durability. Measurements of dn/dT are reported on the ordinary ray of sapphire in the 4 mu m regio n for the first time. Accurate temperature and frequency dependent ref ractive index models can now be constructed from visible measurements of the refractive index, far-infrared reflectance measurements, thermo -optic coefficient measurements, and infrared measurements of the abso rption coefficient. Visible measurements determine the contribution to the refractive index from electronic transitions. Far-infrared measur ements determine the contributions from fundamental lattice vibrations (phonons). Infrared absorption data are used to determine parameters in a multiphonon sum band model. By applying the Hilbert transform to this multiphonon absorption model, a model for the multiphonon refract ivity is obtained. Two- and three-phonon contributions to the refracti ve index are important for an accurate model that includes temperature dependence. Results for the ordinary- and extraordinary-rays are obta ined. (C) 1998 Elsevier Science B.V. All rights reserved.