Optical materials for near infrared Wollaston prisms

The optical characteristics of birefringent materials transmitting in the near IR (0.9 - 2.5 mu m) are analyzed. Wollaston prisms with large beam separation and virtually free from lateral chromatism - e.g. with fields of view > 1 arcmin and image elongation < 0.3 '' - could be manufactured using AgGaS2 or LiYF4 (YLF). These materials are used for non linear applications and may therefore find another interesting application in IR focal plane instruments. From the optical point of view the best material is AgGaS2 but YLF may be practically more convenient because of its lower refractive index (n similar to 1.45), better thermo-mechanical properties and lower price.
New measurements of the transparency of Calcite at room and cryogenic temperatures are also presented, these show significant absorption (similar or equal to 0.4 cm(-1)) of the ordinary ray already at lambda = 2 mu m and very strong bands (absorption coefficient a > 3 cm(-1)) beyond 2.25 mu m. The absorption does not decrease significantly when the crystals is cooled, CaCO3 should not be therefore used in instruments working beyond 2.0 mu m.