Abstract

The endless search for stable low index materials (n ~ 1.4 to 1.5) for the infrared has focussed on a temporary solution with ThF₄. However, ThF₄ is not ideal environmentally and, further, it presents handling problems because of its radioactivity. For some applications it can produce increased detector noise. Other fluoride compounds possess the low index value required for high efficiency AR coating applications, but are not useable in thick layers because of their large tensile stress and/or moisture vulnerability. The Group IIA fluoride compounds suffer in varying degrees from low density or excessive absorption out to 12 μm in QW layers. Substrate temperatures in excess of 250°C are required to achieve a packing density within 90% of the bulk value. These high temperatures often cause other stress problems in multilayer combinations.

© 1984 Optical Society of America