White Paper: Optical Properties Of Single Crystal Materials

By Ronald Kappler, Meller Optics, Inc.

There has been very little written of a practical nature regarding the information needed when specifying single crystal materials for an order- or quotation. This article will hope to address that lack of practical information in as simple a way as possible. Over the years we have had many customer contacts and drawings testifying to the need for some basic information. Some of the non-technical people in optics do not realize that crystal orientation must be specified, even if the specification is "none required". Even some of the technical people in optics do not realize that a non-cubic material could have a serious impact on their optical system by introducing unwanted polarization effects if the proper orientation has not been specified. Almost all of the education and training received by technical and nontechnical personnel is geared toward glass-like materials. Usually information about crystals is learned only when there is a current need.

The similarity is that most single crystal materials can be optically fabricated to the same geometrical and optical accuracies as can optical glasses. Two of the differences are that they are almost always more sensitive to fracturing upon rapid heating or cooling, and they have a much larger range of hardness and solubility than optical glasses. To offset their thermal and environmental problems, most of these materials have a spectral range much wider than glass. Also most of the time smgle crystal materials require the specification of one or more of the crystal axis to get the desired results from the crystal optic.