Fundamental Study of Ultra-Precision Machining and Near Surface Damage Evolution in Single Crystal Fluorides for Advanced Optics

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Single crystal calcium fluoride is one of only a few materials that can be used for optics (lenses) which require the transmission of ultraviolet light. This is a result of its exceptionally low absorption in the ultraviolet spectral region. Optics produced from calcium fluoride are typically machined using an ultra-precision machining process with single crystal diamond tools, referred to as diamond turning. This machining process introduces near surface damage which increases the material's absorption of ultraviolet light. The mechanisms responsible for the degradation of optical quality caused by machining are not scientifically understood. This lack of understanding is limiting applications where diamond turning, as opposed to polishing, is required, for example, for nanometer precision aspheric or free-form optics. This study will contribute to new fundamental understanding of both the nature and extent of subsurface damage introduced into these brittle, optical materials by diamond turning. It is envisioned that the findings of this work will be applicable to a range of single crystal alkaline earth fluorides which are finding emerging uses in optical applications. The project is an international collaboration between Oklahoma State University (OSU), the Laboratory for Precision Machining (LFM) at the University of Bremen, Los Alamos National Laboratory (LANL) and Carl Zeiss Jena GmbH. NSF is only providing funding for the work done by OSU, but leverages research facilities not available in the US to the benefit of US innovation in several several sectors, most notably the ultraviolet optics used for lithograph in integrated circuit production. This represents a very large industry in the United States, and the results of this fundamental research could provide significant tangible benefits to the domestic economy. The international collaboration also provides a unique exposure of an American graduate student to manufacturing research at the highest levels of sophistication in both Germany and the US. The research objective of this research study is to test the hypothesis that the degradation in optical performance of single crystal calcium fluoride, which has been finished by ultra-precision machining, is directly related to the nature and extent of the near surface damage introduced. Ultra-precision machining experiments based on linear planing with round nose single crystal diamond tools will be used to determine the critical depth of cut to produce a non-fractured surface for a given crystal orientation. Based on these findings, linear planing and face turning experiments will be performed to generate surfaces for which the surface and subsurface state can be evaluated. Channeling Rutherford backscattering spectrometry (channeling RBS), cross-sectional transmission electron microscopy (XTEM) and high resolution transmission electron microscopy (HR-TEM), and x-ray diffraction (XRD) will be used to provide a quantitative characterization of the subsurface. The optical performance of the surface will then be assessed by measuring transmissivity using Ultraviolet-Visible (UV-Vis) spectroscopy and birefringence using a polarimeter allowing for the establishment of a quantitative link between spectrally resolved optical performance and subsurface damage to be made.