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Comparison of surface and bulk scattering in optical coatings

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Abstract

In many cases it was shown that light scattering from optical coatings takes its origins in the interface irregularities of the stacks. This result is mainly due to a "substrate effect", consisting in the reproduction of the initial surface at each interface of the multilayer [1]. However the improvement of polishing techniques permits today to obtain substrate roughnesses less than 0.1 nm for certain materials; in these conditions the substrate effect is largely reduced and intrinsic properties of the thin film materials may be studied. The residual light scattering from these high quality filters originates from random irregularities characteristic of the materials and may be present in their bulk or still at their interfaces. Therefore an accurate characterization of materials requires to have at our disposal an adequate tool that enables to display the origin of scattering (surface or bulk) in the stacks. For this we use vector theories of surface and bulk whose basic principles will be briefly described. Numerical calculation then permits to emphasize the essential differencies between these two phenomena and the conclusions permit to choose the right experiments to realize. In other words, can we use wavelength, incidence or polarization to detect the origin of scattering? Is there any particular stack more adequate for this study? What is the relative amount of surface and bulk scattering? All these points will be clearly discussed.

© 1992 Optical Society of America

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