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4 November 1994 Overcoming low index limitations in antireflection coatings with additional thickness
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Proceedings Volume 2253, Optical Interference Coatings; (1994)
Event: 1994 International Symposium on Optical Interference Coatings, 1994, Grenoble, France
It has been shown empirically that the lack of very low index of refraction materials is the major limitation to achieving a very low reflectance coating over a very broad spectral band. It has been shown in the same work that additional coating thickness can be employed to make up for this deficiency to a certain extent. Thicknesses which are an order of magnitude thicker than the minimum necessary for a reasonable very broad band antireflection (AR) coating can reduce the reflection to about one half that of the minimum thickness case. This result is empirically predictable to a satisfactory degree, but the underlying reasons for this have not been clear. This paper explores the principles which contribute to the understanding of this effect of additional thickness by empirical and comparative means. The Fourier viewpoint adds to the understanding of variations from the ideal design. We have previously shown that the ideal AR coating, when any and all indices are available, would be a smooth inhomogeneous `step-down' in index from the substrate to the medium. The form of the index profile is approximately a Gaussian decay from the substrate to the medium or similarly described by Southwell as a `quintic function'. When the medium is a vacuum or air and the lowest available index is represented by a real material such as MgF2 at index 1.38 rather than values very close to that of the medium, the discontinuity from the smooth step-down profile causes a reflection residual that cannot be overcome by adjustments in the rest of the smooth profile. Additional thickness and the appropriate index profile can be used to reduce, but no eliminate entirely, this residual reflection. We discuss our observations and findings on these effects in more detail in the body of this paper.
© (1994) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Ronald R. Willey "Overcoming low index limitations in antireflection coatings with additional thickness", Proc. SPIE 2253, Optical Interference Coatings, (4 November 1994);


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