2 November 1984 Complex Index Interference Films On Metal Substrates
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Abstract
A spectral reflectance model for a homogeneous complex index film on a metal substrate is presented which illustrates the importance of interference in producing selective absorption. In this model the real part of the film's index of refraction (n) and the film thickness (d) are chosen to satisfy the normal quarter wave condition at a shoulder wavelength of approximately 1-2 microns, while the imaginary part of the film's index (k) is chosen so as to match amplitudes of a front and back surface reflected wave. In order to isolate the effects of interference, all optical constants are assumed to be independent of wavelength and all surfaces are considered smooth. The conditions under which zeroes in the reflectivity occur are investigated with a vector diagram technique and the effect of varying these conditions on the spectral reflectivity is studied. A tradeoff between the solar absorptance and the width of the visible-infrared transition region is shown to be a natural consequence of the conditions placed on (n,k) of the interference film. The effect of grading the film's index on this tradeoff can be treated analytically without approximating the film as a stack of homogeneous layers. This analytical approach is outlined and preliminary results are presented.
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Keith A. Snail, "Complex Index Interference Films On Metal Substrates", Proc. SPIE 0502, Optical Materials Technology for Energy Efficiency and Solar Energy Conversion III, (2 November 1984); doi: 10.1117/12.944790; https://doi.org/10.1117/12.944790
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