High Reflection Coatings
Melles Griot offers a wide variety of high-reflection coatings for mirrors, beamsplitters, polarizing beamsplitters, dichroic mirrors, bandpass filters, and rejection filters. Some of these coatings are applied to optics as requested; others are offered only as an integral part of specialized optical elements. High-reflection coatings are ordered in the same way as anti-reflection coatings, namely by appending the three-digit coating suffix to the catalog number of the part being ordered. High-reflection coatings may be applied to the outside of a component, such as a flat piece of glass, to produce a first-surface mirror. Alternately, they may be applied to an internal surface to produce a second-surface mirror, such as a prism. High-reflection coatings can be categorized as either metallic or dielectric coatings. |
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Metallic Coatings |
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Metallic coatings are used primarily for mirrors and are not classified
as thin films in the strictest sense. They do not rely on principles
of interference, but rather on the optical properties of the coating
material. However, metallic coatings are often overcoated with thin
dielectric films to increase reflectance over a desired range of
wavelengths or angles of incidence. In these cases, the metallic
coating is said to be "enhanced." Overcoating metallic coatings with a hard, single, dielectric layer of half-wave optical thickness improves abrasion and tarnish resistance but only marginally affects optical properties. Depending on the dielectric used, such overcoated metals are referred to as durable, protected, or hard coated. The main advantages of metallic coatings are broadband spectral performance, insensitivity to angle of incidence and polarization, and low cost. Their primary disadvantages are lower durability, lower reflectance, and lower damage threshold. |
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Dielectric Coatings |
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High-reflectance dielectric layers work on the same principles as
dielectric antireflection coatings. Quarter-wave thicknesses of
alternately high- and low-refractive index materials are applied to
the substrate to form a dielectric multilayer as shown in the figure
below. By choosing materials of appropriate refractive indices, the
various reflected wavefronts can be made to interfere constructively
in order to produce a highly efficient reflector. |
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A simple quarter-wave stack |
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The peak reflectance value is dependent upon the ratio of refractive
indices of the two materials, as well as the number of layer pairs.
Increasing either increases the reflectance. The width of the reflectance curve (versus wavelength) is also determined by the film index ratio. The larger the ratio, the wider the high-reflectance region. In contrast to antireflection coatings, the inherent shape of a high-reflection coating can be modified in several different ways. The two most effective ways of modifying the performance curve are to use two or more stacks centered at slightly shifted design wavelengths, or to slightly perturb the layer thickness within a stack. Reflectance of such films can easily be made to exceed the highest metallic reflectances over limited wavelength intervals. Such films are effective for both s- and p-polarization components and over a wide angle-of-incidence range. At oblique incidence, reflectance is markedly reduced. Because of the materials chosen for the multilayer, durability and abrasion resistance of such films are normally superior to those of metallic films. |
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| Optics Guide Copyright 2002 Melles Griot Inc. |