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Thin films are thin coating layers present in many different technologies. Despite their widespread use, most of the focus is around how they provide a barrier to various stimuli, or how they can be used as a conductive medium. However, there is another class of thin films that we will discuss, and these are optical thin films.
Optical thin films are a very thin layer of materials that are designed to enhance the transmission, reflection, and/or polarization properties of an optical component. They can be applied to simple substrates such as a glass lens, or more complex optical parts.
The thickness and atomic layers vary from coating to coating, as the thickness has a big effect on how the optical film works, with a particular emphasis being that the different thicknesses can be tuned to be effective to certain wavelengths of light. While the thickness will be different from application to application, to be effective against any wavelengths of visible light, the coating needs to be at least 500 nm in thickness. The class of thin film coatings that can be applied to optical components also extends out to anti-reflective coatings.
Optical thin films are applied to the surface of optical components because it is very rare that a surface will exhibit an ideal optical behavior on its own. The thin film is there to enhance these properties and pretty much every optical component in use today has had some kind of optical coating applied to it. Like any coating, they are applied to the outside surface of the optical component, so they are often designed to not only improve the optical properties of the surface but to also improve their resistance to corrosion and degradation as well.
Optical thin films are often more complex than other types of coatings and are usually composed of multiple atomic layers. These layers are usually a mixture of metallic and dielectric layers, all of which have different thicknesses depending on the desired properties of the coating. At the nanoscale, the difference in the refractive index between the different deposited layers, the air between the layers and the surface of the optical component all affect the way that the light reflects, refracts and transmits into the optical component.
In effect, how well the thin film coating is designed is dependent upon the number of layers in the film, the thickness of the film, and the refractive index difference between the different layers—with the key to a successful coating utilizing the interference effects caused by the differences in refractive index effectively.
Thin film coatings can possess up to hundreds of different layers to yield the desired optical effect, and the calculations to determine the ideal layer sizes and composition can take days to perform using a computer. As computing capabilities have become greater over the years, it has led to more complex and more effective optical coatings being produced.
The fabrication of optical thin films is not that much different to the fabrication of other thin films, or the creation of ultra-thin nanomaterials. The main way to create an optical thin film is to use deposition techniques.
Physical vapor deposition (PVD) techniques are the mostly widely utilized and include methods such as evaporative deposition, ion beam sputtering and plasma sputtering to generate layers of the film.
Atomic layer deposition (ALD)—which is technically a type of chemical vapor deposition (CVD)—is another method outside of PVD that is used in the generation of these films. The fabrication of these films is not the easiest task, but the thin films must be atomically precise to provide the required benefits and is the reason why these methods are used. The specific choice of fabrication method is often dependent on many factors, including deposition time, surface geometry, coating stress, coating performance, and the cost of each method.
As mentioned above, anti-reflective coatings are a big application area, but other examples include color separation filters, telescope mirrors coatings, energy-saving glass, halogen lights, and surface plasmon resonance (SPR) detectors, to name a few examples.
Although, almost all optical components in high-end optic and photonic devices will have some type of optical coating. The effects are also seen in nature such as when a thin film of oil has been spilled and colored patterns are observed, to the observable color patches on both plants and insects (most commonly butterflies).
References and Further Reading
Abrisa Technologies: https://abrisatechnologies.com/2015/08/what-is-a-thin-film-optical-coating/
Edmund Optics: https://www.edmundoptics.com/resources/application-notes/optics/an-introduction-to-optical-coatings/
University of Arizona, College of Optical Sciences: https://wp.optics.arizona.edu/winter-school-workshop/wp-content/uploads/sites/16/2015/09/Thin-Film-Optics-Macleod-1.pdf
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