An anti-reflection coating is a dielectric thin film coating with a specific refractive index and thickness. The coating is applied to an optical surface to reduce the reflectivity/reflectance of that surface due to Fresnel reflections in a specific wavelength range. This article discusses the role of dielectric thin films in optical coatings, specifically anti-reflective coatings.
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What is an Anti-Reflection Coating?
The reflected waves from various optical surfaces are canceled out by each other through destructive interference. Anti-reflection coatings played a leading role in the initial stages of optical thin film technology development.
The major advantages of anti-reflection coatings deposited on lens surfaces include improvement in optical system transmittance, specifically in systems containing several air-to-glass interfaces, and reduction in spurious images and background illumination in the image plane. Anti-reflection coatings are used in several optical systems, such as photovoltaic cells, camera objectives, optical windows, and displays.
Role of Dielectric Thin Films in Anti-Reflection Coatings
Dielectric materials in the form of thin films are utilized extensively in anti-reflection coatings as they can control the propagation of light and decrease the amount of stray light in the system. Dielectric thin films can also be used as filters or low-loss reflectors to transmit specific radiation frequencies selectively.
Commercially available dielectric materials, such as aluminum oxide, cadmium sulfide, ceric oxide, lanthanum fluoride, lithium fluoride, magnesium oxide, silicon monoxide, thorium oxide, zirconium dioxide (ZrO2), and thorium oxyfluoride are used for anti-reflection coatings.
The interference coating performance does not deteriorate over the incidence angle ranges involved in common optical systems in which aperture and field semi-angles in these systems are restricted to less than 25o. However, polarization effects must be considered for special conditions where good performance is necessary over a larger range of incidence angles.
An anti-reflection dielectric thin film coating consisting of a single quarter-wave layer of a material with a refractive index closer to the geometric mean value of refractive indices of two adjacent media can be designed for normal incidence. The coating can cancel two reflections of equal magnitude that emerge at two interfaces by destructive interference.
However, the single-layer coating approach has several limitations, including the need to identify dielectric coating material with a suitably low refractive index, specifically where the bulk medium possesses a low refractive index.
For instance, thin films of cryolite with a low refractive index can be damaged easily, while films of magnesium fluoride are robust with a high refractive index. Moreover, the single-layer coating is only effective in a limited angular range and bandwidth.
The limitations of single-layer dielectric thin film coatings can be overcome using multiple layers in which the number of adjustable parameters, such as thickness and refractive index, are higher, and the refractive indices can be selected corresponding to readily available materials.
Although multi-layer coatings are used in several applications, the high cost of applying these coatings has limited the use of anti-reflection coatings with more than two layers to extremely specialized cases.
Complicated designs of multi-layer dielectric film coatings are used when anti-reflective properties are needed for a very large wavelength range or no suitable medium can be identified for a single-layer coating.
In such multi-layer designs, a general trade-off is made between a large bandwidth and a low residual reflectance. For instance, V coatings deliver a good performance only in a narrow bandwidth of 10 nm, while broadband coatings deliver moderate performance over a wide wavelength range.
The stringency of the specifications is increased while the design parameters remain within the practicable limits when the number of dielectric thin film layers is increased in an anti-reflection coating.
The wavelength range over which the coating is effective increases with the rising number of layers in the coating. However, the coating can behave as an inhomogeneous film if the total coating thickness is fixed.
Studies Using Dielectric Thin Films as Anti-Reflection Coatings
Reflection losses primarily reduce the efficiency of all photovoltaic device types, with the first reflection loss occurring at the glass-air interface of the photovoltaic module. Almost 4% of the solar energy is lost at this surface when no light trapping mechanism is used in the module.
Most commercial cadmium telluride (CdTe) solar modules currently lack a light-trapping mechanism to address the reflection losses at the glass-air interface. In a study published in the IEEE Journal of Photovoltaics, researchers designed and deposited a broadband multilayer dielectric thin film coating on the thin-film CdTe solar cell glass surface to minimize the reflection losses.
The coating contained four dielectric layers of alternating silicon dioxide (SiO2) and ZrO2) thin films. High-rate-pulsed dc magnetron sputtering was used to deposit the layers. The findings of this study confirmed an increase in transmission by 2-5% over the spectrum used by the thin-film CdTe solar cell. Additionally, the weighted average reflection decreased from 4.22% to 1.24%, while the absolute efficiency increased by 0.38%.
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References and Further Reading
Lissberger, P. (2002). Optical applications of dielectric thin films. Reports on Progress in Physics, 33, 197. https://iopscience.iop.org/article/10.1088/0034-4885/33/1/305
Kaminski, P. M., Lisco, F.,Walls, J. M. (2014). Multilayer Broadband Antireflective Coatings for More Efficient Thin Film CdTe Solar Cells. IEEE Journal of Photovoltaics, 4, 1, 452-456. https://ieeexplore.ieee.org/document/6637049
Schubert, M. F., Mont, F. W., Chhajed, S., Poxson, D. J., Kim, J. K., Schubert, E. F. (2008). Design of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials by genetic algorithm. Optics Express, 16, 5290-5298. https://opg.optica.org/oe/fulltext.cfm?uri=oe-16-8-5290#:~:text=Co%2Dsputtered%20and%20low%2Drefractive%2Dindex%20materials%20allow%20the,angles%2C%20and%20includes%20material%20dispersion.
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