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Antireflective coatings are applied to many optical materials, from everyday glasses to imaging instrumentation, to help the surface absorb light so that it doesn’t interfere with the user. In this article, we look at what antireflective coatings are and how different analyses can be used to see if they are performing efficiently.
What are Antireflective Coatings?
Antireflective coatings act a barrier between incident light and the surface of a lens, or other optical component, to reduce the reflection of light. These coatings are used on everyday products such as glasses, but they can also be used on components in imaging systems to improve the efficiency of the unit and the contrast of the image. Even photolithography areas benefit from antireflective coatings, as the coating helps to remove image distortions on the substrate.
Depending on the application, these coatings do vary; and for high-tech applications such as laser amplifiers, the coating must be of the order of 10-4 in thickness. The many types of antireflective coating include index-matching, single-layer interference, multi-layer interference, absorbing and circular polarizing coatings. Antireflective coatings even occur in nature, and moth’s eyes are one such example of where a nanoscale film enables the moth to efficiently see in the dark – and scientists have since recreated the effects using biomimicry approaches.
How Antireflective Coatings work
There are two main ways in which antireflective coatings work, and this is dependant on whether a thick film or a thin film is applied to the surface. There are three main components to an antireflective coating, which are air, the coating and the glass substrate. Antireflective effects using a thick film occur because of the difference in the index of refraction between the coating and the layers either side of it. Thick film coatings are not dependent on the specific thickness, but the coating needs to be thicker than a wavelength of light to work.
On the other hand, antireflective effects from thin film coatings arise when the coating is between a quarter and a half of a wavelength of light. This effect causes a destructive interference which reduces the reflection on the surface. For thin films, the angle at which the light strikes the surface is often important in determining how much light is absorbed.
Analyzing Antireflective Coatings
There are various methods to determine how efficient a coating is performing, and thus, can define it success. There are also no issues with analyzing very thin coatings.
UV-Vis spectroscopy is a common method for determining the absolute reflectance and relative specular reflectance of an antireflective coating. A UV-Vis spectrometer can perform both reflection and absorption measurements to determine how well the coating is performing.
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These methods can be performed on a wide range of materials, including materials that are at either end of the spectrum and exhibit a low or high reflectance. This method can be used on lenses, LCDs and laser mirrors, but an absolute reflectance attachment or a specular reflectance attachment is required.
There are some slight differences between the approaches. As the names suggest, absolute reflectance obtains absolute values and the data obtain by relative specular reflectance are relative to a known reference material – commonly an aluminum reflecting mirror. The obtained values will differ, even when used on the same material. This is because the reflectance of the reference is never 100% and the value is often dependent on the wavelength used. A common reflectance for the reference is between 80 and 95%, but the results are usually repeatable if the same reference mirror is used.
On the high-tech end of the spectrum, i.e. for components used in lasers, there is another method. This is known as the threshold-shift method. This is more of a mathematical method than a physical method. In this method, the relationship between the threshold current and the laser length is measured before and after the coating is deposited. To perform this method, the laser length, the gain and the effective refractive index must be known to perform the required calculations.
This method can be performed without knowing the parameters of the coating, but this process is a lot more complex and requires many other methods to work. This method can only be used for components with a high reflectivity, but it can be used on films that are of the order of 10-3. By knowing all the parameters and relevant equations, the reflectivity can be deduced using the difference in the threshold current before and after the coating has been applied.
Penn State University: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.498.2244&rep=rep1&type=pdf
“Laser diode facet modal reflectivity measurements”- Repasky K. S., et al, Applied Optics, 2000, DOI: 10.1364/AO.39.004338