An anti-reflective coating (AR coating) is a dielectric thin film coating applied to an optical surface to reduce the reflectivity (also commonly referred to as reflectivity) of light in a specific wavelength range due to Fresnel reflection on that surface. Examples of applications for such coatings include eyewear, optical systems such as camera objectives, optical windows, displays, and photovoltaic cells. In most cases, the basic working principle is that reflected waves from different optical interfaces cancel each other out to a large extent through destructive interference.
It is worth noting: there are also anti-glare surfaces, which suppress reflections in a completely different way, by diffuse scattering of microscopic rough surfaces. Such surfaces are suitable for some displays and viewing ports, but are generally not suitable for laser applications and should be carefully distinguished from anti-reflective surfaces.
Single-layer anti-reflective coating
In the simplest case, an anti-reflective thin film coating designed for perpendicular (normal) incidence is one-quarter of a single wavelength and the refractive index of the material is close to the geometric mean of the refractive indices of two adjacent media. In this case, two waves of equal magnitude will appear at the two interfaces, canceling each other out through destructive interference.
The limitations of this approach are twofold:
- It is not always possible to find a coating material with a suitable refractive index, especially if the base material has a relatively low refractive index (e.g. plastic optics).
- Single-layer coatings only work in a limited bandwidth (wavelength range) and a limited angular range.
Multi-layer coating
If a medium cannot be found suitable for a single coating, or if anti-reflective properties are required over a very wide wavelength range (or for different wavelength ranges at the same time, or at different angles of incidence), more complex designs can be used, often using numerical techniques and implemented in appropriate thin film design software. This multilayer design is generally characterized by a trade-off between low residual reflectivity and large bandwidth. The so-called V-coat has high performance only in a narrow bandwidth (10 nm order), while a broadband coating provides moderate performance over a wide wavelength range.
In addition to these characteristics, it is also worth paying attention to the increase in the tolerance for error: there are complex coating designs, and only very precise manufacturing can achieve high performance. Therefore, increasing the error tolerance is an important aspect to consider in the design. Multilayer anti-reflection coatings are commonly used in optical glasses and crystals, but can also be used in plastic optics.
Design Methodology
For simple types of anti-reflective coatings with very few thin film layers, there are analytical design rules. For more complex designs, numerical optimization algorithms can be used. The resulting design is often not easy to understand, as the anti-reflective properties are caused by complex interference of reflections from various interfaces.
Gradient index coating
Gradient refractive index coatings (or graded refractive index coatings) achieve more feasibility, with a gradual change in the composition of the layered material. In the simplest case, a smooth exponential transition between two optical materials over a length range of several wavelengths can suppress reflections fairly well over a wide spectral and angular range. However, this is difficult to achieve for surfaces close to air because the refractive index of all solid materials is significantly different from that of air. One solution is to use nano-optics in a subwavelength pyramid structure or similar form. This structure, which can be referred to as a photonic metamaterial, simulates a smooth transition to a refractive index of 1 by smoothing the amount of solid material in a plane parallel to the surface. However, there are also solutions that do not have nano-optics, in particular the integration of gradient exponential layers into multi-layer coatings. Will have good broadband anti-reflective properties over a wide angular range without the need to use materials with a very small refractive index.
Coating with a strong absorbent layer
An unusual anti-reflective coating consists of a very thin layer of strongly absorbing material. The thickness may be as small as a few tens of nanometers, i.e. much less than the thickness typically required for non-destructive AR coatings. Because the strong and virtual part of the propagation constant of this medium causes a significant phase transition. The incident light is largely absorbed by this structure and will not be transmitted. Due to the combination of subwavelength structures, this anti-reflective structure is known as a photonic metamaterial.
Application of anti-reflection coatings
Anti-reflective coatings are often used on optical components to reduce optical losses and sometimes the harmful effects of reflected beams. In most cases, anti-reflection coatings are used for optical interfaces with an area of at least a few square millimeters. However, it is also possible to manufacture such coatings at the ends of optical fibers, and ion beam sputtering is known to achieve relatively high damage thresholds.
Impairment threshold
In addition to the reflective properties, the optical damage threshold of the anti-reflective coating is also important, e.g. in the components of lasers switched with Q. Depending on the material combination, the damage threshold of the anti-reflection coating can be higher or lower than that of the substrate. Even for a given coating material, the damage threshold can vary greatly depending on the manufacturing technology. Ion beam sputtering is known to allow for relatively high damage thresholds.
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