Anti-reflection Coatings

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

An anti-reflection coating (AR coating) is a dielectric thin-film coating applied to an optical surface to reduce the reflectance (also often called reflectivity) of that surface due to Fresnel reflections – at least in a certain wavelength range. Examples of the application of such coatings are spectacles, optical systems like camera objectives, optical windows, displays and photovoltaic cells.

In most cases, the basic principle of operation is that reflected waves from different optical interfaces largely cancel each other by destructive interference.

Note that there are also anti-glare surfaces, which suppress reflections in an entirely different way: by diffuse scattering from a microscopically rough surface. Such surfaces are suitable e.g. for some displays and viewing ports, but normally not for laser applications, and should be carefully distinguished from anti-reflection surfaces.

Single-layer Anti-reflection Coatings

In the simplest case, an anti-reflection thin-film coating designed for normal incidence consists of a single quarter-wave layer of a material the refractive index of which is close to the geometric mean value of the refractive indices of the two adjacent media. In that situation, two reflections of equal magnitude arise at the two interfaces, and these cancel each other by destructive interference.

The limitations of this approach are twofold:

• It is not always possible to find a coating material with suitable refractive index, particularly in cases where the bulk medium has a relatively low refractive index (e.g. in the case of plastic optics).
• A single-layer coating works only in a limited bandwidth (wavelength range) and a limited angular range.

Multilayer Coatings

If no suitable medium for a single-layer coating can be found, or if anti-reflective properties are required for a very broad wavelength range (or for different wavelength ranges simultaneously, or for different angles of incidence), more complicated designs may be used, which usually have to be found using numerical techniques, implemented in suitable thin-film design software. A general trade-off of such multilayer designs is between a low residual reflectance and a large bandwidth. So-called V coatings have a high performance only in a narrow bandwidth (order of 10 nm), whereas broadband coatings offer moderate performance but in a wide wavelength range.

Two layer pairs of TiO₂ and SiO₂ are used. The coating design was made with the software RP Coating.

Apart from those properties, the tolerance to growth errors may also be of interest: there are sophisticated coating designs which reach a high performance only for very precise manufacturing. The growth error tolerance is therefore an important aspect to be considered in the design.

Multilayer anti-reflection coatings are often applied to optical glasses and crystals, but they can also be used for plastic optics.

Design Methods

Analytical design rules exist for simple types of anti-reflection coatings with very few thin-film layers. For more sophisticated designs, numerical optimization algorithms similar to those described in the article on dielectric mirrors can be used. The resulting designs are normally not easily understood, as the anti-reflection properties result from a complicated interference of the reflections from various interfaces.

Designing AR Coatings

For designing multilayer interference coatings, a flexible simulation and design software is indispensable. It must not only be able to calculate all relevant optical properties for a given design, but assist you in finding a suitable design for achieving given target properties. The RP Coating software is an ideal tool for such work, as it is particularly flexible. For example, you can define a figure of merit, formulating the optimization goal, of any conceivable kind.

Gradient Index Coatings

A wide range of possibilities arises from gradient index coatings (or graded-index coatings) [2, 3, 11], where the composition of a layer material is gradually varied (as in rugate filters). In the simplest case, a smooth index transition between two optical materials over a length scale of a few wavelengths can suppress fairly well the reflection over a wide spectral and angular range. This is difficult to realize, however, for surfaces next to air, since all solid materials have a refractive index significantly different from that of air. One solution is to use nanooptics in the form of sub-wavelength pyramid structures or the like (moth eye structure), see e.g. Refs.  [1], [2] and [7]. Such structures (which can be called photonic metamaterials) imitate a smooth transition of the refractive index to 1 by smoothly reducing the amount of solid material in a plane parallel to the surface. However, there are also solutions without nanooptics, in particular the integration of gradient index layers into a multilayer coating. This allows for good broadband anti-reflection properties in a wide angular range without using materials with a very small refractive index.

Coatings with Strongly Absorbing Layers

An unusual type of anti-reflection coating is one consisting of a very thin layer of some strongly absorbing material. The thickness can be only some tens of nanometers, i.e., far less than usually required for lossless AR coatings, as strong imaginary components of the propagation constant of such media lead to substantial phase changes. The incident light is largely absorbed by such structures, rather than transmitted. Such anti-reflection structures are called photonic metamaterials due to the combination of sub-wavelength structures, although simple interference phenomena are sufficient for understanding their characteristics [13].

Applications of AR Coatings

Anti-reflection coatings [3] are often used for optical components to reduce optical losses and sometimes also the detrimental influence of reflected beams. The residual reflectance for a given wavelength and angle of incidence is often on the order of 0.2%, or less (in a limited bandwidth) with careful optimization. For application on prescription glasses, the achievable suppression of reflections is significantly lower, since the coating must operate in a wide wavelength range and for a wide range of incidence angles. AR coatings are also used on laser crystals and nonlinear crystals. In such cases, additional challenges can arise from anisotropic thermal expansion e.g. of lithium triborate (LBO) crystals.

In most cases, AR coatings are used on optical interfaces with an area of at least a few millimeters squared. However, it is also possible to produce such coatings on the ends of optical fibers, sometimes even in jacketed and connectorized assemblies. There are various technical difficulties, e.g. related to outgasing of polymer jackets in a vacuum chamber and to the limited number of fiber ends which can be treated in one batch, but specialized sputtering processes have been developed which mitigate these problems. The coating performance can be as good as for normal bulk surfaces, at least for simple coating designs with only fewer layers.

Damage Threshold

Apart from the reflection properties, the optical damage threshold of anti-reflection coatings can be of interest, for example for use in components for Q-switched lasers. Depending on the material combination, an AR coating can have a higher or lower damage threshold than the substrate material.

Even for given coating materials, the damage threshold can vary considerably depending on the fabrication technique. Ion beam sputtering is known to allow for relatively high damage thresholds.

More to Learn

Encyclopedia articles:

• reflectance
• dielectric coatings
• dielectric mirrors
• Fresnel reflections
• black coatings

Bibliography

(Suggest additional literature!)

Suppliers

The RP Photonics Buyer's Guide contains 142 suppliers for anti-reflection coatings. Among them:

Vortex Optical Coatings

We produce a wide range of AR coatings with different designs like V coatings and broadband coatings, covering the spectral range from 300–6000 nm. Custom AR coatings are not a problem – we design and manufacture on site, all part of our rapid turnaround service, we like to think we offer the best service out there, typical one week!

UltraFast Innovations

UltraFast Innovations (UFI) offers broadband anti-reflection coatings suitable for ultrafast laser devices, with numerous options for different wavelength ranges and angles of incidence.

OPTOMAN

OPTOMAN offers high-performance Ion Beam Sputtered anti-reflective coatings with reflectance per surface down to R < 0.01%. IBS coatings are optimized for temperature and humidity-independent performance, high damage threshold (> 168 J/cm² @ 1064 nm, 9.8 ns, 100 Hz, 223 μm). Single wavelength, multiwavelength, or broadband coatings can be designed for the spectral range from 200 nm to 5000 nm.

With absorption being at low level (<1 ppm per coated surface at 1064 nm), AR coated windows and lenses feature enhanced lifetimes.

In-stock windows and lenses with various ROC's and coatings are available on OPTOSHOP.

Knight Optical

For maximum transmission, Knight Optical offers a range of AR coatings to reduce surface reflections. Available as single layer MgF₂ coatings, or multi-layer coatings, we can offer AR coatings centred to a specific wavelength or over a broadband wavelength range. With AR coatings from UV to IR, Knight Optical can help with your custom order.

Shanghai Optics

Shanghai Optics provides custom anti-reflection coatings with appropriate materials and optimized configuration on a substrate material specified by the customer, with specified transmittance and a specified wavelength region.

Perkins Precision Developments

Perkins Precision Developments (PPD) manufactures custom IBS coatings, including anti-reflection coatings. PPD utilizes Ion Beam Sputtering coating technology because it is ideal for complex spectral designs, high power Nd:YAG and fiber lasers and applications where it is critical to minimize losses from absorption and/or scatter. IBS thin films have densely packed micro-structures resulting in stable, easy to clean optics that are insensitive to environmental changes such as heat, humidity and pressure. Our AR coatings exceed damage thresholds of 25 J/cm² or sometimes even >60 J/cm² at 1064 nm.

Focuslight Technologies

Focuslight Technologies offers AR coatings from DUV (248 nm) to the infrared and a great selection of coating tests and analysis, including spectrometer and LIDT test. Focuslight can coat substrates up to 140 mm × 140 mm at the moment. Capacity available! Customization acceptable!

Shalom EO

Hangzhou Shalom EO offers custom AR coatings, including optical coating design and manufacturing of anti-reflection coatings. Various AR coatings of multiple-layer or single-layer structures are available, like broadband AR (BBAR) coatings, laser line V coatings, dual/multiple wavelength AR coatings, etc. We can tailor the AR coatings that suit different wavelengths and optical substrates.

Hangzhou Shalom EO utilizes the efficient and cost-effective ion-assisted deposition electron beam (IAD e-beam) technique, which enables the creation of denser films with reduced scatter compared to evaporative deposition alone. In our class 1000 clean room coating workshop, we employ leading-edge facilities like SHINCRON MIC-1350TBN coating machines and ultrasonic cleaning machines to clean and fabricate the optical coatings, and we conduct strict inspection for the critical parameters of each batch of optical coatings using up-to-date metrological instruments like PerkinElmer Lambda 1050+ spectrometer, Ultrafast Innovations GOBI white light interferometer.

Besides, Shalom EO also supplies a series of custom optical coatings, comprising high-reflective coatings (both dielectric and metallic), optical filter coatings, beamsplitter coatings, and optical coatings optimized for high-power/ultrafast systems.

EKSMA OPTICS

The advanced ion-beam sputtering (IBS) thin-film coating facility of EKSMA Optics is ready to meet the most critical demands on laser optics. EKSMA Optics optical engineers can design and produce complex low loss and high LIDT anti-reflection coatings to suit your application. By employing IBS technology we can deposit highly reproducible dielectric thin film coatings with precisely controlled spectral parameters.

Avantier

Anti-Reflection (AR) coatings are one of widely used optical thin-film coatings that can be applied to the surfaces of optical elements to reduce reflection and to improve transmittance within some spectral region. We provide AR coatings with appropriate materials and optimized configuration on a substrate material specified by the customer, with specified transmittance and a specified wavelength region.

Ecoptik

Ecoptik can produce single-layer or multilayer anti-reflection coatings on various kinds of optical elements, using different materials. We achieve reflectivities <0.2% or even better at many wavelengths from 355 nm to 1550 nm, and <0.5% at 284 nm or 193 nm, for example.

We also offer dual wavelength AR coatings, e.g. for 532 nm and 1064 nm.

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