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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-04-23

Definition: optical thin-film coatings for reducing reflections from surfaces

An anti-reflection coating (AR coating) is a dielectric coating applied to an optical surface in order to reduce the optical reflectivity of that surface in a certain wavelength range. In most cases, the basic principle of operation is that reflected waves from different optical interfaces largely cancel each other by destructive interference.

Single-layer Anti-reflection Coatings

In the simplest case, an antireflection 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.
• A single-layer coating works only in a limited bandwidth (wavelength 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), more complicated designs may be used, which usually have to be found using numerical techniques. A general tradeoff of such multilayer designs is between a low residual reflectivity and a large bandwidth.

Figure 1: Reflectivity curve for a numerically optimized anti-reflection coating on a BK7 glass substrate for 1064 nm and 532 nm. Two layer pairs of TiO₂ and SiO₂ are used.

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.

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 layers.

Gradient Index Coatings

A wide range of possibilities arises from gradient index coatings, where the composition of a layer material is gradually varied. 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. [5]), which simulate 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.

Applications

Anti-reflection coatings [1] are often used for optical components in order to reduce optical losses (and sometimes also the detrimental influence of reflected beams). The residual reflectivity for a given wavelength and angle of incidence is often of 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.

Damage Threshold

Apart from the reflection properties, the damage threshold of anti-reflection coatings can be of interest, for example when intracavity components for Q-switched lasers require such coatings. 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.

Bibliography

See also: dielectric coatings, dielectric mirrors

Category: photonic devices