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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-08

Definition: thin-film coatings made of transparent dielectric materials, e.g. for laser mirrors or anti-reflection coatings

Dielectric coatings, also called thin-film coating, consist of thin (typically sub-micron) layers of transparent dielectric materials, which are deposited on a substrate. Their function is essentially to modify the reflective properties of the surface. They can be used for highly reflecting laser mirrors or partially transmissive output couplers, for dichroic mirrors (treating different wavelengths differently), for anti-reflection coatings, for various kinds of optical filters (e.g. for attenuation of certain wavelength regions), beamsplitters, heat reflectors, solar cell covers, and thin-film polarizers. As interference of multiple reflections is at the heart of the principle of operation, such coatings are also called interference coatings. While simple single-layer coatings are often used e.g. as anti-reflection coatings, dielectric mirrors normally use dozens of thin-film layers, sometimes even more than 100.

In many cases, the coating substrate is some kind of glass, with a wide transparency range and high optical quality (low bubble content), a very smooth surface (after proper polishing), and high durability. However, dielectric coatings can also be applied to crystalline materials, e.g., as anti-reflection coatings on nonlinear crystals for nonlinear frequency conversion and Pockels cells, or on semiconductor devices such as edge-emitting laser diodes, vertical cavity surface-emitting lasers, and photodiodes. A further area of increasing importance is the fabrication of dielectric coatings on polymers (plastic materials), as plastic optics are increasingly used due to their competitive properties, e.g. in terms of price and the ease of fabricating non-spherical surfaces (e.g. for aspheric lenses).

Fabrication and Choice of Materials

This section applies mostly to coatings for glasses and crystalline materials, even though some aspects also apply to polymer optics. The fabrication of dielectric mirrors is usually based on one of the following techniques:

• Electron beam deposition involves the evaporation of material in a crucible by heating with an electron beam, which is generated from a hot filament and focused with a magnetic field. In the vacuum chamber, the evaporated material moves to the substrate, which can be covered with a mechanical shutter as soon as the right amount of material has been deposited. The target substrate is heated to improve the quality.
• A similar method uses evaporation by resistive heating of the crucible.
• Ion-assisted deposition (IAD) essentially works like e-beam evaporation, but involves an additional ion beam (consisting of oxygen and/or argon) which hits the target substrate. The comparatively high energy of the ions allows a reordering of the deposited material, leading to denser coatings, even without heating the substrate. The method works well for oxide coatings (e.g. SiO₂ or TiO₂), but not for fluorides, which tend to disassociate.
• Ion beam sputtering (IBS) uses an ion beam hitting a metal or metal oxide target to sputter material to the target substrate. It generates fairly uniform, non-porous coatings with good adhesion and very low surface roughness (possibly below 1 Å), and is well reproducible. However, this method is relatively slow, requires expensive equipment and materials, and is less flexible than other methods.

In any case, one starts with some homogeneous substrate material such as BK7 glass, fused silica, or CaF₂. Common coating materials are oxides such as SiO₂, TiO₂, Al₂O₃, and Ta₂O₅, and fluorides such as MgF₂, LaF₃ and AlF₃. The layers obtained are usually amorphous, with a density which can (depending on the fabrication technique) deviate from that of bulk material by more than 10%. Electron-beam deposition typically generates materials with lower densities, and thus also a lower refractive index. Such porous coatings have microvoids which can fill up with water when exposed to humid air; in effect, the refractive index and thus the whole properties of the coating depend on the humidity. Ion-assisted deposition and particularly ion beam sputtering achieve a higher density and accordingly a lower dependence on humidity. The optical damage threshold can also depend on the fabrication method.

Materials with a high refractive index contrast need to be used for high reflectivity mirrors, and particularly when a large reflection bandwidth is required. However, the chosen materials should also allow for fabrication with high optical quality and should have high stability under given environmental conditions (concerning laser wavelength and intensity, operation temperature, humidity, etc.).

Important aspects for the selection of a fabrication technique are

• the suitability for given coating materials
• the precision of the layer thickness values (which may be increased with automatic control involving in-situ growth monitoring)
• the optical quality of the deposited layers (influencing e.g. the scattering losses)
• the ability of the coatings to withstand high optical intensities
• the uniformity of layer thickness values over a larger area
• the consistency (reproducibility) and stability of obtained refractive indices
• the required substrate temperature
• the time required for the growth

For example, ion-assisted deposition produces TiO₂ films which are more compact and thus more stable and homogeneous and have a higher refractive index, compared with e-beam evaporation, which however is a faster process. The reason is essentially that TiO₂ has a tendency to grow in low-density nanostructures, which can be destroyed (compacted) by irradiation with high-energy ions.

In-situ growth monitoring is crucial for obtaining precisely controlled layer thickness values. One uses the fact that the optical reflection or transmission properties can be used during the process to monitor the thickness of the currently grown layer, so that the growth process for a layer can be stopped at exactly the right time. A challenge is that the growth temperature usually differs considerably from the intended operating temperature of the coating, and accurate temperature-dependent refractive index data are often not available.

Apart from the basic fabrication method, the process parameters such as the substrate temperature and growth rate can also be important for the quality. The details are often confidential proprietary information of the fabricators.

The requirements on the substrate material vary, depending on the application. For highly reflective mirrors, important technical aspects can be the surface roughness, but also the thermal expansion coefficient (which is ideally similar to that of the coating materials) and the thermal conductivity. (In high-power lasers, residual absorption in the coating can cause some bulging of the mirror surface, inducing thermal lensing.) For mirrors with partial transmission of the light (e.g. for output couplers of lasers, or dichroic mirrors), it is also important to have good transparency of the substrate in the relevant wavelength range in addition to high optical quality. The back side may have to be given an anti-reflection coating.

For AR coatings on laser crystals and particularly on nonlinear crystals with anisotropic thermal expansion, but also for substrates with small curvature radii or for optical devices exposed to some chemicals, it can be a challenge to obtain coatings which are sufficiently stable. Optical damage, often occurring at microscopic defects, can also be a problem for devices operating with high optical intensities.

Dielectric mirrors can also be made of crystalline semiconductor materials, grown e.g. with molecular beam epitaxy (MBE) or with metal-organic chemical vapor deposition (MOCVD). Such mirrors are typically parts of some larger structures, such as vertical cavity surface-emitting lasers.

The article on dielectric mirrors explains in some detail how the optical properties of dielectric coatings can be calculated, and which aspects are important for designing such structures.

Bibliography

See also: dielectric mirrors, dichroic mirrors, anti-reflection coatings

Category: photonic devices