定义：由透明电介质材料制作的薄膜涂层，例如，激光反射镜涂层或者抗反射涂层。

电介质涂层也称为薄膜涂层或者干涉涂层，包含很薄的透明电介质材料层（其典型厚度为亚微米），电介质涂层需要涂覆在衬底材料上。电介质涂层的功能是利用多个光学界面处反射光的干涉效应改变表面的反射性质。它们可用作高反射率的激光反射镜、部分透射的输出耦合器、二向色性反射镜（对于不同波长表现不同）、抗反射涂层、各种光学滤波器（例如，用来衰减特定波长区域的光）、分束器、热反射器、太阳能电池罩和薄膜偏振片。单层涂层通常用作抗反射涂层，而电介质反射镜通常需要很多层（有时大于100层）。一种典型的反射镜为简单的布拉格反射镜，当然还有很多非常复杂的发射镜设计。 
一般电介质涂层都是包含多层具有不同折射率的涂层。也有的渐变折射率涂层可用到梳状滤波器中，其中折射率是连续变化的。这可以通过在材料生长过程中逐渐改变其化学组分来得到。 

大多数情况下，涂层衬底会选用某种玻璃，选取的玻璃通常具有很大的透明范围和很高的光学质量（气泡很少），平滑的表面（抛光后获得）以及很高的耐用性。电介质涂层也可以涂覆在晶体材料上，例如，在非线性晶体中涂覆抗反射涂层用于非线性频率转换和普克尔斯盒，或者涂覆在半导体器件（例如边发射激光二极管、垂直腔表面发射激光器和光电二极管）上。另一个非常重要的领域是在聚合物（塑料材料）上制备电介质涂层，塑料光学的应用越来越广泛，主要是由于其在价格以及制备非球表面等方面的优势。 

电介质涂层的材料性质与制备方法（如下所述）和制备参数（例如，衬底温度或各组分的分压）密切相关。与普通的具有相同化学组分的体材料相比，薄膜层的密度和折射率更小。如需将电介质涂层制备在特定的镀膜机器上，需要知道机器在镀膜情况下的折射率值，因为不同情况下该值会有较大的差别。 

制备过程和材料选择 
这部分主要是针对在玻璃或晶体材料上镀膜的情况，当然也有一些内容适用于聚合物的情况。制备电介质反射镜通常需要用到以下技术中的一种： 

• 电子束沉积通常是采用热灯丝产生电子书来加热坩埚，使坩埚中的材料蒸发，然后再采用磁场使其会聚。在真空容器中，蒸发的材料向衬底移动，当材料沉积的量足够时马上在衬底上覆盖机械快门。加热目标衬底可以提高镀膜质量。对于普通的涂层材料来说，得到的薄层通常具有多个孔，因此会降低其密度以及折射率。由于水可能充满孔洞，而温度升高后水会挥发，所以涂层的光学性质随温度的变化变大。这是截止滤波器设计时面临的一个很大的问题。 
• 另一个类似的蒸镀方法是通过电阻丝加热坩埚。 
• 离子辅助沉积（IAD）工作原理类似于电子束蒸镀方法，但是需要采用一束额外的离子束（包含氧离子或氩离子）入射到目标衬底上。离子具有相对高的能量可以使沉积的材料重新排序，即使不加热衬底也可以得到密度更大的涂层。这种方法非常适宜于氧涂层（例如，SiO2或TiO2），与IBS方法制备的涂层具有类似的质量。但是，IAD不适用于氟化物材料，因为该材料易于分离。 
• 离子束溅射（IBS）采用第二个灯丝使离子中和，然后射向金属或金属氧化物目标物使材料溅射到衬底上。离子的通量和能量可精确独立的控制。IBS方法可以得到相当均匀、无孔的涂层，并且具有良好的附着性，很低的表面粗糙度（可能小于1 Å），可重复性强。但是，制备过程花费时间长，需要昂贵的设备和材料，并且与其它方法相比，这种方法比较不灵活。 
• 等离子体增强反应溅射（APRS）是需要溅射薄金属涂层，而后涂层在一个单独的容器中氧等离子体区域被氧化。对于不同的涂层材料需采用单独的磁控管源。APRS可得到同时具有高精度和高密度的涂层（与IBS类似），并且速度快（与蒸镀技术可比拟）。 

在以上任意一种技术中，开始都需要使用各向同向衬底材料，例如BK7玻璃，熔融石英或者CaF2。常见的涂层材料为氧化物，例如SiO2、TiO2、Al2O3和Ta2O5，或氟化物，例如MgF2、LaF3和AlF3。得到的涂层通常是无定形的，其密度通常与体材料差10%以上（与制备技术有关）。电子束沉积得到的密度更小，因此折射率也更小。这种多孔涂层具有微洞，可以填充水暴露在潮湿的空气中；实际上，涂层的折射率和其全部性质都与湿度有关。离子辅助沉积方法，尤其是离子束溅射方法可以得到更高的密度，湿度对其影响也更小。涂层的光学损伤阈值也与制备方法有关。 

高折射率差的材料可以用在高反射率的反射镜中，尤其是需要很大反射带宽的情况下。但是，选择的材料也必须要保证可以得到很高的光学质量以及在制定的环境条件下能够具有很高的稳定性（主要是指激光波长和强度，工作温度，湿度等）。 
在选择制备方法时需要考虑一下几点： 

• 给定涂层材料的适用性 
• 涂层厚度能达到的精度（若生长时采用实时自动控制可以提高） 
• 沉积层的光学质量（会影响散射损耗等） 
• 涂层能够经受高光强的能力 
• 在很大区域内层厚度的均匀性 
• 折射率的一致性（可重复性）和稳定性 
• 所需的衬底温度 
• 生长所需的时间 

例如，采用离子辅助沉积方法得到的TiO2涂层更加紧凑和更加稳定均匀，并且与电子束蒸镀相比，折射率更高，但是电子束蒸镀方法更加快速。原因主要是TiO2更倾向于在低密度的纳米结构中生长，如果辐照高能粒子时则会破坏该过程而压紧该材料。 

生长过程中实时控制在需要得到精确控制的薄层厚度时非常必要。它主要是利用在生长过程中的光反射和透射性质来控制生长层的厚度，因此在恰当的时间生长过程可以立刻停止。遇到的一个问题是生长时的温度与涂层需要工作的温度通常有很大差别，而我们又不能得到折射率随温度变化的准确关系。

 除了基本的制备方法外，生长过程参数（例如，衬底温度和生长速度）对于涂层质量也非常重要。而这些细节则是生产商涉及所有权的机密资料。 

不同应用对衬底材料的要求不同。若用作高反射镜时，需要考虑表面粗糙度和热膨胀系数（理想情况下与涂层材料本身类似），还有热导率。（用在高功率激光器中时，涂层的吸收效应会使反射镜表面隆起，引起热透镜效应。）对于部分透射的反射镜来说（例如，用做激光器的输出耦合器，或者二色性反射镜），需要很高的光学质量并且需要其衬底在相关波长范围内具有很高的透明度。其背面需要采用抗反射涂层。 

对于激光器晶体上的抗反射涂层来说，尤其是具有各向异性热膨胀系数的非线性晶体，还有具有衬底的曲率半径很小或者光学器件需要暴露在一些化学品中时，很难得到非常稳定的涂层。光学损伤（通常发生在微小的缺陷处）对于工作在高光强情况下的器件也是一个严重的问题。 

电介质反射镜也可以由结晶半导体材料制备得到，采用分子数外延（MBE）或者金属有机气相沉积（MOCVD）方法生长。这种反射镜通常为一些更大结构器件的一部分，例如垂直腔表面发射激光器。 

电介质反射镜词条中详细解释了电介质涂层的光学性质如何计算，该结果在设计结构时非常重要。

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

Alternative terms: thin-film coatings, interference coatings, multilayer coatings

More general term: optical coatings

More specific terms: anti-reflection coatings, mirror coatings

Dielectric coatings, also called thin-film coatings or interference coatings, 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 by exploiting the interference of reflections from multiple optical interfaces. 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), beam splitters, heat reflectors, solar cell covers, and thin-film polarizers. While simple single-layer coatings are often used as anti-reflection coatings, dielectric mirrors normally use dozens of thin-film layers, sometimes even more than 100. A typical mirror design is the simple Bragg mirror, but there are many more sophisticated mirror designs.

The typical kind of dielectric coating consists of discrete layers with substantially different refractive indices. However, there are also gradient-index coatings for rugate filters, where the refractive index is varied continuously. That can be achieved, for example, by gradually varying the chemical composition of the material during growth.

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).

The material properties of the dielectric films can substantially depend on the used fabrication method (see below) and fabrication parameters such as the substrate temperature or partial pressures of various substances. Compared with the ordinary bulk materials of nominally the same chemical composition, thin-film layers often have a reduced packing density and a reduced refractive index. When designing a dielectric coating for fabrication on a certain coating machine, one should have the refractive index data for that machine under the used conditions, as the differences can be substantial.

Dielectric coatings are often applied to optical glasses, but they can also be used for plastic optics.

Designing Multilayer Coatings

The reflection properties of dielectric multilayer coatings can be adapted to many different applications by using suitable coating designs. A coating design involves the choice of dielectric materials (typically, two different ones) and of all the layer thickness values. The article on dielectric mirrors contains some details on design methods.

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. For typical coating materials, the obtained thin films tend to be somewhat porous, leading to a reduced density and subsequently to a reduced refractive index. The optical properties can then exhibit a significantly increased temperature dependence, as water may fill the pores, and may be driven out of the coating at elevated temperatures. This can be a problem for some sensitive steep-edge filter designs, for example.
• 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 ions) 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), which can have a similar quality as those obtained with IBS (see below). IAD is not suitable, however, for fluoride materials, which tend to disassociate.
• Ion beam sputtering (IBS) uses an ion beam which, after neutralization with a second filament, hits a metal or metal oxide target to sputter material to the substrate. The flux and energy of the ions can be controlled independently and precisely. IBS generates fairly uniform, non-porous coatings with good adhesion and very low surface roughness (possibly below 1 Å), and is well reproducible. However, it is relatively slow, requires expensive equipment and materials, and is less flexible than other methods.
• Advanced plasma reactive sputtering (APRS) involves the sputtering of thin metal films, which are subsequently oxidized in a separate oxygen plasma region of the chamber. Separate magnetron sources are used for the different coating materials. APRS combines a high precision and high density of the coatings (similar to that of IBS) with a high speed (comparable to that of evaporation techniques).

In any case, one starts with some homogeneous substrate material such as BK7 glass, fused silica, or CaF₂, which is often pre-processed with method for preparing a very clean surface. 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 anti-reflection 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.

Bragg 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) (→ crystalline mirrors). Such mirrors are often 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.

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