An Introduction to Optical Coatings
This is Sections 4.1, 4.2, and 4.7 of the Laser Optics Resource Guide.
Optical coatings are used to enhance the transmission, reflection, or polarization properties of an optical component. For example, about 4% of incident light will be reflected at each surface of an uncoated glass component. An anti-reflection coating could be applied to reduce the reflection at each surface to less than 0.1% and a highly reflective dielectric coating could also be applied to increase reflectivity to more than 99.99%. An optical coating is composed of a combination of thin layers of materials such as oxides, metals, or rare earth materials. The performance of an optical coating is dependent on the number of layers, their thickness, and the refractive index difference between them. This application note discusses optical coating theory, different types of common coatings, and coating manufacturing methods.
Thin film optical coatings are typically created by depositing dielectric and metallic materials, such as Tantalum Pentoxide (Ta2O5) and/or Aluminum Oxide (Al2O3), in alternating thin layers. In order to maximize or minimize interference, they are typically λ/4 optical thickness (QWOT) or λ/2 optical thickness (HWOT) of the wavelength of the light used in the application. These thin film layers alternate between high index of refraction and low index of refraction, thereby inducing the interference effects needed (Figure 1).
Figure 1: In a three-layer broadband anti-reflection (BBAR) coating, the correct choice of λ/4 and λ/2 thicknesses of coatings results in a high transmission and low reflection loss
Optical coatings are designed to enhance the performance of an optical component for a specific angle of incidence and polarization of light such as s-polarization, p-polarization, or random polarization. Using the coating at a different angle of incidence or polarization than what it is designed for will result in a significant degradation in performance. Sufficiently large deviations in incidence angle and polarization can result in a complete loss of coating function.
Optical Coating Theory
The Fresnel equations of refraction and reflection must be understood in order to comprehend optical coatings. Refraction is the change in direction of a wave’s propagation as it passes from one optical medium to another and is governed by Snell’s law of refraction:
n1 is the index of refraction of the incident medium, θ1 is the angle of the incident ray, n2 is the index of the refracted/reflected medium, and θ2 is the angle of the refracted/reflected ray (Figure 2).
Figure 2: Light moving from a low index medium to a high index medium, resulting in the light refracting towards the interface normal
The angle of a ray anywhere in a multilayer thin film coating consisting of plane parallel surfaces of different refractive indices can be found using Snell’s law. The internal angle of the ray in the film is independent of the film order or the location of the film in the stack because Snell’s law applies at each interface (Figure 3):
Figure 3: The refracted angle of a ray at any layer in a multilayer thin film coating consisting of plane parallel surfaces is independent of the layer order and can be found using Snell's law
The exiting ray in Figure 3 will be parallel to the incident ray because n1 = n4. Optical coatings on curved surfaces are not truly plane parallel structures due to the curvature of the optic. However, this approximation is still valid due to the thinness of the coatings.1
The law of reflection states that the angle of a reflected ray, with respect to the surface normal, is of equal magnitude to the angle of incidence, but of opposite direction with respect to the surface normal.
If the angle of incidence of a ray passing from one medium to another with a lower refractive index is larger than the critical angle of a material (θC) defined by the ratio of the two refractive indices, total internal reflection will occur and the ray will be completely reflected (Figure 4). The angle of refraction equals 90° when the incident angle is exactly equal to the critical angle.2
Figure 4: Demonstration of total internal reflection (TIR) where the incidence angle is larger than Θc
Where ts and tp are the amplitude transmission coefficients for s- and p-polarization, rs and rp are the amplitude reflection coefficients for s- and p-polarization, n1 and n2 are the refractive indices of the two optical media, θ1 is the incident angle, and θ2 is the transmitted or reflected angle. At normal incidence, θ1 and θ2 are 0 making all cosine terms 1 and the amplitude coefficients the same for both polarization states. This makes intuitive sense as there is no distinction between the s- and p-polarization states at normal incidence.
Reflection occurs when light strikes electrons on the surface of the material it is entering. The electrons absorb and re-emit the light with some energy loss. Shiny, highly reflective mirrored materials have more electrons with free mobility, leading to maximum reflection and minimal transmission.
There are several technologies used to apply optical coatings including evaporative deposition, plasma sputtering, ion beam sputtering, and atomic layer deposition (Table 1).
|Evaporative||Evaporative with IAD||Plasma Sputtering||IBS||ALD|
|Coating Stress||Low||Medium||High||Very High||High|
|Repeatability||Medium||Medium||High||Very High||Very High|
|Process Time||Slow||Slow||Intermediate||Very Slow||Very Slow|
|Non-Flat Geometry Capabilities||Better||Better||Good||Bad||Best|
Table 1: Comparison of different coating technologies (IAD: ion assisted deposition, IBS: ion beam sputtering, ALD: atomic layer deposition4
During evaporative deposition, source materials in a vacuum chamber are vaporized using heating or electron-beam bombardment. The resulting vapor condenses onto the optical surfaces and precise control of heating, vacuum pressure, substrate positioning, and rotation during the vaporization process results in uniform optical coatings of specific designed thicknesses. Evaporative deposition accommodates larger coating chamber sizes than the other technologies described in this section, and is typically more cost effective. The relatively gentle nature of vaporization creates loosely packed or porous coatings. These loose coatings suffer from water absorption, which changes the effective refractive index of the layers and results in a degradation of performance. Vaporization cannot be precisely controlled in evaporative deposition, therefore layer thickness cannot be as precisely controlled as when using other techniques such as ion beam sputtering. However, the advantage of these loosely packed coatings is that they are relatively stress-free. Evaporative coatings can be enhanced using ion beam assisted deposition (IBAD or IAD), where an ion beam is directed at the substrate surface, increasing the adhesion energy of the source material to the surface and creating denser, stronger coatings.
Plasma sputtering covers a range of technologies known by a variety of names including advanced plasma sputtering and magnetron sputtering. The general concept is rooted in the generation of plasma. The ions in this plasma are subsequently accelerated into the source material, striking loose energetic source ions, which then sputter onto the target optic. While each type of plasma sputtering has its own specific properties, advantages, and disadvantages, these technologies are grouped together because they have a common operating concept. The differences within this group are much smaller than with the other coating technologies discussed in this section. Plasma sputtering offers a middle ground of price and performance between evaporative deposition and ion beam sputtering.
Ion Beam Sputtering (IBS)
During ion beam sputtering (IBS), a high-energy electric field is used to accelerate a beam of ions (Figure 5). This acceleration imparts the ions with significant kinetic energy (~10-100 eV). Upon impact with the source material, source material ions from the target “sputter” and create a dense film upon contact with the optical surface.5 A main advantage of using IBS coating instead of evaporative deposition is that the growth rate of individual coating layers, energy input, and oxidation level are much more precisely monitored and controlled. This level of control allows for high reproducibility of coating batches and minimizes layer thickness errors, which ensures consistent coating performance with the designed spectral and phase parameters.5 IBS coatings are much smoother than those deposited using other coating techniques, making IBS the only coating technology that can produce “super mirrors” featuring a reflectivity above 99.99% and coatings with less roughness than the beginning substrates. The high density of IBS coatings makes them robust and improves their chemical resistance, increasing the lifetime of the coating and enabling them to withstand harsher environments. The refractive index of each layer can also be varied during IBS, which further improves the level of process control.5 IBS is known for its precision and repeatability, and is the premier coating deposition technique for high performance laser optics coatings. Disadvantages of IBS include higher costs compared to other techniques because of longer cycle times and stress generated in the optics, which can lead to deformation and optical aberrations.
Figure 5: During ion beam sputtering (IBS), a strong electric field accelerates ions from an ion gun onto the target, which releases more ions that deposit a dense thin film coating on the rotating substrates
Atomic Layer Deposition (ALD)
Unlike evaporative deposition, the source material for atomic layer deposition (ALD) does not require evaporation from a solid, but is provided directly in the form of a gas. Despite the use of gases, elevated temperatures are often still used in the vacuum chamber. During ALD, the precursors are delivered in non-overlapping pulses where each pulse is self-limiting. The chemical design of the process is such that only a single layer can adhere per pulse and the geometry of the surface is not a limiting factor. The result is an extraordinary level of control for layer thicknesses and designs. This results in a slow rate of deposition and a high cost per coating run. However, the chambers used for ALD are typically quite large and can coat many optics in a single run. ALD is also independent of line-of-sight, meaning that it can be used to coat optics with unusual geometries that would be difficult to coat through other methods.
Figure 6: During atomic layer deposition (ALD), individual thin film layers are deposited by exposing the optics to different gaseous precursors, which results in a high level of control of layer thickness independent of the surface geometry of the optics
- Willey, Ronald R. Field Guide to Optical Thin Films. SPIE Optical Engineering Press, 2006.
- Greivenkamp, John E. Field Guide to Geometrical Optics. SPIE Optical Engineering Press, 2004.
- Paschotta, Rüdiger. Encyclopedia of Laser Physics and Technology, RP Photonics, October 2017, www.rp-photonics.com/encyclopedia.html.
- Vandendriessche, Stefaan. “No One-Size-Fits-All Approach to Optical Coatings.” Photonics Spectra, Photonics Media, December 2016.
- “IBS Mirror Coatings for Highly Demanding Applications.” Photonics News, Laser Components Group, August 2016, www.lasercomponents.com/uk/news/ibs-mirror-coatings-for-highly-demanding-applications