An Introduction to Optical Coatings
An optical coating is composed of a combination of thin film layers that create interference effects used to enhance transmission or reflection properties within an optical system. The performance of an optical coating is dependent upon the number of layers, the thickness of the individual layers, and the refractive index difference at the layer interfaces. Many common types of coatings are used on precision optics, including Anti-Reflection (AR) Coatings, High Reflective (Mirror) Coatings, Beamsplitter coatings, and Filter coatings such as shortpass, longpass, and notch filters. Anti-reflection coatings are included on most refractive optics and are used to maximize throughput and reduce unwanted reflections. High Reflective coatings are designed to maximize reflectance at either a single wavelength or across a broad range of wavelengths and are most often used to create mirrors. Beamsplitter coatings are used to divide incident light into known transmitted and reflected light outputs. Filters are found in a large number of life science and medical applications and are used to transmit, reflect, absorb, or attenuate light at specific wavelengths. Edmund Optics also offers a variety of Custom Coatings to meet any application need.
Optical Coatings are designed for a specific incident angle and a specific polarization of light such as S-polarization, P-polarization, or random polarization. Using the coating at a different angle of incidence than what it is designed for will result in a significant degradation in performance, and sufficiently large deviations in incidence angle can result in a complete loss of coating function. Similarly, using a different polarization than the design polarization will generally yield undesirable results.
Optical Coatings are 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 quarter-wave optical thickness (QWOT) or halfwave optical thickness (HWOT) of the wavelength of the light used in the application. These thin films alternate between layers of high index of refraction and low index of refraction, thereby inducing the interference effects needed. Refer to Figure 1 for a sample illustration of a broadband anti-reflection coating design.
Figure 1: In a three layer BBAR coating light, the correct choice of quarter wave and half wave thicknesses of coatings results in a high transmission and low reflection loss
Coatings control the reflection and transmission of light through the mechanism of optical interference. When two beams propagate along coincident paths and their phases match, the spatial location of the wave peaks also match and will combine to create a larger total amplitude. When the beams are out of phase (180° shift) their overlay will result in a subtractive effect at all the peaks, causing the combined amplitude to decrease. These effects are known as constructive and destructive interference respectively.
The relations that dictate the total reflectance of a multi-layer thin film structure are given in Equations 1 - 4:
While the wavelength of light and angle of incidence are usually specified, the index of refraction and thickness of layers can be varied to optimize performance. Changes in any of these will have an effect on the path length of the light rays within the coating, which in turn will alter the phase values as the light travels. This effect can most simply be explained through the example of a single-layer anti-reflection coating. As light propagates through the system, reflections will occur at the two interfaces of index change on either side of the coating. In order to minimize reflection, we would ideally have 180° phase shift between these two reflected portions when they recombine at the first interface. This phase difference directly corresponds to a λ/2 shift of the sinusoid wave, which can best be accomplished by setting the optical thickness of the layer to λ/4. Refer to Figure 2 for an illustration of this concept.
Figure 2: A 180° Phase Shift between two reflected beams results in destructive interference and as a result there is no reflected beam
Index of refraction not only influences optical path length (and thus, phase), but also the reflection characteristics at each interface. The reflection is defined through Fresnel's Equation (Equation 5), which provides the amount of reflection that will occur from the refractive index change at an interface at normal incidence.
The final parameter that must be taken into account is the incident angle of the light the coating is to be designed for. If the incident angle of the light is altered, the internal angles and optical path lengths within each layer will be affected; this will influence the amount of phase change in the reflected beams. When a non-normal incidence is used, S-polarized and P- polarized light will reflect differently from one another at each interface, which will cause different optical performances at the two polarizations. It is this phenomenon that allows for the design of polarizing beamsplitters.
During evaporative deposition, source materials in a vacuum chamber are either 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 vaporization result in uniform optical coatings of specific designed thicknesses. The relatively gentle nature of vaporization creates coatings that are loosely packed or porous. These loose coatings suffer from water absorption which changes the effective refractive index of the layers, resulting in a degradation of performance. Evaporative coatings can be enhanced using Ion Beam Assisted Deposition, where an ion beam is directed at the substrate surface. This increases the adhesion energy of the source material to the surface, creating denser, stronger coatings that also have more stress.
Ion-Beam Sputtering (IBS)
In Ion Beam Sputtering (IBS), a high energy electric field is used to accelerate a beam of ions. This acceleration imparts the ions with significant kinetic energy, and upon impact with the source material they “sputter” it loose from the target. These sputtered source material ions are in their turn energetic and creating a dense film upon contact with the optical surface. IBS is a well-established technology known for its precision and repeatability.
Figure 3: In the Ion-Assisted E-Beam Deposition Process, an ion gun is aimed at the optical surface to increase the adhesion and density of the coating
Plasma sputtering covers a range of technologies with varying names including Advanced Plasma sputtering and Magnetron sputtering. The general concept consists of the generation of a 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, we group these technologies together because they have a common operating concept and the differences within this group are much smaller than the differences with the other coating technologies discussed in this article.
Atomic Layer Deposition
Unlike evaporative deposition, the source material for Atomic Layer Deposition (ALD) does not need to be evaporated from a solid, but is provided directly in the form of a gas. Despite the use of gases, the elevated temperatures are often still used in the vacuum chamber. In ALD, the precursors are delivered in non-overlapping pulses and 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, but for the same reason a slow rate of deposition.
Subwavelength Structured Surfaces
Surface structures smaller than the wavelength of light have been a research topic in optics since the discovery of the textured pattern on moth’s eyes. While surface texturing is still evolving as a technology, it entails modifying the structure of a substrate’s surface as opposed to depositing alternating layers of high and low refractive index materials as in traditional thin film coating. The features on the textured surfaces can either be random or periodic such as the moth’s eye pattern. Subwavelength structured surfaces can be manufactured using photolithography for the periodic patterns or using modified plasma etching for the random patterns.
The manufacturing process involved for optical coatings is both labor and capital-intensive as well as time-consuming. The factors that influence the cost of a coating include the number of optics being coated, the type of optic, the size of the optic, the number of layers in the coating, and the number of coated surfaces on the optic. The deposition process used to apply the coating also plays a huge factor in determining coating cost as well as the coating performance. Furthermore, a great deal of preparation work is necessary to ensure the highest level of quality in every coated optic.
Cleaning and preparation of optics prior to coating is a vital part of the process. An optical element must have a clean surface for the coating to adhere. Also, any stains on the substrate that are not removed beforehand can often be enhanced once a coating is applied. For this reason, Edmund Optics® has meticulous cleaning processes in place that ensure a consistent, high quality final product.
Many different coating deposition technologies exist, and each has their advantages and disadvantages. Edmund Optics® implements many of these coating deposition techniques. Contact us to determine which coating technique is best suited to your application.