Optical Coatings Principles and Technology

Optical coatings are mainly utilized to change the transmittance, reflectance, polarization, or absorbance properties of optical components. The optic being coated is known as the substrate. The coating is deposited in a high vacuum, using any one of the processes described below. Coating materials comprise dielectrics, semiconductors or metals. The most common dielectric materials are oxides, sulfides and fluorides. Selenides and tellurides are mainly used for specialized applications.

Evaporation

In this process, the coating material is changed from solid to vapor form, and then condensed on to the optic surface. Electron beam ion bombardment and thermal vaporization are the most common techniques of vaporization.

Thermal Vaporization

In this method, the coating material is resistance heated in a platinum, molybdenum, or tungsten “boat” for high temperature dielectrics and metals. For some low temperature dielectrics, indirect heating with a refractory filament is sufficient.

Electron Beam Ion Bombardment

This method of evaporation is versatile and reduces contamination, which results from interaction between the crucible and the coating material. In this process, the coating material is heated by direct electron bombardment. Then, a variable voltage and current electron beam is focused on a very small moveable spot in a multipocket, water cooled, copper crucible. The heating is localized and temperatures are kept sufficiently high to evaporate refractory materials.

Sputtering

This technique is used for thin film deposition, and is specifically useful for large area substrates. In a plasma chamber, the substrate and a target of the material to be deposited are held close together. Then, the substrate is cooled and heavy ions from the plasma hit the target, knocking out tiny particles of the target material. These particles condense on the cooled substrate. Sputtering can rapidly create homogeneous coatings over large areas, and uses the deposition material more efficiently than that of the evaporation method.

Thickness Control

In optical coating, the control of layer thickness is very important. Two principal methods are used to control layer thickness: interference monitoring, and crystal oscillator monitoring. Interference monitoring proves useful for depositing materials with a well defined refractive index. This technique can be used on dielectrics and certain semiconductor materials. Crystal oscillator monitoring is often utilized to control the thickness of metal layers, and for dielectric coatings with several layers.

Reflection from Uncoated Optics

Refraction and partial reflection of a ray incident at angle i in an interface between two media.

Figure 1. Refraction and partial reflection of a ray incident at angle i in an interface between two media.

When light falls onto a smooth or polished surface between two transparent media, as shown in Figure 1, part of the light penetrates the second media and gets refracted, while the remaining light is reflected at the interface. The relationship between the angle of refraction (r) and the angle of incidence (i) is given by Snell’s law:

    n1 sin i = n2 sin r ............................. (1)

Where n1 and n2 are indices of refraction of the media, as illustrated in Figure 1.

Reflectance depends on the angle of incidence and polarization of the incident light, and the indices of refraction of the materials involved. In an optical system with 8 glass (n2 = 1.5) optical elements, there will be a reflection loss of 47.9% or 1- (0.96)16, and possible ghost images. Anti-reflection coatings can reduce both the reflection loss and ghost images.

Single Layer Anti-reflection Coatings

Transparent thin film coating showing reflection from the air-dielectric-glass interfaces.

Figure 2. Transparent thin film coating showing reflection from the air-dielectric-glass interfaces.

1/4 wave thick AR coating of magnesium fluoride on a fused silica substrate reduces the reflection from 4% to < 2.0% at the design wavelength

Figure 3. 1/4 wave thick AR coating of magnesium fluoride on a fused silica substrate reduces the reflection from 4% to < 2.0% at the design wavelength

Figure 2 displays a 1/4 wave optical thickness of a transparent dielectric material deposited on the glass surface. At normal incidence, there will be reflection from the dielectric and air-dielectric glass boundaries. In case the dielectric material exhibits an index lower than that of glass, the two reflections will be from a medium with an index greater than the one in which the light was passing. Due to the 1/4 wave optical thickness of the dielectric layer, the two reflections will be 180° out of phase with each other and cause destructive interference. The 1/4 wave optical thickness of dielectric reduces the total reflectance, as shown in Figure 3.

Wavelength and Angle of Incidence Dependence

Since the optical thickness can only be 1/4 wave for a single wavelength, the reflectance depends on the wavelength of the incident light. In addition to wavelength dependent, the change in reflectance that occurs with the application of a thin film coating also depends on the angle of incidence.

Multi-Layer Anti-Reflection Coatings

A number of techniques can be used to enhance the efficiency and wavelength range of anti-reflection coatings. Most of these methods involve the use of multi-layer coatings. One of the simplest techniques is the double quarter layer, where two quarter wave layers of different materials are employed. Another technique of multi-layer anti-reflection coating involves the use of a single dielectric, but with layers having different thicknesses.

Dielectric Reflector Coatings

If the dielectric layer has a refractive index (n3 = 2.32) higher than the glass substrate, the reflectance of the quarter wave layer is 0.314, which is much higher than the 0.042 from the bare substrate. If a second layer with an index lower than the first layer is added, it acts as an anti-reflection coating for the first layer, and thus reduces the reflectance. If a third layer is deposited with a material having an index higher than the second layer, it acts as a reflector layer to the second layer and increases the total reflectance. This way, the reflectance can be built up from 0.042 from the glass surface to nearly 1.

Typical transmittance of a multi-layer dielectric reflector coating at 0 and 45° incidence.

Figure 4. Typical transmittance of a multi-layer dielectric reflector coating at 0 and 45° incidence.

Figure 4 shows the reflectance of a typical multi-layer reflector. Maximum reflector coatings at different wavelengths can be applied to improve the range of high reflectance. Orion Instruments uses this technique on its broadband reflectors.

Metal Reflector Coatings

In metal reflector coatings, the main factors are hardness, reflectance, durability, uniformity, adherence, and long-term stability. Metallic reflector coatings from Oriel Instruments are deposited under clean, high vacuum conditions. The optic to be coated is initially cleaned in a series of ultrasonic baths, then air dried in a dust-free environment. Next, in a vacuum chamber, the optic is heated and again cleaned by ion bombardment in a glow discharge.

Conclusion

Optical coatings are used to change the reflectance, transmittance, polarization or absorbance properties of optical components. Oriel Instruments offers a wide range of metal coatings, such as bare aluminum, gold, aluminum with silicon monoxide overcoat, aluminum with magnesium fluoride overcoat, and silver with magnesium fluoride overcoat.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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