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Optical coating materials

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Author : Trunnano
Update time : 2022-09-29 15:14:06
Optical coating refers to the process of plating one (or more) metal (or dielectric) film on the surface of optical parts. The purpose of coating on the surface of optical parts is to reduce or increase the reflection, beam separation, color separation, filtering and polarization of light.
The commonly used coating methods are vacuum coating and electroless coating.

Principle of optical coating:
Vacuum coating:
Vacuum coating mainly refers to the coatings that need to be carried out in a higher vacuum, including vacuum ion evaporation, magnetron sputtering, MBE molecular beam epitaxy, PLD laser sputtering deposition and so on. The plated material is made into a substrate and the electroplated material is used as a target or medicinal material. The substrate is in the same vacuum as the target.
The evaporation coating is usually the target of heating so that the surface components evaporate in the form of free radicals or ions and are deposited on the surface of the substrate by film-forming method (scattering island structure-trapezoidal structure-layered growth).
Sputtered coating
For the sputtered coating, it is easy to understand that the target material is bombarded by electrons or high-energy lasers, and the surface components are sputtered in the form of free radicals or ions, and finally deposited on the substrate surface to form a thin film.
The characteristic of the optical film is that the surface is smooth, the interface between the layers is geometrically segmented, and the refractive index of the film can jump at the interface, but it is continuous in the film, which can be a transparent medium or an optical film.
Absorption medium: it can be normal uniform or non-uniform. The practical application of the film is much more complex than the ideal film, because the optical and physical properties of the film deviate from the bulk material, and its surface and interface are rough, which leads to the diffuse scattering of the light beam. the mutual penetration between the films forms a diffusion interface, which forms the anisotropy of the film due to the growth, structure and stress of the film, and the film has a complex time effect.

The common optical coating materials are as follows:
1. Metal (alloy): germanium, chromium, aluminum, silver, gold, etc.
Rare metal, non-toxic and non-radioactive, mainly used in semiconductor industry, plastic industry, infrared optical devices, aerospace industry, optical fiber communication and so on. The light transmission range is 2000NM muri-14000NM judicial nasty 4 or more.
Sometimes used in spectroscopes and usually used as a "colloid layer" to enhance adhesion, the colloid layer may be within the range of 550NM, but under the guide of aluminum mirror film, 30NM is an effective value to enhance adhesion.
In the ultraviolet region, it has the best reflective performance among common metals, and the effective thickness of the film is more than 50NM.
If the evaporation rate is fast enough and the substrate temperature is not very high, silver has the same good reflectivity as aluminum, which is the result of massive accumulation at high speed and low temperature, which leads to greater absorption.
Above infrared 100nm wavelength is the material with the highest reflectivity among the known materials.
2. Oxides.
Yttrium trioxide.
Using electron gun evaporation, the properties of the material vary with the film thickness, and the refractive index is about 1. 8% at 500nm. It is very popular as an aluminum protective film, especially relative to the high incident angle in the 800-12000nm area, it can be used as a protective film for glasses and is exposed to moisture for 24 hours.
Cerium dioxide.
Using high-density tungsten boat evaporation, cerium dioxide is evaporated on a 200 °C substrate to obtain a refractive index of about 2.2. there is an absorption band in about 3000nm whose refractive index changes significantly with the change of substrate temperature. Nylon 2.35 (500nm) low absorption thin films can be obtained by oxygen ion plating.
The refractive index is the light transmission range of 2.21500nm. Because of its high refractive index and relative firmness, people like to use this high refractive index material for anti-reflection film, splitter film, cold light film, filter, high reflection film, glasses film, thermal mirror and so on.
Silicon dioxide.
Colorless transparent crystal, high melting point, high hardness and good chemical stability. High purity, using it to prepare high quality Si02 coating, good evaporation state, no collapse point. According to the use requirements, it can be divided into ultraviolet, infrared and visible light. If the pressure is too high, the film will have pores and fragile, on the contrary, the film with too low pressure will be absorbed and the refractive index will increase.
Zirconium dioxide.
The white heavy crystal has high refractive index and high temperature resistance, stable chemical properties and high purity. There are many reasons for using it to prepare high quality zirconia coating without breaking point, which affects the transmittance of one plane lens. The roughness of the mirror will cause the diffusion of the incident light and reduce the light transmittance of the lens. In addition, the optical rotation of the material will also cause some of the incident light sources of which the frequency dissipation is particularly serious. For example, a material that absorbs red light looks green. However, these factors of poor processing can be removed as much as possible.
Hafnium oxide.
The refractive index is about 2.0C when it is evaporated by electron gun on the substrate at 150C. Stable refractive index of 2.05-2.1can be obtained by oxygen ion assisted plating. HFO2 is better than SiO2 as the outer layer of aluminum protective film in the 8000-12000NM region.
3. Fluoride.
Magnesium fluoride.
As an antireflective film with 1x4 wave thickness, it is widely used as a glass optical film, and the transmission performance of about 120NM real ultraviolet radiation to the middle infrared region of about 7000nm is good.
Calcium fluoride and barium fluoride.
Their limitation is the lack of complete compactness. Transmittance shifts to longer wavelengths at high temperatures, so at present they can only be used in infrared films.
Lead fluoride.
It can be used as a high refractive index material in UV. When it is used in 300nm, the refractive index decreases when it comes into contact with molybdenum, tantalum and tungsten boats, so platinum or ceramic dishes are needed.
4. Other compounds.
Zinc sulfide.
The light transmission range with refractive index of 2.35400-13000m has good stress and good environmental durability. Mainly used in spectroscopic film, cold light film, decorative film, filter, high reflective film, infrared film.
Lead telluride.
It is a kind of IR material with high refractive index. As a thin film material, it is transparent in 300--4000NM. In the infrared region, the material is sublimated. The substrate temperature of 250C is beneficial. Health prevention is necessary. It works well when it is up to 40000NM. Other materials are often used in more than ordinary 14000NM infrared edge.
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