How much do you know about the commonly used optical coating materials?
Optical coating is the process of coating the surface of an optical part with a thin layer (or layers) of metal (or dielectric). The purpose of coating on the surface of optical parts is to reduce or increase the light reflection, beam splitting, color separation, light filtering, polarization and other requirements. Commonly used coating methods are vacuum coating and chemical coating.
Optical coating principle:
Vacuum coating: Vacuum coating mainly refers to coatings that need to be applied under higher vacuum, including vacuum ion evaporation, magnetron sputtering, MBE molecular beam epitaxy, PLD laser sputtering deposition and other coatings, so there are two main types of evaporation and sputtering. The material to be coated is made into a substrate, and the plated material is used as a target or drug. The substrate is placed in the same vacuum as the target.
Evaporative coatings are usually heated targets in order to evaporate surface components in the form of free radicals or ions, and deposited on the surface of the substrate by film formation methods (scattering island structure - trapezoidal structure - layer growth), thin films.
② For sputtered coatings, it is easy to understand that the target material is bombarded with electrons or high-energy lasers, the surface components are sputtered in the form of free radicals or ions, and finally deposited on the surface of the substrate to eventually form a thin film.
Optical thin films are characterized by smooth surfaces, geometrically divided interfaces between the layers, a jump in refractive index at the interface, but continuous within the layers, either as a transparent medium or as an optical film.
absorbing medium: it can be normal uniform or normal inhomogeneous, the practical application of the film is much more complex than the ideal film, this is because, when preparing, the optical and physical properties of the film deviate from the bulk material, its surface and interface is rough, which leads to diffuse scattering of the light beam, the interpenetration between the film layers to form a diffusion interface, due to the growth of the film layer, structure, stress, etc., the formation of Due to the anisotropy of the film, the film layer has complex time effects.
1、 Metal (alloy): germanium, chromium, aluminum, silver, gold, etc.
① Germanium
Rare metal, non-toxic and non-radioactive, mainly used in the semiconductor industry, plastics industry, infrared optical devices, aerospace industry, fiber optic communication, etc. Light transmission range 2000NM- 14000NM, n=4 or more.
②Chromium
Sometimes used in spectroscopy and often used as a "gel layer" to enhance adhesion, the gel layer may be in the range of 550NM, but under the aluminum mirror film guide, 30NM is the effective value to enhance adhesion.
③Aluminium
It is one of the best reflective metals in the UV domain, and the effective thickness of its film is 50NM or more.
④Silver
If the evaporation rate is fast enough and the substrate temperature is not very high, silver and aluminum have good reflectivity, which is the result of a large amount of agglomeration at high speed and low temperature, this agglomeration at the same time leads to greater absorption.
⑤Gold
In the infrared wavelength above 100nm is the highest reflectivity of the known materials.
2、Oxides: yttrium trioxide, cerium dioxide, magnesium oxide, titanium dioxide, silicon dioxide, silicon oxide, zirconium dioxide, aluminum trioxide, hafnium oxide, etc.
①Yttrium trioxide
The use of electron gun vapor deposition, the material performance changes with the film thickness, at 500nm refractive index of about 1.8. Used as aluminum protective film its extremely popular, especially relative to the 800-12000nm region of high incidence angle, can be used as a protective film for glasses, and 24 hours exposure to moisture.
Cerium dioxide
Using high-density tungsten boat evaporation, cerium dioxide is evaporated on the substrate at 200°C to obtain a refractive index of about 2.2. There is an absorption band at about 3000 nm and the refractive index changes significantly with the change of substrate temperature, and a low-absorption film with n=2.35 (500 nm) can be obtained by oxygen ion assisted plating.
(3) Titanium dioxide
Refractive index is 2.21, 500nm transmission range, because of its high refractive index and relative sturdiness, people like to use this high refractive index material for anti-reflective film, spectral film, cold light film, filter, high reflective film, glasses film, heat reflective mirror, etc.
④Silicon dioxide
Colorless and transparent crystal, high melting point, high hardness, good chemical stability. High purity, high quality Si02 coating, good evaporation state, no chipping point. It is divided into ultraviolet, infrared and visible light according to the application requirements. If the pressure is too high, the film will be porous and fragile, and if the pressure is too low, the film will have absorption and the refractive index will be large.
Zirconium dioxide
It has high refractive index, high temperature resistance, chemical stability, high purity, and high quality zirconia coating without chipping point. The roughness of the mirror surface can cause diffusion of the incident light and reduce the transmission of the lens. In addition, the material's ability to absorb light can also cause some of the frequencies of the incident light source to be dissipated more severely. For example, a material that absorbs red light will appear green. However, these poor processing factors can be removed as much as possible.
(6) Hafnium oxide
The refractive index is around 2.0, and with the help of oxygen ion plating it is possible to obtain a stable refractive index of 2.05 - 2.1. HFO2 is better than SiO2 as an outer layer of aluminum protective film in the 8000 - 12000 NM region.
Fluoride: thorium fluoride, magnesium fluoride, cerium fluoride, calcium fluoride, barium fluoride, etc.
Among them, magnesium fluoride material features: colorless tetragonal crystalline powder, high purity, with its preparation of optical coatings can improve the transmission rate, no chipping point.
Other compounds: zinc sulfide, lead telluride.
3, fluoride class
①Magnesium fluoride
As 1/4 wave thickness anti-reflection film is commonly used for glass optical film, and about 120 NM real ultraviolet to about 7000 nm in the central infrared region through good performance.
②Calcium fluoride and barium fluoride
The limitation of both of them is the lack of complete densification. The transmittance shifts to longer wavelengths at high temperatures, so they are currently used only in infrared films.
③Lead fluoride
In the UV can be used as a high refractive index material, at 300nm N = 1.998, the material and molybdenum-tantalum, tungsten boat contact refractive index will be reduced, so you need to use platinum or ceramic dishes.
4、Other compounds
①Zinc sulfide
Refractive index of 2.35, 400-13000m light transmission range, with good stress and good environmental durability. Mainly used in spectral film, cold light film, decorative film, filter, high reflective film, infrared film.
②Lead telluride
It is a kind of IR material with high refractive index, which is transparent in 300-4000NM as a film material, and N=5.1-5.5 in the IR region, the material sublimation, substrate temperature 250 degrees Celsius is beneficial, health prevention is necessary, it is effective when used in up to 40,000NM, other materials are often used in more than ordinary 14000NM infrared edge.
