This article mainly introduces optical coatings and their principles, as well as various types of optical coating materials and their characteristics, applications, advantages and disadvantages.

Optical coating refers to the process of coating one (or multi-layer) 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 splitting and color separation of light. , filter, polarization and other requirements. Commonly used coating methods include vacuum coating and chemical coating.

1. Vacuum coating: Vacuum coating mainly refers to coatings that need to be carried out under higher vacuum, including vacuum ion evaporation, magnetron sputtering, MBE molecular beam epitaxy, PLD laser sputtering deposition and other coatings. Therefore, evaporation and sputtering There are two main types. The material to be plated is made into the substrate, and the plated material is used as the target or medicinal material. The substrate is in the same vacuum as the target.
   Evaporative coatings are usually heated targets to cause the surface components to evaporate in the form of radicals or ions and deposited on the surface of the substrate by film-forming methods (scattering island structure – trapezoidal structure – layered growth), a thin film.
2. For sputter-like 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 form a thin film.

The surface is smooth, the interface between the film layers is geometrically divided, and the refractive index of the film layer can jump at the interface, but it is continuous in the film layer, which can be a transparent medium or an optical film.

Absorptive medium: It can be uniform in normal direction or non-uniform in normal direction. The actual film is much more complicated than the ideal film. This is because the optical and physical properties of the film deviate from the bulk material during preparation. The surface and interface are rough, which leads to diffuse scattering of the light beam, and the interpenetration between the films forms a diffusion interface. Due to the growth, structure, stress and other reasons of the film layer, the anisotropy of the film is formed, and the film layer has a complex time effect.

Form

①Germanium

Rare metal, non-toxic and non-radioactive, mainly used in semiconductor industry plastic industry, infrared optical device, aerospace industry, optical fiber communication, etc. Light transmission range 2000NM—14000NM, n=4 or even larger.

②Chrome

Sometimes used on beamsplitters and often as a “glue layer” to enhance adhesion, the glue layer may be in the range of 550NM, but under aluminum mirror film guides, 30NM is an effective value to enhance adhesion.

③Aluminum

In the ultraviolet domain, it has the best reflection performance among ordinary metals, and the effective thickness of its film is more than 50NM.

④Silver

If the evaporation rate is fast enough and the substrate temperature is not very high, silver is as well reflective as aluminum, which is the result of a large build-up at high speed and low temperature, which at the same time leads to greater absorption.

⑤Gold

It is the material with the highest reflectivity among the known materials above the wavelength of 100nm in the infrared.

①Yttrium trioxide

Using electron gun evaporation, the material properties vary with film thickness, and the refractive index at 500nm is about 1.8. It is extremely popular as an aluminum protective film, especially for the high incidence angle in the 800-12000nm region, it can be used as glasses protective film and exposed to moisture for 24 hours.

②Ceria

Using high-density tungsten boat evaporation, cerium dioxide is evaporated on the substrate at 200 ° C, and a refractive index of about 2.2 is obtained, and there is an absorption band at about 3000 nm. The refractive index changes significantly with the temperature of the substrate. Change, with oxygen ion flux plating can obtain n=2.35 (500nm) low absorption film.

③Titanium dioxide

The refractive index is 2.21,500nm light transmission range, because of its high refractive index and relatively sturdiness, people like to use this high refractive index material for anti-reflection film, beam splitter film, cold light film, filter, high-reflection film , glasses film, heat mirror, etc.

④Silica

Colorless and transparent crystal, high melting point, high hardness, good chemical stability. High purity, use it to prepare high-quality SiO2 coating, good evaporation state, no collapse point. According to the requirements of use, it is divided into ultraviolet, infrared and visible light. If the pressure If the pressure is too high, the film will be porous and brittle, and if the pressure is too low, the film will absorb and increase the refractive index.

⑤Zirconium dioxide

White heavy crystalline state, with high refractive index and high temperature resistance, stable chemical properties, high purity, using it to prepare high-quality zirconia coating, no collapse point, affecting the transmittance of a flat lens There are many reasons. The roughness of the light source will cause the diffusion of incident light and reduce the light transmittance of the lens. In addition, the optical absorption and rotation of the material will also cause some of the incident light sources to dissipate particularly seriously. For example, a material that absorbs red light looks It appears green. However, these poor processing factors can be removed as much as possible.

⑥Hafnium oxide

It is vaporized by electron gun on the substrate at 150 degrees Celsius, and the refractive index is about 2.0. It is possible to obtain a stable refractive index of 2.05–2.1 with oxygen ion plating. In the 8000–12000NM region, HFO2 is used as the outer layer of the aluminum protective film better than SiO2 .

● Fluorides: thorium fluoride, magnesium fluoride, cerium fluoride, calcium fluoride, barium fluoride, etc.

Among them, the characteristics of magnesium fluoride material: colorless tetragonal powder, high purity, use it to prepare optical coating can improve the transmittance, no collapse point.

● Other compounds: zinc sulfide, lead telluride.

①Magnesium fluoride

As a 1/4 wave thickness anti-reflection film, it is commonly used as a glass optical film, and has good transmission performance in the middle infrared region from about 120NM real ultraviolet to about 7000nm.

②Calcium fluoride and barium fluoride

Their limitations are the lack of complete densification. Transmittance shifts to longer wavelengths at high temperatures, so currently they can only be used in infrared films.

③ Lead fluoride

It can be used as a high refractive index material in UV, N=1.998 at 300nm, the refractive index will decrease when the material is in contact with molybdenum, tantalum and tungsten boat, so platinum or ceramic dish is required.

①Zinc sulfide

The refractive index is 2.35, the light transmission range of 400-13000m, with good stress and good environmental durability. Mainly used in beam splitting film, cold light film, decorative film, filter, high reflection film, infrared film.

②Lead telluride

It is an IR material with high refractive index. As a thin film material, it is transparent at 300–4000NM. In the infrared region, N=5.1-5.5. The material sublimates. The substrate temperature is 250 degrees Celsius. , It works well up to 40000NM, and other materials are often used at the edge of infrared rays exceeding the ordinary 14000NM.