Sapphire, as an optical material, is widely used in fields such as infrared optical windows and detector protection windows due to its high hardness, high temperature resistance, and good chemical stability. However, the transmittance of sapphire in the infrared band is not perfect, especially in the mid to far infrared region (such as 3-5 μ m and 8-12 μ m atmospheric windows), where its transmittance decreases due to the absorption of lattice vibrations by the material itself. In order to improve the infrared transmittance of sapphire window panels, coating technology has become a key means. This article will analyze the influence of coating on the infrared transmittance of sapphire window panels from the aspects of sapphire optical properties, coating principles, film system design, and practical application effects.

1、 The optical properties and infrared transmittance limitation of sapphire

Sapphire (α - Al ₂ O3) is a single crystal aluminum oxide material with a light transmission range covering the ultraviolet (about 0.15 μ m) to mid infrared (about 5.5 μ m) wavelength range. In the 1-5 μ m wavelength range, the transmittance of sapphire can reach over 85%, but beyond 5 μ m, the transmittance sharply decreases due to phonon absorption effects caused by lattice vibrations. In addition, there is a Fresnel reflection loss of about 8% on the surface of sapphire, further reducing the effective transmittance.

2、 The principle and function of coating technology

Coating technology uses methods such as vacuum evaporation, magnetron sputtering, or ion beam assisted deposition to deposit one or more optical thin films on the surface of sapphire window plates, mainly achieving the following functions:

1. Anti reflective film (AR film): By using interference effects to counteract surface reflections, it improves the transmittance of specific wavelength bands. For example, designing magnesium fluoride (MgF ₂) or zinc sulfide (ZnS) film layers with an optical thickness of λ/4 in the 3-5 μ m wavelength range can reduce the single-sided reflectivity from 8% to below 1%.

2. Broadband anti reflection film: Using a multi-layer film system with alternating high and low refractive index materials (such as ZnS/YbF3 combination), the anti reflection band can be extended to 8-12 μ m, while suppressing the phonon absorption peak of sapphire.

3. Protective film: such as diamond-like carbon film (DLC), which can maintain infrared transmittance while improving surface hardness and corrosion resistance.

3、 Key factors in membrane design

1. Material selection

① Low refractive index material: MgF ₂ (n= 1.38@4 μ m), SiO ₂ (n= 1.45@4 μ m) is suitable as the outer layer of an anti reflective film.

② High refractive index material: ZnS (n= 2.25@10 μm）、Ge（n= 4.0@10 μ m) can be used to adjust phase interference.

③ Transition layer material: Al ₂ O3 thin film can serve as a buffer layer between sapphire substrate and film layer, reducing the risk of detachment caused by stress.

2. Film thickness control

By accurately controlling the optical thickness of each layer of film (nd=λ/4 or λ/2), constructive interference is utilized to enhance transmission. For example, in the 8-12 μ m wavelength range, using an irregular film system design (such as a gradient refractive index film) can avoid sideband fluctuations caused by single wavelength optimization.

3. Environmental adaptability design

For high temperature or high humidity environments, it is necessary to choose materials with good thermal stability (such as Y ₂ O3) and optimize the film structure.

Coating technology can improve the infrared transmittance of sapphire window panels, enabling them to play a greater role in the infrared field. With the development of technology, the performance boundary of coated sapphire window panels will be further expanded. In practical applications, it is necessary to balance optical performance, environmental tolerance, and cost factors, and design customized coating schemes for specific scenarios.