The core performance of infrared bandpass filters - the transmittance and cutoff depth of specific bands - directly depends on the synergistic selection of their substrate materials and coating materials, which requires comprehensive consideration based on the target wavelength range, environmental tolerance, and cost.

1、 Base material: the cornerstone of optical and mechanical properties

The substrate is the carrier of the filter, and its primary condition is that it must be highly transparent within the target passband and have stable mechanical and chemical properties.

Short wave infrared (SWIR, 1-3 μ m): common optical glass (such as BK7) or fused silica. The latter has an extremely low coefficient of thermal expansion and better environmental stability.

Medium wave infrared (MWIR, 3-5 μ m): The mainstream material is single crystal germanium (Ge), which has a high refractive index and good transparency, but is expensive and fragile; Zinc sulfide (ZnS) and zinc selenide (ZnSe) are excellent alternatives, especially ZnSe, which has a wide transmission range in both MWIR and long wave infrared (LWIR), but relatively low hardness.

Long wave infrared (LWIR, 8-14 μ m): Single crystal germanium remains important, while silicon (Si) is commonly used for 8-12 μ m due to its high hardness and low cost; In addition, crystals such as sodium chloride (NaCl) and potassium bromide (KBr) have an extremely wide range of light transmission, but are prone to deliquescence and require strict protection.

2、 Coating materials: the key to achieving spectral "screening"

The principle of thin film interference is the core of achieving bandpass function, which is composed of alternating evaporation of high refractive index (H) and low refractive index (L) dielectric film layers.

Material pairing: Classic combinations include germanium (Ge) as the H layer and silicon monoxide (SiO) or zinc sulfide (ZnS) as the L layer. For example, in the MWIR/LWIR region, the Ge/ZnS combination is widely adopted due to its excellent refractive index difference and environmental durability.

Breakthroughs and Trends: The advancement of materials science has driven the application of new materials

Oxide substitution: such as titanium dioxide (TiO ₂), tantalum pentoxide (Ta ₂ O ₅), etc. are used in near-infrared to provide higher laser damage threshold and stability.

Amorphous thin film: using amorphous silicon (a-Si), amorphous germanium (a-Ge), etc., can reduce scattering losses caused by grain boundaries, improve optical uniformity and performance.

Durable and sturdy design: By introducing hard materials such as germanium carbide (GeC) and diamond-like carbon (DLC) as protective layers or film components, the wear resistance and corrosion resistance of the filter are significantly improved, making it suitable for harsh environments.

Conclusion:

The design of modern infrared filters has evolved into a systematic engineering. When selecting, first choose the substrate based on the working band, and then select the appropriate coating material system and film design based on passband accuracy, cutoff requirements, environmental hardness (temperature resistance, moisture resistance, impact resistance), and cost. The breakthroughs in materials science, especially the development of new hard, low loss, and high stability coating materials, are driving the development of infrared filters towards higher performance, smaller size, longer lifespan, and wider environmental adaptability.