One Article to Understand: Why Chalcogenide Glass Is the Irreplaceable Core Material for Mid-to-Far-Infrared Optical Filters

One Article to Understand: Why Chalcogenide Glass Is the Irreplaceable Core Material for Mid-to-Far-Infrared Optical Filters

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Ask any infrared optical engineer what the most difficult material selection problem is in the mid-to-far-infrared band, and you will most likely get the same answer: finding a material that is simultaneously highly transparent in the mid-to-far-infrared, mechanically robust enough, and capable of surviving real-world operating environments-is extremely difficult.

Germanium is good, but expensive and band-limited. Fluoride crystals have a broad transmission range, but they are extremely hygroscopic and begin to dissolve even in slightly humid conditions. Silicon performs reasonably well in the mid-infrared, but its transmittance starts to drop noticeably beyond 8m and becomes unsuitable for longer far-infrared wavelengths.

Chalcogenide glass is one of the few systemic solutions to this selection dilemma.

What Is Chalcogenide Glass, and Why Does It Excel in the Mid-to-Far-Infrared?

Chalcogenide glass is an amorphous glass material primarily composed of Group VI elements-sulfur (S), selenium (Se), and tellurium (Te). Typical compositions include As-S, As-Se, Ge-Sb-Se, Ge-As-Se-Te, and various other formulations.

The fundamental reason for its excellent transmission in the mid-to-far-infrared lies in the relatively low bond energy of its chemical bonds.

Infrared transmission is determined by the phonon cutoff frequency of the material-the vibrational frequency of the chemical bonds between atoms determines the absorption edge for infrared light. Lower bond energy and higher atomic mass result in lower phonon frequencies, allowing the material to transmit longer infrared wavelengths.

Chalcogen elements have high atomic masses. When they form chemical bonds with arsenic, germanium, and other elements, the bond energies are low, and the phonon cutoff frequency can extend beyond 10m or even longer. This enables chalcogenide glass to maintain high transmittance across the entire 8-14m long-wave infrared window and beyond. By contrast, oxide glasses such as SiO have high Si-O bond energy, with phonon cutoff frequencies around 4-5m, making them completely opaque in the long-wave infrared-which is exactly why you cannot see anything through ordinary glass with a thermal imager.

Different formulations of chalcogenide glass have different transmission ranges. Sulfur-based systems (e.g., AsS) have a transmission window from approximately 0.6m to 11m. Selenium-based systems (e.g., AsSe) can extend beyond 15m. Tellurium-based systems have even broader transmission, reaching above 20m. This tunability allows chalcogenide glass to cover multiple application bands-from mid-wave to long-wave and even very-long-wave infrared-offering far greater flexibility than any single crystalline material.

Comparison with Germanium: Each Has Its Strengths, but Chalcogenide Glass Offers Unique Irreplaceability

Germanium is the most widely used material in mid-to-far-infrared optics. Its refractive index at 8-14m is approximately 4.0, providing excellent light-gathering capability, making it the mainstream choice for thermal imaging lenses.

However, germanium has three notable limitations:

First, price volatility. Germanium is a rare metal with highly concentrated global supply. Affected by export policies and downstream demand, its price can double or halve within a few years, posing significant supply chain risks.

Second, limited hardness. Germanium has a Vickers hardness of approximately 780HV. In harsh environments with sand, dust, or rain, the surface is susceptible to abrasion and scratching, limiting its long-term service life as a window or filter substrate compared to harder materials.

Third, a finite transmission range. Germanium's transmission window spans approximately 2-15m, with transmittance dropping noticeably beyond 15m.

Chalcogenide glass performs differently across these three dimensions, but its advantage in transmission range is structural-for applications requiring coverage beyond 12m in the very-long-wave infrared, chalcogenide glass is one of the few practical choices. Additionally, as an amorphous material, chalcogenide glass can be mass-produced into complex surface profiles through molding processes, with manufacturing costs lower than precision machining of germanium, making it well-suited for large-scale civilian applications.

DLC Coating: Addressing the Hardness Shortcoming of Chalcogenide Glass

Chalcogenide glass has a notable mechanical weakness: its hardness is low, typically in the range of 100-200HV, far below that of germanium (780HV) and silicon (1000HV). An uncoated chalcogenide glass surface lacks sufficient abrasion resistance in outdoor environments with sand and dust, or in industrial settings requiring frequent cleaning-limiting its service life in harsh conditions.

DLC (Diamond-Like Carbon) coating is the most mature technical solution to this problem.

DLC is an amorphous carbon film with a Vickers hardness typically ranging from 1500 to 3000HV, a low coefficient of friction (approximately 0.05-0.15 in dry conditions), and good transmittance in the mid-to-far-infrared band. When deposited on chalcogenide glass surfaces, it significantly enhances substrate resistance to wear, scratching, and corrosion, while limiting the impact on infrared transmittance to within 1-2 percentage points.

The significance of this combination is that chalcogenide glass provides broadband mid-to-far-infrared transmission, while DLC provides mechanical protection. Together, they integrate two properties that are rarely achieved simultaneously in a single material-"broadband transmission" and "environmental durability"-into a single optical element.

MULTI IR has independently developed DLC-coated chalcogenide glass filters, with processes covering multiple substrate systems including As-S, As-Se, and Ge-Sb-Se. The adhesion and uniformity of the DLC films have been specifically optimized for these substrates. The thermal expansion coefficient matching between different chalcogenide glass substrates and the DLC film varies significantly, directly affecting the adhesion stability of the DLC layer-this is the most technically challenging aspect of DLC coating on chalcogenide glass, and the most difficult part to control for production consistency. Currently, MULTI IR's DLC-coated chalcogenide glass products are shipping in applications including military infrared windows, industrial thermal imaging, and automotive long-wave infrared systems.

Key Selection Criteria for Chalcogenide Glass Filters

Application-level selection typically requires confirming three things:

1. Operating band requirements. Different chalcogenide glass formulations have different transmission windows. For applications in the 8-12m band, As-Se systems are generally sufficient. For needs extending beyond 14m, tellurium-containing or higher-selenium-content formulations are required. If the substrate formulation is mis-selected and transmittance fails to meet specifications, no subsequent coating process can compensate.

2. Mechanical environment requirements. For fixed indoor installations, uncoated chalcogenide glass substrates can be used, with DLC coating adding cost. For outdoor, automotive, airborne, or marine applications involving sand, dust, rain, or vibration, DLC coating is essential. The operating environment determines whether coating is required, as well as the thickness and hardness specifications of the DLC layer.

3. Trade-off between transmittance and blocking. The transmittance of chalcogenide glass filters is affected by both substrate absorption and surface reflection. Anti-reflective coatings can significantly improve peak transmittance, but their design must be considered together with the optical characteristics of the DLC layer-they cannot be optimized independently.

From detector windows in military thermal imaging systems to filter front-ends in civilian gas analyzers, the wide application span of chalcogenide glass in the mid-to-far-infrared field reflects the unique physical properties of this material: very few materials can simultaneously maintain high transmittance, processability, and a reasonable degree of cost control across the entire 8-14m window.

This is not because nobody has looked for alternatives-it is because molecular physics sets a high threshold here, and chalcogenide glass happens to cross it.

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