{"id":622434,"date":"2026-11-05T15:22:36","date_gmt":"2026-11-05T15:22:36","guid":{"rendered":"https:\/\/www.olympiajournal.com\/news\/story\/622434\/choose-the-wrong-substrate-and-the-coating-is-wasted-how-to-select-germanium-silicon-zinc-selenide-or-chalcogenide-glass.html"},"modified":"2026-11-05T15:22:36","modified_gmt":"2026-11-05T15:22:36","slug":"choose-the-wrong-substrate-and-the-coating-is-wasted-how-to-select-germanium-silicon-zinc-selenide-or-chalcogenide-glass","status":"publish","type":"post","link":"https:\/\/www.olympiajournal.com\/news\/story\/622434\/choose-the-wrong-substrate-and-the-coating-is-wasted-how-to-select-germanium-silicon-zinc-selenide-or-chalcogenide-glass.html","title":{"rendered":"Choose the Wrong Substrate and the Coating Is Wasted: How to Select Germanium, Silicon, Zinc Selenide, or Chalcogenide Glass"},"content":{"rendered":"

Hangzhou, Zhejiang, China – <\/strong>November<\/strong> 0<\/strong>5<\/strong>, 2026<\/strong><\/p>\n

Choose the Wrong Substrate and the Coating Is Wasted: How to Select Germanium, Silicon, Zinc Selenide, or Chalcogenide Glass<\/p>\n

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For the same infrared filter, the coating design can be completely identical, but change the substrate and the performance, cost, and lead time all change. The substrate is not a passive carrier. It determines which wavelength band the filter can work in, what environment it can withstand, and what aperture it can support.<\/p>\n

There are only a few commonly used substrates in infrared optics, and their applicable boundaries are quite clear. Understanding these boundaries can save a lot of detours during selection.<\/p>\n

1. Start with the transmission band<\/p>\n

The first threshold for a substrate is its transmission band. If this requirement is not met, none of the other parameters matter.<\/p>\n

Germanium’s transmission range covers roughly 2 to 14 μm, making it the workhorse material for mid-wave and long-wave infrared. Its refractive index is very high, about 4.0. This value has a side effect: single-surface reflection loss is close to 36%, and bare substrate transmittance is very low, so an anti-reflection coating is mandatory before use.<\/p>\n

Silicon’s transmission range is 1.2 to 7 μm, covering near-infrared to mid-wave. Its refractive index is about 3.4, and it also requires AR treatment. Silicon’s mechanical strength and hardness are far higher than germanium’s, and its density is lower, giving it an advantage in weight. The drawback is strong absorption beyond 9 μm, so it cannot be used in the long-wave band.<\/p>\n

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Zinc selenide has a very wide transmission range, roughly 0.6 to 21 μm, covering visible light to extreme far-infrared. Its refractive index is relatively low, about 2.4, so reflection loss is smaller than germanium and silicon. However, its mechanical properties are soft, hardness is low, and it is easily scratched, so the surface requires additional hard protection.<\/p>\n

Chalcogenide glass is a general term for a class of materials based primarily on chalcogen elements such as sulfur, selenium, and tellurium. Its transmission range is roughly 2 to 12 μm, with slight differences among formulations. Its biggest advantage is that it can be molded, allowing aspheric lenses to be mass-produced, which is significant for cost.<\/p>\n

Fused silica and sapphire follow a different path. They transmit well from visible to near-infrared, but become essentially opaque in the mid-to-far-infrared. Therefore, for mid-infrared filters used in gas detection, these two materials are usually used only as windows, not as filter substrates.<\/p>\n

2. Refractive index: it affects more than reflection loss<\/p>\n

Refractive index determines two things: the reflection loss of the bare substrate, and the difficulty of coating design.<\/p>\n

Reflection loss has a clear formula. At normal incidence, single-surface reflectance R = ((n-1)\/(n+1))². For germanium with n about 4.0, R ≈ 36%; for silicon with n about 3.4, R ≈ 30%; for zinc selenide with n about 2.4, R ≈ 17%. After combining both surfaces, bare substrate transmittance is approximately 41%, 49%, and 69%, respectively.<\/p>\n

These numbers demonstrate the necessity of AR coatings. Bare germanium has only about 40% transmittance and is essentially unusable without coating. The effectiveness of an AR coating is directly related to the substrate’s refractive index: the higher the refractive index, the more complex the required coating design. Broadband AR on a high-index substrate often requires multiple layers, and more layers mean higher stress and cost.<\/p>\n

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Refractive index also affects the passband shape of the filter. The filter’s coating stack is an interference structure built on the substrate, and the substrate’s refractive index participates in the equivalent calculation of the entire stack. Applying the same coating parameters to a different substrate changes both the passband position and shape. Therefore, changing the substrate requires redesigning the coating stack; it cannot be directly transplanted.<\/p>\n

This point often causes problems during quotation. If a supplier quotes based on “same coating stack, different substrate,” the delivered spectrum often does not match. The correct approach is to re-optimize for the new substrate. This is design work that requires time and verification.<\/p>\n

3. Thermal performance: the coating is the first layer, the substrate is the second<\/p>\n

The temperature drift of a filter is essentially the change in the optical thickness of the coating with temperature—that is, how the product of refractive index and thickness changes. The first contribution comes from the thermo-optic coefficient and thermal expansion of the coating materials themselves. The second layer is the substrate, which participates indirectly through surface shape changes and coating-substrate thermal mismatch stress.<\/p>\n

This primary-secondary relationship must be clear. Choosing the right substrate can improve temperature drift, but it cannot replace temperature compensation design at the coating level. The following discussion of several materials refers to the influence of the substrate’s refractive index temperature coefficient on the interference condition, not the entire source of temperature drift.<\/p>\n

From the substrate layer’s perspective, germanium’s refractive index temperature coefficient is quite large, about 4×10\u207b\u2074 per degree Celsius, one of the higher values among infrared materials. For filters on germanium substrates, this layer’s contribution is relatively obvious. In scenarios with large ambient temperature variations, this is an item that must be carefully evaluated.<\/p>\n

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Silicon’s refractive index temperature coefficient is much smaller, about 1.5×10\u207b\u2074 per degree Celsius, giving it better thermal stability than germanium. This is why silicon substrate solutions are preferred when high temperature stability is required—provided the operating band falls within silicon’s transmission range.<\/p>\n

Chalcogenide glass has a low refractive index temperature coefficient and good thermal stability, which is one reason it is viewed favorably. For specific formulations and specific bands, dn\/dT can be made very small, and some systems have reported results approaching zero. But this is formulation-dependent behavior, not a universal property of this material class. During procurement, measured curves should still be requested for the specific grade.<\/p>\n

The coefficient of thermal expansion is equally important because it affects stress matching between the coating and the substrate. The greater the difference between the substrate’s expansion coefficient and the coating material’s, the greater the interfacial stress during temperature cycling and the higher the risk of delamination. This is why the same coating stack can be very stable on one substrate but develop reliability problems on another.<\/p>\n

4. Mechanical and environmental performance: determines how long it lasts<\/p>\n

Hardness determines scratch resistance. Here we refer to Mohs scratch hardness, which is a relative value, not an absolute one, and numbers from different sources may vary. Zinc selenide is the softest, about 3, and is easily scratched by most metals and glass, so wiping requires controlled force and consumables. Chalcogenide glass is also relatively soft, with hardness varying considerably by formulation, so a single value is not appropriate. Silicon and germanium are harder; silicon is often given 6.5 to 7, and germanium 6 to 6.5.<\/p>\n

For filters on soft substrates, the coating must assume the function of mechanical protection, and a hard coating is almost mandatory. This is why the combination of chalcogenide glass with a diamond-like carbon (DLC) hard coating is valued by the industry: the substrate handles forming and cost, while the hard coating handles mechanical and environmental resistance.<\/p>\n

But this combination is not easy to execute well. Chalcogenide glass has a coefficient of thermal expansion on the order of 12 to 15 × 10\u207b\u2076 per degree Celsius, while hard coatings are only 2 to 5, a difference of several times. After deposition, residual stress is high, and cracking is likely under thermal cycling. The hard coating itself also has high internal stress, and improper control can cause surface deformation or even substrate fracture.<\/p>\n

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The cause of the adhesion issue must also be stated accurately. The main cause is not that the surface is dirty or easily oxidized, but that chalcogenide glass surfaces struggle to form effective chemical bonding with the hard coating, compounded by the aforementioned thermal mismatch stress. Surface cleaning and activation can improve the interface, but controlling thermal mismatch stress is the core difficulty of this process route and the reason it has not been widely overcome by the industry for a long time.<\/p>\n

In terms of chemical stability, zinc selenide is sensitive to acids and bases, so care must be taken in selecting cleaning agents. Germanium has a tendency to oxidize at high temperatures. The chemical stability of chalcogenide glass depends on the specific formulation, with high-selenium formulations being relatively more stable. These characteristics determine the boundaries of packaging methods and operating environments.<\/p>\n

5. Cost and availability: an often overlooked dimension<\/p>\n

Germanium prices are significantly affected by resource and export policies. Global refined germanium production is highly concentrated, and infrared optics is one of the main consumption sectors, so price fluctuations directly impact cost. Supply chain uncertainty is the primary motivation for many OEMs to consider alternatives.<\/p>\n

Silicon costs much less, large-size wafer supply is ample, and the processing supply chain is mature. This is the economic basis for the rising penetration of silicon-based solutions in mid-wave infrared.<\/p>\n

Zinc selenide is medium-cost; the main difficulty lies in producing large-size, high-quality blanks, and lead times for large-aperture products are often long.<\/p>\n

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The cost advantage of chalcogenide glass is reflected in the molding step. Traditional crystalline materials must be cut, ground, and polished into lenses, with each piece processed individually—especially time-consuming for large apertures and aspheres. Chalcogenide glass can be molded in a single step, enabling high-volume production efficiency. Therefore, in high-volume, small-to-medium aperture aspheric lens scenarios, chalcogenide glass has a clear cost advantage.<\/p>\n

But this advantage has boundaries. Molded shape error can already reach the sub-micron level in conventional aspheric scenarios, with PV around 0.5 μm, and the cost advantage is obvious. The shortcomings are surface roughness, high-order surface form errors, and the ability to directly mold diffractive surfaces and freeform surfaces.<\/p>\n

These aspects still fall short of single-point diamond turning, which can achieve surface roughness below 5 nm. Therefore, for scenarios involving microstructures, the common practice is to mold first and then turn the diffractive surface or polish to compensate—not simply replace turning with molding.<\/p>\n

6. The practical order of selection<\/p>\n

Putting these dimensions into a decision sequence is more effective than comparing item by item.<\/p>\n

Step one: determine the band. Where the operating wavelength falls immediately eliminates more than half of the materials. For mid-wave 3 to 5 μm, germanium, silicon, zinc selenide, and chalcogenide glass are all options. For long-wave 8 to 12 μm, the main choices are germanium and zinc selenide; silicon has already dropped out of range. For far-infrared above 15 μm, zinc selenide is the mainstay; germanium still has engineering usability near 14 μm, but absorption rises with wavelength, and longer bands usually give way to zinc selenide.<\/p>\n

Step two: determine temperature stability requirements. For equipment with a wide operating temperature range and high-precision temperature measurement requirements, the temperature drift coefficient is a hard threshold, and silicon and low-dn\/dT chalcogenide glass have advantages here. For temperature-stable indoor equipment, the weight of this item can be lowered.<\/p>\n

Step three: determine mechanical and environmental requirements. For outdoor, coastal, and wipe-cleaning scenarios, hardness and chemical stability take priority. Soft substrates need enhanced protective coatings, or the material should be changed directly.<\/p>\n

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Step four: determine cost and lead time. After screening with the first three steps, choose the most cost-effective among the remaining options. Note that cost is not just the unit price; it also includes AR coatings, protective coatings, processing losses, and calibration labor caused by batch consistency.<\/p>\n

After these four steps, the choice in most scenarios converges to one or two options. What truly needs weighing is usually the segment between performance and cost.<\/p>\n

MULTI IR has turned this process into a mass-production capability for hard coatings on chalcogenide glass substrates. The value of this direction lies in solving two problems simultaneously: the cost advantage brought by molding, and the shortcomings of soft substrates in mechanical and environmental resistance.<\/p>\n

Specifically, the hardness of this type of DLC hard coating exceeds 18 GPa, cross-hatch adhesion reaches grade 5B, and average transmittance in the infrared band remains above 93%. These three metrics correspond exactly to the three difficulties mentioned earlier: hardness addresses scratch resistance, adhesion addresses cracking risk, and transmittance addresses the optical performance penalty. This process remains a threshold that only a few companies in the industry can mass-produce stably.<\/p>\n

The substitution relationship must also be stated clearly. Chalcogenide glass still falls short of germanium in long-wave transmittance and large-aperture lenses, and its refractive index is lower, so the same optical power requires steeper curvature, adding difficulty to large-aperture designs. For high-end, high-precision scenarios, germanium will still be used in the short term; glass will first capture the mid-to-low-end volume market.<\/p>\n

The essence of selection is not finding a universal material, but finding the optimal solution under the specific constraints of a given scenario. Among the four items—band, temperature drift, mechanical environment, and cost—whichever is the hard constraint determines which way the answer leans.<\/p>\n

MULTI IR – National-level “Little Giant” Specialized and Sophisticated Enterprise. Over 10,000 types of infrared sensitive components in stock. Top 3 globally in comprehensive strength. Product portfolio covers infrared filters, optical coatings, infrared sensors, and other core categories. Lead drafter of the infrared filter industry standard.<\/p>\n

Website: www.mirhz.com<\/a> | Global Site: www.miroptech.com<\/a><\/p>\n

About Us<\/strong><\/p>\n

Founded in 2007, Hangzhou MULTI IR Technology Co., Ltd. is an optoelectronic technology enterprise integrating R&D, production, and sales. Its products are widely applied in aerospace, medical care, AR\/VR, display imaging, photography, and other fields, steadily holding the position of the world’s largest spot supplier of optical components.<\/p>\n

Media Contact<\/span>
Company Name:<\/strong> HANGZHOU MULTI IR TECHNOLOGY CO., LTD.<\/a>
Contact Person:<\/strong> Media Relations
Email:<\/strong> Send Email<\/a>
Country:<\/strong> China
Website:<\/strong> https:\/\/www.miroptech.com\/<\/a><\/p>\n

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Hangzhou, Zhejiang, China – November 05, 2026 Choose the Wrong Substrate and the Coating Is Wasted: How to Select Germanium, Silicon, Zinc Selenide, or Chalcogenide Glass For the same infrared<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"_links":{"self":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/posts\/622434"}],"collection":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/comments?post=622434"}],"version-history":[{"count":0,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/posts\/622434\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/media?parent=622434"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/categories?post=622434"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/tags?post=622434"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}