Infrared Optics

An essential condition for optical elements to work with infrared light is that transparency (i.e., propagation with low absorption and scattering losses) is obtained for optical materials – particularly for elements like lenses and prisms, where propagation lengths can be significant, but often also for dielectric coatings.

Many optical materials which are transparent in the visible optical range also exhibit good transparency in the near infrared, but not for longer wavelengths (mid and far infrared). For example, fused silica exhibits rapidly increasing absorption losses from about 2 μm on.

Such behavior with a relatively sharp infrared absorption edge is typical; it results from multiphonon absorption. This process sets in where the photon energy is only a small multiple of the maximum phonon energy, so that the energy of a photon can be converted to that of a few phonons. For shorter optical wavelengths (higher photon energies), such processes have higher orders (i.e., involve more phonons) and rapidly become very weak. Equally, it helps if the material is chosen such that it has low phonon energies, i.e., relatively slow vibrations of its lattice. Typically, that is the case for materials with relatively heavy constituents. At the same time, such materials often exhibit a small band gap energy, which results in strong absorption for shorter wavelengths: both edges of the transparency range are shifted towards longer wavelengths. As a result, such materials often exhibit strong absorption in the visible spectral region. Some of them look yellow or orange due to absorption only in the blue region, while others are even completely opaque.

Infrared materials can be (mono)crystalline, glasses, semiconductors or metals. Some typical materials used for infrared optics are described in the following:

• Fused silica (SiO₂) is an excellent optical material, but usable only up to roughly 2 μm wavelength. Crystalline quartz (also SiO₂) is similar. Some other materials, which are common for optics indivisible spectral range, are also suitable for the near infrared – for example, glasses like BK7 (a borosilicate crown glass).
• Sapphire is one of the hardest and most durable optical materials (but difficult to process) and is transparent from the UV up to about 6 μm. It also exhibits an excellent thermal conductivity.

• Various fluorides such as calcium fluoride (CaF₂), barium fluoride (BaF₂), magnesium fluoride (MgF₂) and lithium fluoride (LiF) are quite common as crystalline materials and partly also for dielectric coatings. For example, MgF₂ is often used for anti-reflection coatings. Fluorides generally offer a wide transparency range from the near ultraviolet to the mid infrared. There are also fluoride glasses, e.g. fluorozirconate, fluoroaluminate and fluoroindate glasses.
• Quite wide transparency ranges are also obtained with some selenides like zinc selenide (ZnSe) and zinc sulfide (ZnS); these robust materials are often applied for CO₂ laser optics. They are also available as rare-earth-doped gain media, particularly with chromium doping. There are also tellurides, particularly cadmium telluride (CdTe) for use at 12 μm to 25 μm.
• Cesium bromide (CsBr) and cesium iodide (CsI) are also suitable for application at very long wavelengths (the latter even up to about 40 μm). Potassium chloride (KCl) has a particularly broad usable wavelength range from the UV to the mid infrared (about 0.3 μm to 21 μm). Sodium chloride (NaCl) is more durable but very fragile. Such ionic substances are soluble in water and strongly hygroscopic.
• Further, there are some chalcogenide glasses containing substances like sulfur, selenium, arsenic, germanium and silver. They are typically sold under certain trade names, not specifying the exact chemical composition. Such glasses can be used in glass molding processes.
• Semiconductors like silicon, germanium and gallium arsenide, which are completely opaque in the visible region, exhibit good infrared transparency. They have rather high refractive indexs.
• Various metals can be used for reflecting coatings of infrared mirrors, namely copper, aluminum, silver, gold and stainless steel, as well as various metal alloys such as chrome–nickel. They are normally made as first surface mirrors.

It is important to have a wide range of such materials, since various properties need to be considered for applications:

• Beginning with the most essential optical properties, the transmission wavelength bands are quite different, often excluding a whole range of materials for a certain application. The materials also have very different values of refractive index and chromatic dispersion. Many infrared materials exhibit quite strong optical nonlinearities.
• The possibility of getting of broadband anti-reflection coatings depends on refractive indices and the availability of suitable coating materials.
• Some materials have very high thermal expansion coefficients and thermo-optic coefficients (e.g. dn/dT), which in combination with residual absorption can lead to strong thermo-optic effects such as thermal lensing.
• There are big differences concerning mechanical robustness: some products are extremely fragile or quite soft, while others are robust and durable. Different optical fabrication techniques are required for cutting and surface finishing, and the results in terms of practically achievable surface quality (e.g. scratch–dig specifications) also differ significantly.
• Some materials are quite hygroscopic and can thus be used only in humidity-free environments.
• Some infrared materials (often those for the longest wavelengths) contain highly toxic substances such as thallium and cadmium, which is also a problem in the context of ROHS conformity, while others are not creating any environmental hazards.

Note that there are not only infrared bulk optics, but also mid-infrared fibers, e.g. fluoride fibers.

Naturally, scattering processes are relatively weak at long optical wavelengths. For example, the intensity of Rayleigh scattering – scattering at objects which are far smaller than the wavelength – scales with the inverse fourth power of the wavelength. Therefore, scattering losses are usually not a serious concern for infrared optics – very much in contrast to ultraviolet optics – although the homogeneity of infrared materials is often not perfect.

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