Property Management: A Review of Optical Materials

Lynn Savage, lynn.savage@photonics.com

Optical materials are as old as the natural quartz and obsidian crystals early civilizations used to change the way people saw the world. Artificial materials came later, with mirrors and other optics produced following the growth of art glass in Egypt, India and China. Today, the prospects for choosing the perfect material to make lenses for endoscopes and cameras, beam shapers for laser systems, or segments of a giant telescope mirror are wide-ranging and often daunting, even for seasoned pros.

In general, optical materials can be broken down into categories, such as transmissivity vs. reflective or glass vs. ceramic vs. metal. Nonetheless, they are typically discussed only as the sum of their properties, or in terms of whether they exhibit the qualities necessary for the task at hand.

Reflective optics need less specialized materials because, by nature, light should not be passing through them. Instead, attributes such as ease of polishing, the ability to accept a coating and durability under heat count the most. Refractive index, homogeneity and the number of bubbles or similar features in the glass are most important to transmission optics.

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Optical material properties

The innate (or engineered) properties of an optical material directly affect the selection process. Several factors must be considered, but the most important properties are:

• Index of refraction
• Dispersion
• Transmission value at any given wavelength
• Thermal expansion coefficient
• Density
• Hardness
• Elasticity
• Cost

The first three indicate how well a material will transmit, reflect or otherwise manipulate a light beam. A material’s thermal resistance, density, hardness and elasticity all relate to how well an optic will withstand the rigors of the environment in which it is used. And of all of these, the cost will tell you where else on your checklist you must make compromises in quality. Nearly everywhere, demanding better specs for your optic will equal a higher price.

“Client education is very much an issue,” said Brandon Light of !%Optimax Systems Inc.%! in Ontario, N.Y. “You want to move beyond the refraction index” when discussing the specifications with your optics supplier.

To Light and other engineers, the material selection is the fundamental consideration. The most important qualities of a material – beyond the refraction index – are not easy to pin down.

“We work with whatever meets the design and application needs, with particular attention given to relative cost and melt frequency,” said Gregg Fales of !%4064%%Edmund Optics%! in Barrington, N.J. He and Light also cite macroeconomic factors – such as the availability of a material – as having at least a modicum of importance.

Various glasses are getting more expensive because of the shrinking availability of rare-earth materials imported from China, Light said. Overall, the variety of glasses is dwindling. For example, New York-based Corning Inc. stopped producing calcium fluoride glass, leaving only two factories making it – one in Germany and one in Japan – to meet global demand.

Calcium fluoride is used for microlithography applications because of its compatibility with high-energy lasers. Courtesy of Schott AG.

Optical engineers consider specific requirements for each new situation. Making the right match is only a matter of understanding those needs well, knowing what is available and prioritizing one’s desired results.

The following is a rundown of the characteristics of some of the most often used materials, from glass to metals and beyond:

BAK1: A common borosilicate glass used for lenses and other transmission optics, BAK1 has moderate abilities and comes at a moderate cost. It is similar to other borosilicates, such as BK7, but has a somewhat better transmission in the near-ultraviolet range.

Barium fluoride (BaF₂): Used chiefly for windows in spectroscopy applications, BaF₂ is not found in a native state, and so all of it must be synthesized, making it relatively expensive to produce. It cleaves easily, is etchable and polishes well, all features that aid processing. It is highly susceptible to thermal shock, however.

BK7: Another common borosilicate, BK7 is a relatively hard material with good scratch resistance. It can be formed into optics with high homogeneity; thus, a low bubble or inclusion content. It also is easy to work with but not recommended for temperature-sensitive applications, such as precision mirrors.

Synthetic quartz glass is highly scratch-resistant and birefringent in nature. Courtesy of Schott AG.

Cadmium telluride (CdTe): CdTe is used in IR transmission applications, such as spectroscopy, up to 20 μm, as well as for IR filter substrates, low-power CO₂ lasers and some solar photovoltaic applications. It is highly toxic, however, and thus expensive to manufacture and problematic for dismantling and recycling systems that use it.

Calcium fluoride (CaF₂): Featuring low absorption, CaF₂ has a high damage threshold that lends itself to use as excimer laser optics. With a high thermal expansion coefficient and low index of refraction, it can be trusted to remain stable even without antireflection coatings when used to make windows, lenses, prisms and other optics.

Cesium bromide (CsBr): Mainly used in radiation-detection settings, CsBr can be found in some optical applications, including beamsplitters and prisms in broadband spectrophotometers, as well as for x-ray fluorescence screens and absorption-cell windows.

Crystal quartz (SiO₂): Featuring high transmittance in the infrared to ultraviolet range, SiO₂ is used primarily to make wave plates and polarizers because of its birefringent nature. It also is hard and scratch-resistant.

Fused silica: Generally free of inclusions and bubbles, fused silica offers a very low thermal expansion coefficient, high homogeneity and a high energy damage threshold, making it very suitable for both transmission and reflection optics. It is used in energetic laser applications, photolithography and light pipes as well as standard optics such as prisms, mirrors and lenses. UV-grade fused silica offers these benefits for deep-UV applications.

Suitable for both reflection and transmission optics, fused silica has a very low thermal expansion coefficient and high damage threshold. Courtesy of Schott AG.

Gallium arsenide (GaAs): In its basic form, GaAs is a semiconductor useful for making LEDs and solar panels. Optical-grade GaAs is used in infrared transmission. It is more expensive than germanium or zinc selenide, but it is hard and durable with a low absorption coefficient.

Germanium (Ge): Used primarily in infrared applications, Ge is a high-refractive-index material commonly used for attenuated total reflection prisms for spectroscopy, as a beamsplitter that doesn’t require coatings, as a substrate for optical filters and for lenses. It is best suited for low-power applications, including thermal imaging.

Lithium fluoride (LiF): Used mainly for applications in the ultraviolet range, LiF has the best transmittance in the UV of all available materials. It also is used for x-ray monochromator plates, but it is very fragile and hard to process.

Magnesium fluoride (MgF₂): As is LiF, magnesium fluoride is used mostly for UV optics. It also is found in infrared applications and excimer laser systems because of its resistance to thermal and mechanical shock and its high energy damage threshold. The material sometimes is used in biological and military imaging applications as well. Grown in vacuum and very durable, MgF₂ is slightly birefringent and easy to work to high tolerances.

Potassium bromide (KBr): It is most commonly used for infrared applications, such as lenses, beamsplitters and windows for gas and liquid sample cells in Fourier transform infrared spectrophotometers. A very soft material, KBr also is water-soluble and susceptible to degradation from atmospheric moisture.

Pyrex: Another borosilicate, Pyrex has too low homogeneity and too high bubble content and other inclusions to be considered an optical glass, so it is more often used for lab equipment than for optics. Nevertheless, its low thermal expansion coefficient and resistance to heat shock and high temperatures in general means that some use the material for common nontransmission optical applications, such as windows and mirrors, where toughness matters.

Rutile (TiO₂): Typically used as a whitener for pigments, TiO₂ also can be employed as a high-numerical-index (in the visible range) material that can be used between metallic layers in an optical system, such as between two coupling prisms or waveguides. Because of its high numerical aperture, it also is extensively used as a durable thin-film coating for beamsplitters, mirrors and other optics.

Sapphire (Al₂O₃): Tough and durable, artificially grown sapphire crystals are used extensively for optical windows and substrates in a wide variety of wavelengths, primarily infrared and ultraviolet. Sapphire often is found in lightguides, prisms, windows and other optics required in extreme environments.

Silicon (Si): Silicon is used for optical windows and filter substrates, mostly in infrared applications. Its applications run a very wide gamut, however, with optically relevant devices as small as nanowires to as large as 18-in. wafers.

Schott's glass ceramic Zerodur is used as the primary mirror in space-bound telescopes. Courtesy of Schott AG.

Zerodur: A glass ceramic made by Schott, Zerodur offers a thermal expansion coefficient near zero (fused silica is next closest at 0.52). With a high number of inclusions, however, it is best suited for mirror substrates rather than transmission optics.

Zinc selenide (ZnSe): Ubiquitous in infrared imaging and biomedical applications, as well as for prisms in attenuated total reflection spectroscopy accessories, ZnSe has a high index of refraction and high thermal shock resistance. It is fairly soft but offers low absorption, so it is best for high-power CO₂ laser applications at 10.6 μm.

Zinc sulfide (ZnS): Used for infrared imaging where rugged optics are necessary, such as IR viewers with military applications and as windows in IR spectrometers. The material can be treated in such a way as to be clear rather than sulfur yellow. This permits use in visible wavelengths as well, but at the expense of durability.