INFRARED OPTICS ALON and spinel excel as infrared optical materials MOHAN RAMISETTY, LEE GOLDMAN, SURI SASTRI, UDAY KASHALIKAR, NAGENDRA NAG, and SREERAM BALASUBRAMANIAN The two infrared (IR) optical mate- rials aluminum oxynitride (termed ALON by Surmet), which has a cu- bic spinel crystal structure, and mag- nesium aluminate spinel (commonly called spinel) possess several desir- able and unique properties. 1 Chief among them are durability, chemi- cal resistance, high hardness, and high strength, along with broadband spectral transparency (UV through mid-wave IR). Although the mate- rials have been studied in laboratories for de- cades, they have only recently become com- mercially available (see Fig. 1). Growing interest in the de- fense and optics industries has driv- en the market for these materials in recent years. Moreover, their com- bination of mechanical and optical properties makes them extremely at- tractive in many other areas. Both materials look the same as glass, so what is so special about ALON and spinel? There are sever- al facets to the answer. First, ALON and spinel transmit over a broad- er range of wavelengths; they are transparent in spectral regions where glass is as opaque as steel. Second, their excellent mechanical proper- ties provide them with extreme en- vironmental durability, making them about ten times more abrasion resis- tant than glass. They also survive much higher temperatures and more aggressive chemical environments than glass can withstand. Another transparent ceramic ma- terial, sapphire (alumina in single- crystal form), is also hard, durable, and transparent. So what makes ALON and spinel different from sapphire? In order to be transpar- ent, sapphire must be grown as single-crystal boules or sheets, and subsequently cut, ground, and polished into final com- ponents. On the other hand, by virtue of their cubic crystal structure, ALON and spinel are transparent in their polycrystal- line form, allowing them to be produced using conventional ALON and spinel, two transparent ceramics used in IR optics, each exhibit a unique combination of properties; a comparison allows the optical engineer to select the right material for the job. FIGURE 1. Examples of ALON and spinel components produced by Surmet are shown, including a component with a metal grid incorporated into it (upper right). (Courtesy of Surmet) Reprinted with revisions to format, from the August 2014 edition of LASER FOCUS WORLD Copyright 2014 by PennWell Corporation