Fundamental Optical Phenomena in Infrared Window Materials

Abstract

A wide variety of high-energy lasers and associated applications (1) and optical systems with very long path lengths [e.g. fiber optics (2)J have emerged in recent years. These developments have led to corresponding requirements for highly transparent window materials (3). Ideally, a window material would transmit light without any effect on either the light beam or the window. Obviously, no such ideal material exists or is really required; moreover, specific types of beam modi­ fications are often not only tolerable but desirable. Thus, for example, we may require a material which refracts light, modulates light intensity, modifies polariza­ tion, or the like, yet acts as a window in all other respects. Our principal emphasis here is on properties of materials for totally passive windows, although those appropriate for active components are considered briefly. We are concerned with the fundamental optical phenomena of absorption, refraction, and photo­ elasticity in window materials. The latter phenomena underly or contribute to more complex optical effects such as thermally induced lensing (4), laser-induced interference (5), laser-induced breakdown (6), ilnd self-focusing (7), for example. Typical observations of laser-induced lensing and interference in infrared materials are indicated in Figure 1. The primary emphasis here is on bulk infrared materials, especially those for 10 11m (C02) and 3-5 11m (chemical) laser applications. For this case, one is then concerned with the highly transparent frequency regime in insulating or semiconducting solids, which lies well above the principal lattice absorption bands, but well below the onset of the electronic absorption edge. Clearly, in order to achieve high transparency over an extended frequency region, materials with low lattice resonances Wo and high-energy gap frequencies Wg are desirable. It is useful to correlate the latter properties with molecular weight and with families of compounds of the same structure, as pointed out by Hilton (8). The graphs in Figure 2 vividly indicate the decrease in wyand increase in Wo as