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Thermally evaporated thin films of TlI have been found to grow epitaxially on KCl(100) surfaces equally with two mutually perpendicular orientations. We present x-ray and optical data to show that the TlI(004) plane grows with its bo axis parallel to each of the two KCl 〈110〉 directions that lie on the (100) surface. This unusual double epitaxial microstructure with mutually perpendicular orientations results in optical scattering by the diffraction of light passing from one biaxially birefringent orientation to another. Due to the highly desirable properties of TlI as an optical coating material for high powered infrared laser components, several techniques have been devised to minimize the optical scattering and improve film homogeneity. Data are presented on the use of a thin buffer layer which severely reduces the epitaxial growth of TlI resulting in homogeneous films similar to those deposited on glass. Epitaxial TlI films may also be made with no visible light scattering by alternating thin layers of TlI with those of another material. This alternating layer method takes advantage of the polymorphism of TlI in thin layers to produce films in the cubic phase. We show by x-ray and optical measurements that each TlI layer possesses the CsCl structure at room temperature.

Efficient T1I /KCl /T1I antireflection (AR) coatings (< 0.1 % reflectance per surface), which are highly resistant to laser radiation damage (< 0.04 % absorption per AR coating at À = 10.6 µm) have been fabricated. The three -layer AR coatings were designed by replacing one of the layers of a two -layer AR coating with a Herpin equivalent structure. Thickness of the layers of the resultant three -layer AR coatings can be varied continuously over ranges defined by the indices of the coating materials and the substrate. The ability to vary the thickness of the layers enables the optimization of the coating design with respect to one or more requirements, for example, low absorption, environmental durability or ease of fabrication.The TlI films deposit with an orthorhombic structure, the orientation of which is dependent on the KC1 substrate orientation. Therefore, the refractive index of the TlI films depends on the substrate orientation. The influence of the substrate orientation on the TlI refractive index can be eliminated by coating the KC1 substrate with a thin layer (rv200 A) of SrF2 prior to depositing the TlI film.