Previous papers at this meeting have highlighted the potential afforded by molecular beam techniques in producing optical thin film structures with a high resistance to laser induced damage. These techniques have been recently applied to the growth of polycrystalline thin films of candidate fluoride materials suitable for use over a wide range of laser wavelengths. Here the chief materials of interest have been barium and lead fluoride. Despite sharing a common vaporisation process, significant differences exist in the behaviour of the two molecules on condensation. In the case of barium fluoride, the surface morphology is rough in films produced at room temperature and it is necessary to deposit at significantly elevated temperatures to obtain smooth fi1ms even in UHV. In comparison, lead fluoride films have an exceptionally smooth morphology, even when produced at ambient temperatures. Such factors have an important influence on the laser damage thresholds of the films.
In multilayer designs, some control of columnar microstructure is also possible by a stratification technique where two differing component materials are used to prevent microstructure propagation. The coating is built up by alternating the materials throughout the coating until the desired thickness is achieved. By continuously varying the relative thicknesses of the individual laminations, it is possible to produce graded index coating designs. These allow the exploration of interface effects, hitherto difficult using optical techniques.