Optical properties of thin films — Where to?

The optical behaviour of a surface carrying a transition layer is discussed, as a problem in scattering. The results are given of the application of electromagnetic theory to the case of a single parallel-sided film. Methods are given for extending this treatment to the case of multiple parallel-sided layers. Both analytical and graphical solutions are considered. Features governing the design of certain multilayer systems are given. A survey is made of the many methods now available for the determination of the thickness and optical constants of materials in the form of thin films. The range, accuracy and conditions of application of the various methods are summarized. A selection of the results obtained on a range of metallic and dielectric films is made and the basis of interpretation of these results is given. It is shown that the apparently anomalous variations of the optical constants of metal films with film thickness can be attributed to the granular nature of such films. Although less spectacular variations are observed in the properties of dielectric films, it is seen that for some materials the optical behaviour suggests the presence of marked inhomogeneity and anisotropy. Films formed chemically and electrolytically are briefly mentioned. Recent methods of controlling the thickness of films during deposition are given, together with a selection of the more important applications of thin films in optics. These include anti-reflecting and high reflecting systems, narrow-band, wide-band and cut-on multilayer filters, the use of thin films in polarizing systems and the application of thin films in heat-absorbing systems.

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