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Mussett, Edward Anthony (1957) Some optical properties of evaporated films and their measurement. Doctoral thesis, Northern Polytechnic.
An investigation is made into certain optical properties of thin, vacuum evaporated, dielectric films. In particular, the variation of refractive index which occurs in a direction normal to the plane of the film has been studied. Evidence for the occurrence of such inhomogeneities is reviewed and some possible causes discussed. The material chosen for the experimental study was zinc sulphide and the investigation of the non-uniformities proceeded along two lines. Firstly, measurements were made on a series of films of both their optical and their metrical thicknesses to determine if these are related according to the theory for a single homogeneous layer on a backing plate. Certain deviations of the experimen tal data have suggested the presence of boundary layers in the zinc sulphide films though the optical stability of these films was found to be extremely good. The magnitude of the inhomogeneities depends upon the evaporation conditions; for zinc sulphide, the overall refractive index has been found to be dependent on the source temperature during the evaporation. Also, theory predicts that the optical and the metrical thicknesses are not exactly proportional but that small perturbations from linearity occur; a study is made of the phase retardation of the transmitted beam and the theoretical behaviour has been experimentally confirmed.
The second approach has yielded a more powerful method for detecting film inhomogeneities and involves measuring the Brewster angle for the film. A general study is made of this much-used method for determining the refractive index and it is shown that the presence of boundary layers in the film can shift the apparent Brewster angle considerably. This process has been used in reverse; by measuring the shift of the Brewster angle the inhomogeneities in the zinc sulphide films have been estimated. The results have explained why abnormal dispersions are often measured when the Brewster angle method is used to find the refractive index of a thin film.
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