Spectral Characteristics of an Inhomogeneous Film 1. Reflectance and Transmittance Spectra for Different Film Refractive Index Profiles

The reflectance and transmittance spectra of an optical thin film exhibit a number of maxima and minima and the envelopes of these extrema are used in the ‘envelope method’ to determine the refractive index, extinction coefficient and thickness of the film. This method works quite well so long as the film is homogeneous. But when the film is inhomogeneous, the determination of the refractive index profile, i.e. variation of refractive index with depth, from the transmittance/reflectance spectra is not a straightforward task, even when the film is weakly absorbing. In order to aid this determination (which is the subject of the following paper), the trends of the reflectance/transmittance extrema envelope curves with increasing wavelength have been studied for different types of film inhomogeneities and compared with the results for a homogeneous film having a refractive index equal to the average refractive index of the inhomogeneous film. The earlier studies in this direction have been extended and a more comprehensive study undertaken.