Interference enhancement of the sensitivity and the signal-to-noise ratio in differential spectrodensitometry of surface layers with normally reflected light

A simple method for studying a superthin surface layer (SL) on an interference film through the measurement of differential reflection-a relative variation of the reflectivity caused solely by the SL-is considered. The effect of the SL is analyzed by perturbation method. Within the framework of the quasi-local approximation for the refractive index of the SL, a formula describing the SL contribution to the ordinary Fresnel coefficient for reflection from an interference film with sharp interfaces (without an SL) is obtained. The utilization of an interference film in differential spectrodensitometry (spectroscopy) is shown not only to allow a significant enhancement of the sensitivity of this method and improvement of the signal-to-noise ratio of photometric measurements but also to open up new possibilities for resolving the inverse problem of determining SL parameters.