Transmission spectra of systems with thin films: classical analog of the Fano effect

This work is devoted to the analysis of the shape of transmission spectra of stratified systems with a deposited thin adsorbed layer. It is demonstrated, that the transmission spectra are in general determined by coherent light scattering of the molecules in the deposited layer and interference of this scattered light with the incident one. As a result, the shape of the dispersion of both imaginary and real parts of the layer's dielectric function determines the transmission spectrum instead of only the imaginary part as in the case of transmission spectra of adsorbed layers on a transparent bulk substrate. The fact that such spectra can not be constructed from the absorption spectra of the stratified system and the deposited film alone was often the reason for seeking of new interactions in such systems. We demonstrate that this non additive property of the spectra has to be expected for such systems in general also in the absence of such interactions and that the position of the minimal transmission in a normalized spectrum does not correspond to the position of an absorption line. It is shown that the shape of such spectra is described by an asymmetrical line like a Fano line and that the origin of such a shape can be associated with the physics underlying a Fano resonance. A new approach for the determination of the dielectric function of deposited films is proposed on the basis of the presented analysis. It is demonstrated experimentally and theoretically that the deposition of a thin absorbing layer can give rise to an increase in transmission of an initial system. Conditions of such an increase are presented.