A thin absorbing layer at the center of a Fabry-Pérot interferometer

(Received 7 September 1993, accepted in final form 9 December 1993)

Abstract
The influence of a dye (Rhodamin B) dissolved in solution or adsorbed at an interface at the center of a Fabry-Pérot interferometer on the transmission was investigated both experimentally and theoretically. We show that (i) spectra of extremely thin films are measurable at sub-monolayer concentration (one monolayer reduces the transmission by %), (ii) the transmission is sensitive to the location of an extremely thin film within nm range and (iii) the absorption coefficient is determined quantitatively by comparison between theory and experiment thus permitting one to estimate the local concentration. The results are relevant for the use of dye probes to monitor changes of organic interfaces in the Surface Forces Apparatus as well as generally to detect changes of interfaces between adjacent surfaces.