Comparative analysis of absorbance calculations for integrated optical waveguide configurations by use of the ray optics model and the electromagnetic wave theory

Focusing an the use of planar waveguides as platforms for highly sensitive attenuated total reflection spectroscopy of organic thin films, we extend t he ray optics model to provide absorbance expressions for the case of dichr oic layers immobilized on the waveguide surface. Straightforward expression s are derived for the limiting case of weakly absorbing, anisotropically or iented molecules in the waveguide-cladding region. The second major focus i s on the accuracy of the ray optics model. This model assumes that the intr oduction of absorbing species, either in the bulk cladding or as an adlayer on the waveguide surface, only causes a small perturbation to the original waveguide-mode profile. We investigate the accuracy of this assumption and the conditions under which it is valid. A comparison to an exact calculati on by use of the electromagnetic wave theory is implemented, and the discre pancy of the ray optics model is determined for various waveguide configura tions. We find that in typical situations in which waveguide-absorbance mea surements are used to study organic thin films (k(l)/n(l) less than or equa l to 10(-1), h/lambda approximate to 10(-2)) the discrepancy between the ra y optics and the exact calculations is only a few percent (2-3%). (C) 2000 Optical Society of America.