Coupling efficiency of fluorescent molecules to a sensing waveguide

Optical biosensors are often based on fluorescence analysis of chemical reactions where one reagent is immobilized on the surface of a suitable substrate. The application of a thin optical waveguide at the substrate surface enables a highly efficient and selective excitation of fluorescent molecules in close proximity to the waveguide surface via the evanescent field of the guided mode. Due to the coupling of the fluorophores to the evanescent field, a substantial part of the fluorescence light is coupled back into and collected by the waveguide, where the coupling efficiency depends on position, environment and orientation of the fluorophores. The utilization of this signal for fluorescence detection and analysis could allow a significant simplification of the employed optical instrument. In this study, we present a fundamental investigation of the fluorescence collection efficiency by the waveguide by theoretical and experimental means and explore the suitability for fluorescence analysis techniques.