Technique for determining the angular orientation of molecules bound to the surface of an arbitrary planar optical waveguide

A technique to determine the angular orientation of a molecular assembly bound to the surface of a planar optical waveguide of arbitrary structure is described. The approach is based on measuring the absorption dichroic ratio by using the waveguide evanescent fields with orthogonal polarizations (TE, TM) and the same mode order to probe two molecular assemblies, (i) a reference sample composed of an isotropic orientation distribution of dipoles and (ii) a sample of interest. The isotropic sample is used to characterize the waveguide structure, which then allows the orientation parameters of a molecular assembly under investigation to be determined from a measured dichroic ratio. The method developed here is particularly important for applications in gradient-index and multilayer planar waveguide platforms because in those cases the extension of previously reported approaches would require a full experimental characterization of the guiding structure, which would be problematic and may yield inaccurate results.