Ellipsometric characterization of the optical constants of polyfluorene gain media

Variable-angle spectroscopic ellipsometry (VASE) has been applied to five polyfluorene gain media. The ellipsometric data have been analyzed using an electronic model based don critical points of zero dimension, i.e. an exciton model. The optical constants of thin-film samples on spectrosil B substrates have been deduced and are used to characterize the waveguiding conditions in these asymmetric slab structures. The exciton model that we have used leads to a small correlation of the parameters and accurate fits. The good match with normal-incidence transmission spectrophotometry data and surface profilometry determinations of thickness gives further confidence in the suitability of this model. Based on our measurements, we calculate the cutoff thicknesses for he fundamental TE-guided modes in our silica-polymer-air structures to lie within the range of 40 nm to 70 nm (depending on the specific polymer), and demonstrate corresponding confinement factors (T) between 37% and 68%. The calculated thickness dependence of the cut-off wavelength agrees well with experimental data for the thickness dependence of the peak amplified spontaneous emission (ASE) wavelength and is, therefore, consistent with previous explanations of the ASE spectral shifts.