The change in refractive index of poly(9,9-dioctylfluorene) due to the adoption of the β-phase chain conformation

The focus of this work is to study the refractive index in thin films of blue light-emitting poly(9,9-dioctylfluorene) (PFO) that contain extended rigid chains (β-phase). Using a post-deposition exposure to toluene vapour, β-phase chains were induced in glassy PFO films that had been spin coated from a range of solution concentrations to provide thicknesses from 50–200 nm. With the aid of absorption spectroscopy, a semi-empirical calculation of the change in refractive index due to the presence of β-phase chains is undertaken. Over the spectral region where optical gain occurs, the refractive index of β-phase PFO is found to be appreciably larger than for as-spin-coated glassy PFO. Further development and explicit evaluation of the Kramers–Kronig relations lead to analytic and closed form expressions describing the change in index Δ n which are used to highlight the key contributions to index change. We find that the spectral response of Δ n is largely governed by the emergence of a characteristic absorption band associated with the presence of β-phase chains. The changes in refractive index are experimentally studied by tracking the spectral shifts of optical resonances observed from films spin coated directly onto sub-wavelength 1D grating structures. These measurements are supported and compared with rigorous calculations of the spectra, which incorporate the full optical descriptions for glassy PFO and β-phase PFO. The excellent agreement confirms the accuracy of our semi-empirical extraction approach. Similar polymer-coated grating structures are examined under lasing conditions and can also be used to support the expected spectral response and the decline of Δ n for longer wavelengths.