Photo-induced changes in the complex index of refraction in conjugated polymer/fullerene blends

We report steady-state photo-induced absorption (PIA) and photo-induced reflectance (PIR) in films of $\mathrm{poly[2-methoxy-5-(}{2}^{\prime }\mathrm{-ethyl-hexyloxy)-}p\mathrm{-phenylene\; vinylene}]$ (MEH-PPV) and poly[2,5-bis cholestanoxy-1,4 phenylene vinylene] (BCHA-PPV) blended with fullerenes. Absorption from the metastable charge-transferred state is probed by PIA; the modulated absorption spectrum causes changes in the real part of the index of refraction, $\mathrm{\Delta n,}$ which can be measured directly by PIR. The charge transfer gives rise to pronounced features in $\mathrm{\Delta n,}$ including vibrational structure in the mid-infrared, and broad spectral windows with negative $\mathrm{\Delta n}$ in the mid- and near-infrared. Our measurements over a wide spectral range (0.05 eV–1.9 eV) allow quantitative comparison of $\mathrm{\Delta n}$ obtained from PIR with that obtained from Kramers–Kronig transformation of the PIA data. We find good agreement throughout the infrared, indicating that our method for measuring $\mathrm{\Delta n}$ is useful as an analytical tool for optical characterization and for prediction of optical spectral ranges for nonlinear optical response.