PHOTOINDUCED CHANGES IN THE COMPLEX INDEX OF REFRACTION IN CONJUGATEDPOLYMER FULLERENE BLENDS/

We report steady-state photo-induced absorption !(PIA) and photo-induc ed reflectance (PIR) in films of poly[2-methoxy-5-(2'-ethyl-hexploxy)- p-phenylene vinylene] (MEH-PPV) and poly[2,5-bis cholestanoxy-1,4 phen ylene vinylene] (BCHA-PPV) blended with fullerenes, Absorption from th e metastable charge-transferred state is probed by PIA; the modulated absorption spectrum causes changes in the real part of the index of re fraction, Delta n, which can be measured directly by PIR. The charge t ransfer gives rise to pronounced features in Delta n, including vibrat ional structure in the mid-infrared, and broad spectral windows with n egative 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 Delta n obtained from PIR with that obtained from Kramers-Kronig trans formation of the PIA data, We find good agreement throughout the infra red, indicating that our method for measuring Delta n is useful as an analytical tool for optical characterization and for prediction of opt ical spectral ranges for nonlinear optical response. (C) 1998 American Institute of Physics. [S0021-9606(98)52304-5].