Electronic and Raman-type nonlinearities of thin films of oligomeric paraphenylene vinylenes Original Research Article

Electronic and Raman-type third-order susceptibilities χ(3) of thin films of oligomeric paraphenylene vinylenes OPV-n (n = 1−5) were determined by nondegenerate four-wave-mixing techniques. These two contributions and third harmonic susceptibilities differ considerably with respect to their magnitude but show a similar increase with chain length. The different susceptibilities were calculated on the basis of a Hückel approach taking into account the increasing delocalization of π electrons with chain length n. The ratios of the different contributions are quantitatively explained by a dominating electronic two-photon (TP) process due to a TP transition near 300 nm and by the Raman resonance of a vibrational mode at 1588 cm−1 exhibiting most of the electron-phonon coupling strength (ECC mode).